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The significance of the so-called law of cephalocaudal differential growth.

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T H E SIGNIFICANCE O F THE SO-CSLLED LAW O F
CEPHALOCAUDAL DIFFERENTIAL GROWTH
B. F. KIKGSBURY
Department of Histology and Embryology, Stwnson Hall, Cornell University,
Ithaca, New Pork
During the past ten years the writer has, in lectures on
the development of the body, stressed as an important feature
of developmental transformation what he usually styled the
‘law of cephalocaudal differential growth. ’ Although embryological workers have heen quite familiar with the fact
that the differentiation of a preponderating portion of the
body proceeds as a ‘wave’ in the head-to-tail direction, comment on the fact or discussion of its sigiiificance seems t o
have been slight. The somites furnish the most obvious illnstration, and the statement of Minot (’11,p. 2) that “the
development of segments begins at the cephalic end and progresses tailwarcl ; hence so loiig as the development of segments continues various stages of their differentiation may
be found in a single embryo, the more advanced stages being
cephalad from the less advanced” may be taken as expressing
the generally recognized fact. The more cephalic somites,
in fact, will he quite completely differentiated before the most
caudal have actually appeared.
What is true of the somites seems to be equally true of the
structures formed from them. The myotomic muscles differentiate in a cephalocaudal sequence. The growth of the vertebrae conforms to the cephalocandal growth law (Calkins,
’23), although in the process of their ossification the sequence
appears to be not strictly followed (cf. Nall, ’06). Furthermore, the neural tube (spinal cord) is formed (grows and
Law of anteroposterior development (Child) ; law of cephalocandal growth
(Calkins) ; law of developmental dircctioii (Scammon), etc.
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T H E ANATOMICAL
RFCORD, VOL
27, KO. 5
306
B. F. KINGSBURY
differentiates?) in a head-to-tail direction ; the spinal ganglia
in their appearance seem to conform to the rule and doubtless
likewise in their growth and differentiation. The notochord
appears t o be continuously formed by appositional growth
at its caudal end (Keibel, ’99) and its differential change to
proceed cephalocaudally. The differentiation of the nephrotomic material, as is of course fully appreciated, advances
caudally ; the pronephros antedates the mesonephros, the
most cephalic portion of the mesonephros has begun its degeneration before the caudal portion has attained its development. The mesonephric duct grows caudally, etc. The first
branchial cleft, pouch, and arch to form are the most cephalic.
In the arm bud differentiation proceeds in a cephalocaudal
direction (Harrison, ’21, p. 5, for Amblystoma). The arm
bud antecedes the leg bud in its appearance and growth and
(somewhat) in its differentiation. The more cephalic muscles
of the trunk respond to stimulation first and the muscles of
the upper extremity before those of the lower extremity
(Carey, ’22, p. 6). Other illustrations might be offered. T t
is undoubtedly correct t o state that the body wall throughout
the neck and trunk in a general way exhibits the characteristie progression of differentiation in a cephalocaudal direction
in accordance with the underlying developmental principle
which the writer has expressed under the convenient though
inadequate term of the law of cephalocaudal differential
growth.
Scammon (’23, p. 23) terms it the law of developmental
direction, and gives us the following concise but quite adequate statement of it: “While each part passes through its
own cycle of changes these changes as a whole tend to follow
what is known as the law of developmental direction; for it is
generally found that development (including growth and differentiation), in the long axis of the body, appears first in
the head region of the body and progresses toward the tail
region and similarly development in the transverse plane
begins in the mid-dorsal region and progresses lateroventrally (in the limbs proximodistally).”
CEPHALOCAUDAL DIFFERENTIAL G R O W T H
307
As has been intimated, the significance of the ‘law’ has
received scant attention. The explanation of the existence of
such cephalocaudal sequence seems, however, to be quite
simple. The bodily material exhibiting in its development
this feature is laid down by a process of differential growth
at the caudal end. The more cephalic portion of the body is,
therefore, older and accordingly farther along in differentiation and growth; regions farther caudad are younger and less
advanced in growth and differentiation. At successive intervals of time a given developmental stage occupies thus progressively lower and lower (i.e., more caudal) levels, and
hence the apparent wave, progression, or direction. However,
it at once becomes clear that cephalocaudal differentiation or
cephalocaudal growth-growth in a head-to-tail directionhas no existence in fact, and the ‘law’ becomes but a convenient formula or phrase in the description of observed fact.
Possibly it would not be quite correct to say that development
appears first in the head region and progresses toward the
tail region. Graham Kerr (’19, p..195) has recognized the
law in its most significant features in the following pertinent
paragraph :
. . . . it is necessary to bear in mind that the Vertebrate in early
stages develops from before backwards and that the growth in length
by the addition of new segments takes place a t its hinder end where
there is a mass of actively growing embryonic tissue forming a kind
of ‘growing point.’ The tissue of this, although to the eye quite
undifferentiated, contains the elements which form all the various
tissues such as the nerve cord, notochord, myotomes, alimentary canal,
etc. As growth goes on these gradually become differentiated out,
the differentiation always proceeding from before backwards. . . . .
we see the typical Vertebrate structure, including the alimentary
canal . . . extending right back practically to the tip of the tail:
it is only a t the extreme tip that the various organs merge together
into undifferentiated embryonic tissue.
,
This lucid statement requires no comment or supplement
save in one respect. Kerr here gives no suggestion that he
appreciates the embryological significance of the ‘growing
point’; nor does he elsewhere in the volume clearly indicate
308
B. F. KINGSBURY
that he recognizes-as lie doubtless did-that the region of
differential growth to which he refers is the blastoporal lip2
and subsequently the ‘tail bud’ into which through growth it
is transformed. 0. Hertwig (’92, ’11) and Reihel ( ’11)3
particularly, among others, have adequately considered this
aspect of development. So concliisive is the evidence when
fully considered, that differential growth iii and from the
blastoporal lip (primitive streak) may, in the opinion of
the writer, be accepted as an establislied fact in the pattern
of vertebrate differentiation. Differential transformation
continuiiig in the material so laid down by growth in the
region of tlie blastopore (primitive groove), cephalocaudal
progression in derelopmeiit follums as ail iiievitable result.
Scammon ( ’23), in the passage quoted above, has described
the development as appearing ‘first in the head region.’
Graham K e r r ( ’19), from a rather teleological viewpoint,
comments on this fact i n considering the vertebrate h e a d 4
From the mode of differential growth the more cephalic portion begins its differential growth first, and therefore, at any
given stage, may be expected to be further along in both
growth and cliffereiitiation. The development of tlie head
requires, however, rather special consideration, and only briclf
comment can be made here. Yet it is obviously important iii
connection with the subject of this paper to ascertain as
* It is perhaps quite unnecessary to point out that in the I c h t h y p s i d a with
increased yolk the blastoporal lip becomes the margin of the blastoderni, while
in the Sauropsida and RZanim:tlia it is repreuented by the primitive streak or by
a blastopore converted into a primitive streak. The tail bud, derived from the
hlastoporal lip (primitive streak) through continled growth, is of course poorly
named, as Keihel has pointed out.
”Page
47, in the American edition; page 53, German edition, and earlier
discussions.
‘Pnge 499. ‘‘It has already been pointed out that organs of great complexity
in the adult tend t o be laid down at a n early stage of individual development,
time being thus obtained f o r the development of their complex detail. It is
perhaps in direct relation to this principle t h a t the highly complex head-region
of the Vertebrate, which comes t o assume control over most of the activities of
the individual, develop particularly early in ontogeny-the various developmental
processes making themselves a s a rnle first apparent in the head region and
spreading thence tailwarcls along the trunk. This fact is of practical importance
to the embryologist for in the case of segmentally repeated organs it enables
him t o find a series of developmental stages within the body of a single eniliryo. ”
CEPHALOCAUDAL DIFFERENTIAL GROWTH
309
nearly as possible the region where the cephalocaudal progression in differentiation begins, or, in other words, the
region that is differentially oldest. As will appear subsequently, this is not altogether an easy matter to determine,
since the marked growth expansion that characterizes the
early development of the head is necessarily accompanied by
considerable alterations in the early material arrangement.
It is, however, at once obvious when the early development
of the head is examined that the cephalocaudal progression or
'wave' of differential growth does not begin either at the
definitive o r the primitive end of the body. Tn the elasmobranch, according to Scammon ('ll), the mandibular somite
is only formed after four or five postotic samites have appeared. The hyoid somite soon follows, while the premandibular (and so-called anterior ' somite ') appears only some
time subsequently. The closure of the neural tube is accomplished first at a point considerably caudad to the anterior
end of the neural plate and advances thence both forward
and backward, the closure toward thb anterior end being accompanied by the marked growth expansion of the cranial
neural plate and tube which seems to postpone correspondingly the completion of closure.
It is obvious that while differential growth is adding to the
body caudally, determining thus the cephalocaudal progression, the more anterior material destined t o form the head
is undergoing a marked growth expansion in the opposite
direction, and exhibiting in different portions both a cephalocaudal and a caudocephalic direction in differentiation.;
5 h l r . Adelmann and myself ('24) have attempted to indicate the general
pattern of cranial morphogenesis, but for no form has there been drawn up a
correlated picture of the detailed growth transformations ; while any determination of a n orderly sequence of histogenetic events has hardly beed made. According t o the study of Mall ('06) ossification begins first in the mandible (and
clavicle) the ossification in the head bones antedating the appearance of ossification in the trunk (though not in the extremities). On the other hand, the
differentiation of the cranial muscles clearly occurs only some time subsequent
to the appearance of the myotomic muscles of the trunk. The general trend of
ossification in the head, beginning with the mandible, is eaudocephalic; I know
of no observations permitting any statement as to any recognizable sequence in
the differentiation of the muscles.
310
B. F. KINGSBURY
Brachet ( ’21, p. 352), having particular reference to segmentation in the paraxial mesoderm, apparently places a sharp
boundary between cephalocaudal progression (in the trunk)
and a caudocephalic progression (in the head) at the caudal
limit of the head-a boundary of considerable theoretical importance to him in his conception of vertebrate development,
since it separates ‘cephalogenesis’ from ‘notogenesis.’ The
writer is inclined to regard it as questionable whether such
a sharp boundary may be drawn between a region of forward
progression and one of backward progression. The location,
furthermore, of such a boundary between head and trunk
appears not fully supported by the facts. Cephalocaudal
progression begins apparently well within the territory of
the future head and certain structural features, for example,
the branchial region, follow the head-to-tail sequence in their
development. I n the opinion of the writer, the level of the
mandibular arch may be taken as marking the cephalic limit
of the territory within which the law of cephalocaudal progression might be expected to hold. However, the cranial
portion of the neural tube, paraxial mesoderm, and (possibly)
notochord appear to exhibit the law slightly, if at all; possibly
the great forward growth and expansion of this material
may be a factor. Anterior to the level of the notochordthat is, in the territory of the prechordal portion of the heada more o r less well-defined caudocephalic progression in
growth and differentiation seems to prevail. It may again
be emphasized that the ‘law’ but expresses a time difference
in the growth and differentiation of the regions compared,
and is without other or deeper significance. There is, I think,
some suggestion that marked growth is attended by postponed or retarded differentiation, and vice versa.
A cephalic ‘growing point,’ comparable to the caudal growing point in which neural tube, notochord, mesoderm, and
entoderm are confluent and from which they become differentiated through growth, does not characterize vertebrate development. The mesoderm, notochord, and entoderm remain
for some time confluent in the prechordal plate (protochordal
CEPHALOCAUDAL DIFFERENTIAL GROWTH
31.1
plate, completion plate), but the material that becomes the
anterior portion of the neural plate is early separated, maintaining apparently, in the region of the primitive infundibulum, a close association or possibly even fusion with the
prechordal plate. It can hardly be doubted that a high degree
of differentiation accompanies the growth in these two layers
(anterior-prechordal-neural
plate, prechordal plate), particularly in the former. Forward growth with an accompanying ‘overgrowth’ and down-folding adds to the complexity of the picture. The importance of determining the
growth pattern in the region of the prospective head is obvious. It can hardly be said, however, as is sometimes done,
that the head is developed first, the development of the trunk
following.
The growth expansion in the transverse plane of the body
may be briefly considered. Although it has been said that
there is a progression in development from the middorsal to
the midventral line, it is clear that in describing a dorsoventral sequence in direction of growth and differentiation
we are dealing with facts of quite different significance from
those determining the cephalocaudal sequence in development.
There is no zone of differential growth that moves ventrally
and lays down ectoderm and mesoderm (and entoderm) as
is the case in the differential growth from the primitive streak
and tail bud. It is mainly the transformations in the mesoderm that are responsible for the formulation of a ‘law of
dorsoventral growth.’ This finds its basis in the quite wellestablished origin from the somites of the somatic musculature of trunk (and neck?) together with the skeletal constituents and presumably a large portion at least of the
related connective tissue.
The ventral body wall, in its main constituents at least,
originates well dorsally and moves, relatively, ventrad in the
process of growth. However, growth expansion from the
somites is likewise dorsad and mediad as well as ventral, and
the expansion is both dorsal and ventral to the neural tube.
The dorsal body wall is clearly older and bulks larger in
312
B. F. KINGSBURP
any given stage than the ventral body wall. It is only relatively late that the ventral body wall from the symphysis
menti to the symphysis pubis attains its completion. There
is, however, this agreement between the cephalocaudal progression and the dorsoventral progression : both but express
time differences in developmental events as determined by
the mode of origin of the material concerned.
The progressive growth differentiation of a large portion
of the body in and from the primitive streak, thus determining
the so-called law whose significance we have been considering,
has far-reaching effects on the developmental pattern. Certain of these may be briefly commented on:
1. Closely linked to the time difference in the progress of
differentiation is the growth aspect. The eyes which so early
begin their development with the expansion of the apical
(prechordal) portion of the neural plate, and the head as a
whole, are relatively enormous in size in the early embryo.
The legs as compared with the arms are correspondingly
small. It: is only as maturity is approached that the final
bodily proportions are attained, as is so well shown by the
figures of Stratz ( ’09), frequently copied.
2. While in presomite stages demarcations are indeterminate, it is nevertheless clear that the material ahead of thc
primitive streak is mainly, at least, prospective head. When
four somites have been formed (occipital somites) and thc
prospective caudal limit of the head is more definite, tmothirds (approximately) of the total length including primitive streak is occupied by prospective head. Even after
twenty-three somites have been formed and the liver diverticulum has appeared, that structure and the septum transversum are scarcely caudal to the level of the head. Gradually, as more material is differentiated caudally (from the
tail bud) and that already formed expands, the body wall
‘pushes forward’ so that the visceral structures occupy progressively lower and lower levels in relation to the somatopleure, giving the phenomenon often spoken of as the ‘descent
of the heart,’ or, more broadly, the ‘descent of the viscera.’
CEPHALOCAUDAL D I F F E R E N T I A L GROWTH
313
There are included the heart, lungs, diaphragm, stomach,
liver, pancreas, and, clearly, a portion of the intestine. Laryngeal structures and derivatives of the metapharynx share
in the ‘descent, to a certain extent. Each of these organs
possesses, of course, its own individual growth expansion,
but it may be clearly recognized that the ‘descent’ is mainly
but the expression in a plastic mass of the forward crowding
of the dorsal-somatic-material
as it is formed and grows ;
while this early and marked relative displacement in a cephalocaudal direction of the developing structures above mentioned is determined by the mode of differential growth from
the primitive streak (and tail bud). The relative developmental shiftings in the caudal portion of the body of visceral
organs and body wall are less easily determined-or at least
have been less determined. There is, however, cephalocaudal
descent, as well as also relative displacement in ail opposite
direction.
3. Inasmuch as the ventral body wall grows in a dorsoventral direction, and the more cephalic portion, from its
mode or origin, is farther along in development, a two-fold.
effect may be expected: a ) a ‘closure’ of the ventral body
wall in a cephalocaudal direction; b ) accompanying the ventral
growth a relative displacement caudally as the growth expansion in the meantime carries the associated dorsal material
cephalad, giving in a degree a relative ‘descent, of ventral
body-wall material. These expected results are in part realized : The sternum forms and closes in a cephalocaudal direction ; the ribs inclining more and more caudally as the process
advances. The relative ventrocaudal movement of the abdominal body musculature may be mentioned.
I n the neck the picture is obscured by some uncertainty as
to the source of the ventral neck muscles as well as insufficient
knowledge of the general growth expansions. It is further
complicated by the branchial region, certain phases of whose
transformations have been touched upon by myself (’15) and
by Congdon ( ’21). It is clear that the heart and the metapharyngeal (more caudal) portion of the region become
314
B. F. IiINGSBUBY
buried through the ventral growth of the body wall, while at
the same time the relative forward movement of the body
wall produces the effect of an ‘engulfment’ of the heart and
associated structures by the growing body wall closing over
them. For this phenomenon I earlier (’15) employed the
term ‘growth eddy,’ the term being coined mainly as an aid
in analyzing the transformations in the metapharynx with
particular reference t o the factors determining the thymus.
Finally, it may be stated that, while the facts appertaining
to the sequence of differentiation are here in large part more
difficult to determine, there is, I believe, no clear indication
that the viscera whose parenchyma is of entodermal origin
exhibit the ‘law.’ The entoderm is not derived by differential growth from the primitive streak or tail bud-or only in
a minor degree.
I n following through the general results of cephalocaudal
differential growth, we come thus to questions of fact, to gaps
in our knowledge of the actual growth changes which must
of necessity be filled before any comprehensive picture of the
growth transformations may be established. It requires no
prolonged discussion to indicate the importance of determining such facts as a basis for broad generalizations as to
developmental factors, although frequently an appreciation
of any such need seems t o be lacking. The primary analysis
of development is in terms of growth and differentiation. Not
only do these present broad groups of problems, but it is
essential that the pattern of differential growth be ascertained. By ‘pattern’ is not meant a predetermined arrangement of ‘organ-forming’ substances as a mosaic, which determine differentiation. It is obvious that the a priori assumption of the existence of such substances begs the question, since the fundamental problem is precisely that which
determination of the nature
such a theory postulates-the
and pattern of differential growth. Since the first interpretations of His, such theoretic prelocalizations or predeterminations have not advanced embryology; nor has the latest
attempt to map out predetermined regions in the blastoderm
CEPHALOCAUDAL D I F F E E E N T I A L G R O W T H
315
-that of Corning (’21)-aided the determination of the developmental pattern whose importance he seemingly does not
appreciate. Corning places anterior to the neural plate in
caudocephalic succession, hypophysis, heart, liver. I n its
fundamental aspects the developmental pattern is, and of
necessity must be, alike in all vertebrates. An examination
of the development in other vertebrate groups alone suffices
to indicate how unlikely are such localizations. An analysis
of the growth differentiation process alone may determine
whether a structure is presumptively there in advance of its
actual appearance.
The inadequacy of theoretic interpretations of a different
category is revealed by an appeal to the actual facts of developmental transformation. I refer to the assumption somotimes made that the head is formed first, and then the body;
put phylogenetically, that the head corresponds to the original
organism to which the body has been added. Stated somewhat
differently, the phylogenetic suggestion is attractive, but
with the recognition that such a body space as the pericardial coelom is associated primarily with material that
becomes head, it is evident that head and body are both established through a differential growth whose pattern it is important to determine. The inadequacy of the conception of
the head as a segmental structure is revealed when the pattern
of the cephalic growth is considered.
It may be safely stated that the law of cephalocaudal progression and the general resultants of the mode of differential
growth which it expresses are illustrated in the development
of all vertebrates. This hardly needs stressing, since, of very
necessity, the development of all vertebrates conforms to a
fundamental plan or pattern, as I have previously (’22, p. 466)
insisted-quite unnecessarily, perhaps.
A somewhat different interpretation of the law of cephalocaudal differential growth has been made by Child. In his
fascinating book “ Senescence and Rejuvenescence” he says
(p. 204) :
316
B. F. KINGSBURY
As regards animals, the so-called law of anterior-posterior development indicates the existence of a metabolic gradient along the mein
axis of the organism during embryonic development. This 'law' is
merely the statement of the observed fact of embryology that in
general the first parts to become morphologically visible are the
apical or anterior regions, and these are followed in sequence by
successively more posterior o r basal parts. In other words, that region
of the egg or early embryo which has the highest rate of metabolism
gives rise to the apical or head region, which in consequence of the
higher rate, becomes differentiated in advance of other parts, and these
follow in sequence along the axis. This fact of embryology is familiar
to every zoologist, and its significance as the expression of a gradient
in dynamic activity along the axis cannot be doubted, although, so
far as I am aware, no one has called attention to it.
Quite similar passages might be reproduced from later publications ( '20, p. 154; '21, p. 28). However, to quote further
from this or other works of this author would be unnecessary, since his interpretations are well known. Such apical
or head region of higher metabolic rate is also a region of
metabolic dominance, setting the pace and determining the
metabolism of more remote regions along the primary axial
gradient or along secondary gradients corresponding to a
dorsoventral falling off in metabolic rate. Built upon the
foundation of experiment and the deductions therefrom, there
has been constructed a theory of the organism wherein-to
mention a single feature-the dominance of the head region
remains as the physiological dominance of the brain in bodily
control.
The interesting and suggestive theoretic interpretations of
Child (cf. '20, '21) need not be reviewed here, since they
introduce problems outside the scope of this paper. As ascertained for a large number of living forms by Child and his
associates, such regions of higher metabolic rate exist in the
egg or embryo or become evident as development proceeds.
As determined in different degrees in different organisms,
such regions of higher metabolism as compared with other
regions possess, a> greater sensitivity to lack of oxygen;
b ) greater reducing power; c ) greater oxygen consumption
and carbon dioxid production ; d ) negativity (through the
GEEHALOCAUDAL DIFFEREK’TIAL GROWTH
317
galvanometer) in electrical potential ; e ) permeability in a
higher degree. Such, indeed, would be the anticipated differences characterizing a region of higher metabolic activity as
compared with one of lower rate. There would seem to be
paralleled within the developing organism itself the differences in metabolism shown in general by the organism as a
whole in earlier, as compared with later stages, of the lifecycle-if the writer interprets correctly the facts that are
available.
Among the vertebrates such analytical methods have been
applied in the development of the frog’s egg by Bellamy
( ’19, ,22) and Hyman and Bellamy (’22)’ and in the development of certain teleosts by Miss Hyman (’21). I n the frog
it was found that the apical region at the beginning of development possessed tlie highest metabolic rate. To this was
added as development progressed a second region of high
metabolism in the blastoporic lip which would seem to possess
(his figure 8) the greater susceptibility. I n tadpoles (Hyman
and Bellamy, ’22) it was found that the posterior end (tail
bud?) had the highest metabolic rate. I n the teleost the
region of greatest susceptibility (presumptive highest metabolic rate) was progressively and successively the central region of the blastoderm, the region of the future ‘embryonic
shield, the anterior portion of the embryonic shield and
axis, to which was added subsequently a ‘secondary’ region
of high susceptibility at the posterior end of the embryo which
persisted as long as “the posterior end continues t o develop
and elongate.” The methods applied, I venture to suggest,
are little calculated to do more than determine regions of high
metabolism characterizing rapid differential growth. From
the morphological side the results attained are quite such as
would be anticipated from the well-established mode of differential growth of the vertebrate embryo. Without discussing
the matter in detail, it may be stated that two regions of
marked differential growth early become evident, preblast oporal and blastoporal, which become more sharply localized
and more widely separated, cephalic and caudal, as the body
3 18
B. F. KINGSBURY
differentiates between them and from them. There are a
number of reasons for regarding these regions as essentially
and primarily one, periblastoporal, which becomes divided
through the changes attending the closure of the blastopore
(cf. Adelmann, ’22),
As far as concerns the law of cephalocaudal differential
growth (lhw of anteroposterior development), the writer is
constrained to agree with Harrison (’21, p. 92), “that such
gradients may well be an expre’ssion of the polarity rather
than its cause”; or, stated more specifically for present purposes, that the cephalocaudal progression in differentiation
and growth but expresses a time or age difference due to the
mode of differential growth-a result of the mode of differential growth rather than a determiner of differential growth
through metabolic dominance.
As simply expressing a time difference in the sequence of
developmental events, the ‘law’ does not in any way furnish
evidence as to factors determining or regulating differentiation. Thus, as long as the blastoporic lip or primitive streak
and subsequently the tail bud, as a region of differential
growth of high metabolic rate, continues to leave behind it
(i.e., ahead of it) as a trail neural tube, notochord, mesoderm,
and apparently some entoderm, it is entirely possible that it
is a ‘center of influence’ on cell territories more remote from
it. Spemann (’23) has indeed experimentally presented evidence of this and terms the blastoporic lip an ‘organixator.’
Such an assumed effect might be due to, a ) the emanation
of chemical substances (autocatalyzers 7 ) of growth promoting or determining potencies, or, b ) , as Child believes, radiation of chemical change of diminishing intensity constituting
a metabolic gradient. It can easily be conceived that the
progressive changes in differentiation might be determined
quantitatively or even qualitatively by their remoteness from
the blastoporic lip (or tail bud). The metabolic gradient
would in that event be in reverse direction to that postulated
by Child.
Returning again to the ‘law,’ it is, I think, clear that so far
at least as this phenomenon of development is concerned an
CEPHALOCAUDAL DIFFERENTIAL GROWTH
319
adequate explanation is not afforded by the assumption that
the primitive streak (and tail bud) is a region of metabolic
dominance determining the differential pattern f o r a greater
or lesser portion of the body. When the whole complex of
developmental transformation is considered, the difficulties
for any interpretation of the ‘law,’, other than that here proposed, as representing simply a time difference in growth
and differentiation, become very great. If the ‘law’ is conceived as representing a metabolic gradient, the degree of
or tendency toward differentiation depending -on the remoteness from the blastoporic lip (tail bud), it must be recognized
that nearly the entire body wall including parts belonging
to the future head would be under its sway, while the apical
or cephalic region of differential growth with high metabolic
rate, extensive as it undoubtedly is, would have a correspondingly restricted field of dominance. Also, it must be recognized that with the completion of differential growth in the
long axis, the mass of undifferentiated material constituting
the tail bud disappears; the region of high metabolic rate
ceases to exist, but the cephalocaudal sequence of differential
change continues without a ‘pace setter.’ If, on the other
hand, it is assumed that the regions of high metabolic rate
promote differentiation so that the degree of differentiation
progressively decreases with increasing distance from such
centers, the apical region would possess an extensive field of
dominance and a cephalocaudal gradient in differentiation
would be an expression of such diminishing influence ; but
the caudal region of high metabolism, that of the primitive
streak and tail bud, would lack the expected field of influence.
The dilemma here is obvious.
It may be pointed out, however, that despite the fact that
a critical consideration of the facts of development reveals
the inadequacy of the metabolic-gradient conception for the
explanation of the law of cephalocaudal differential growth,
it is quite within the limits of possibility that it may nevertheless possess value in analysis of development in other
respects and as a whole. Certain modifications of the original
form of the axial gradient interpretation might be necessary
320
B. F. KINGSBURY
in its application to the vertebrate. Child has not, so far as I
am aware, fully discussed6 the significance f o r his theory of
a second center of high metabolic rate located in the blastoporic lip. It is entirely conceivable that, with the cessation
of growth in the long axis and the disappearance of the caudal
region of high metabolic rate, the cephalic region, in so far
as the brain is concerned, gains a metabolic dominance
through the differentiation of the neuronal systems. Further
consideration would lead to matters entirely beyond the s c o p ~
of this paper. The writer has a keen interest in the fundamental problems that Professor Child is attacking, together.
with a marked appreciation of their enormity.
The discussion in -this paper of the law of cephalocaudal
differential growth here presented does not, of course, bear
on the ultimate problems of the organism. Only certain results of the mode of differential growth are considered while
the processes that underlie development belong to a distinct
aspect of the analysis. What the writer desires to emphasize
in closing' is the importance of ascertaining, as an essential
basis, the developmental pattern made obvious by such
processes.
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' I a m glad t o acknowledge a i d given me
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THE A N A T O M I C A ~ RECORD, VOL.
27. N o . 5
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