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=Part III The Development of Primitive Embryonic Form=
=Part III The Development of Primitive Embryonic Form=
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E. Quantitative and qualitative cleavages and their influence upon later development  
E. Quantitative and qualitative cleavages and their influence upon later development  


==delete==
==The Chordate Blastula and Its Significance==
A. General Considerations
[[Book - Comparative Embryology of the Vertebrates 3-7|7. The Chordate Blastula and Its Significance]]


1. Definitions
A. Introduction


e period of cleavage (segmentation) immediately follows normal fertilization^ or any other means which activates the egg to develop. It consists of
1. Blastulae without auxiliary tissue
a division of the entire egg or a part of the egg into smaller and smaller cellular
entities) In some species, however, both chordate and non-chordate, the early
cleavage stages consist of nuclear divisions alone, to be followed later by the
formation of actual cell boundaries (fig. 62).^The cells which are formed
during cleavage are called blastomere^


cleavage of the egg continues, the blastular stage ultimately is reached.  
2. Blastulae with auxiliary or trophoblast tissue
The blastula contains a cavity or blastocoel together with an associated layer
or mass of cells, the blastodern^ The blastula represents the culmination and
end result of the processes at work during the cleavage period. Certain aspects
and problems concerned with blastulation are considered separately in the
following chapter. However, the general features of blastular formation are
described here along with the cleavage phenomena.


3. Comparison of the two main blastular types


B. History of the concept of specific, organ-forming areas


GENERAL CONSIDERATIONS
C. Theory of epigenesis and the germ-layer concept of development


D. Introduction of the words ectoderm, mesoderm, endoderm


281
E. Importance of the blastular stage in Haeckel's theory of The Biogenetic Law of  Embryonic Recapitulation


F. Importance of the blastular stage in embryonic development


2. Early History of the Cleavage (Cell-division) Concept
G. Description of the various types of chordate blastulae with an outline of their organforming areas


An initial appreciation of the role and importance of the cell in embryonic
1. Protochordate blastula
development was awakened during the middle period of the nineteenth century. It really began with the observations of Prevost and Dumas in 1824 on
the cleavage (segmentation) of the frog’s egg. The latter observations represented a revival and extension of those of Swammerdam, 1738, on the first
cleavage of the frog’s egg and of Spallanzani’s description in 1780 of the first
two cleavage planes, “which intersect each other at right angles,” in the egg
of the toad. Other studies on cleavage of the eggs of frogs, newts, and various
invertebrates, such as the hydroids, the starfish, and nematodes, followed the
work of Prevost and Dumas. The first reported cleavage of the eggs of a rabbit
was made in 1838-1839, a fish in 1842, and a bird in 1847. (See Cole, ’30,
p. 196.) Newport, in 1854, finally founded the new preformation by showing
that the first cleavage plane in the frog’s egg coincided with the median plane
of the adult body (Cole, ’30, p. 196).  


In the meantime, the minute structures of the bodies of plants and animals
2. Amphibian blastula
were intensively studied, and in 1838-1839, the basic cellular structure of
living organisms was enunciated by Schlciden and Schwann. Following this
generalization, many studies were made upon the phenomenon of cell division
in plant and animal tissues. These observations, together with those made
upon the cleaving egg, established proof that cells arise only by the division
of pre-existing cells; and that through cell division the new generation is
formed and maintained. Thus it is that protoplasm, in the form of cells, assimilates, increases its substance, and reproduces new cells. Life, in this
manner, flows out of the past and into the present, and into the future as a
never-ending stream of cellular substance. This idea of a continuous flow
of living substance is embodied well in the famed dictum of R. Virchow,
“Omnis Cellula c Cellula,” published in 1858 (Wilson, E. B., ’25, p. 114).  


The consciousness of life at the cellular level acquired during the middle
3. Mature blastula in birds
period of the nineteenth century thus laid the groundwork for future studies
in cytology and cellular embryology. Much progress in the study of the cell
had been made since R. Hooke, in 1664, described the cells in cork. In
passing, it should be observed, that two types of cell division, direct and
indirect, were ultimately defined. For the latter, Flemming in 1882, proposed
the name mitosis, while the direct method was called amitosis.


3. Importance of the Cleavage-Blastular Period of
4. Primary and secondary reptilian blastulae
Development


The period of cleavage and blastular formation is a time of profound differentiation as well as one of cell division. For, at this time, fundamental
5. Formation of the late mammalian blastocyst (blastula)
conditions are established which serve the purposes of the next stage in development, namely, gastrulation. Experimental embryology has demonstrated


a. Prototherian mammal, Echidna


b. Metatherian mammal, Didelphys


282
c. Eutherian mammals


6. Blastulae of teleost and elasmobranch fishes


CLEAVAGE (SEGMENTATION) AND BLASTULATION
7. Blastulae of gymnophionan amphibia


==Late Blastula in Relation to Certain Innate Physiological Conditions: Twinning==
[[Book - Comparative Embryology of the Vertebrates 3-8|8. The Late Blastula in Relation to Certain Innate Physiological Conditions: Twinning]]


that optimum morphological conditions must be elaborated during the cleavage phase of development along with the developing physiology of the blastula.  
A. Introduction


a. Morphological Relationships of the Blastula
B. Problem of differentiation


There are two aspects to the developing morphology of the blastula, namely,
1. Definition of differentiation; kinds of differentiation
the formation of the blastoderm and the blastocoel.


During cleavage and blastulation, the structure of the blastoderm is elaborated in such a manner that the major, presumptive, organ-forming areas of
2. Self-differentiation and dependent differentiation
the future embryonic body are segregated into definite parts or districts of
the blastoderm. The exact pattern of arrangement of these presumptive, organforming areas varies from species to species. Nevertheless, for a particular
species, they are arranged always according to the pattern prescribed for that
species. This pattern and arrangement of the major, presumptive, organforming areas permit the ordered and symmetrical migration and rearrangement of these areas during gastrulation.


Similarly, the blastocoel is formed in relation to the blastoderm according
C. Concept of potency in relation to differentiation
to a plan dictated by the developing mechanisms for the species. One of the
main functions of the blastocoel is to permit the migration and rearrangement
of the major, presumptive, organ-forming areas during gastrulation. Consequently, at the end of the blastular period, the blastoderm and the blastocoel
are arranged and poised in relation to each other in such a balanced fashion
that the dramatic cell movements of gastrulation or the next period of development may take place in an organized manner.


b. Physiological Relationships of the Blastula
1. Definition of potency


The development of a normal-appearing, late blastula or beginning gastrula
2. Some terms used to describe different states of potency
in a morphological sense is no proof that proper, underlying, physiological
states have been established. A few examples will be given to illustrate this fact:


1) Hybrid Crosses. When the sperm of the wood frog, Rana sylvatica,
a. Totipotency and harmonious totipotency
are used to fertilize the eggs of the ordinary grass frog, Rana pipiens, cleavage
and blastulation appear normal. However, gastrulation is abortive, and the
embryo soon dies (Moore, ’41, ’46, ’47).


2) Artificial Parthenogenesis. In the case of many embryos, chordate and  
b. Determination and potency limitation
non-chordate, in which the egg is stimulated to develop by means of artificial
activation, the end of the blastular stage may be reached, but gastrulative
processes do not function properly. A cessation of development often results.


3) Oxygen-block Studies. In oxygen-block studies, where the fertilized eggs
c. Prospective potency and prospective fate
of Rana pipiens are exposed to increased partial pressures of oxygen from
the time of fertilization to the four- or eight-cell stage, the following cleavages
and the morphology of the blastula may appear normal, but gastrulation does
not occur. Similar oxygen-pressure exposures during the late blastular and
early gastrular stages have no effect upon gastrulation. This fact suggests that


d. Autonomous potency
c. Competence


D. The blastula in relation to twinning


GENERAL CONSIDERATIONS
1. Some definitions


a. Dizygotic or fraternal twins


283
b. Monozygotic or identical twins


c. Polyembryony •


important physiological events accompany the earlier cleavage stages of development (Nelsen, ’48, ’49).
2. Basis of true or identical twinning


Aside from the foregoing examples which demonstrate that invisible changes
3. Some experimentally produced, twinning conditions
in the developing blastula are associated with morphological transformations
is the fact that experimental research has demonstrated conclusively that an
organization center is present in the very late blastula and beginning gastrula.  
The organization center will be discussed later. However, at this point it is
advisable to state that the organization center is the instigator and the controller of the gastr Illative processes, and gastrulation does not proceed unless
it is developed.


The above considerations suggest that the period of cleavage and blastulation is a period of preparation for the all-important period of gastrulation.
E. Importance of the organization center of the late blastula
Other characteristics of this phase of development will be mentioned in the  
chapter which follows.


4. Geometrical Relations of Early Cleavage
==Gastrulation==
| [[Book - Comparative Embryology of the Vertebrates 3-9|9. Gastrulation]]


a. Meridional Plane
A. Some definitions and concepts


The meridional plane of cleavage is a furrow which tends to pass in a
1. Gastrulation
direction which, if carried to completion, would bisect both poles of the egg
passing through the egg’s center or median axis. The latter axis theoretically
passes from the midpolar region of the animal pole to the midpolar region
of the vegetal pole. The beginning of the cleavage furrow which follows the
meridional plane may not always begin at the animal pole (fig. 140D) although in most cases it does (figs. 142A-C; 154A-C; 155A).  


b. Vertical Plane
2. Primitive vertebrate body plan in relation to the process of gastrulation


A vertical plane of cleavage is a furrow which tends to pass in a direction
a. Fundamental body plan of the vertebrate animal
from the animal pole toward the vegetal pole. It is somewhat similar to a  
meridional furrow. However, it does not pass through the median axis of the  
egg, but courses to one side of this axis. For example, the third cleavage planes
in the chick are furrows which course downward in a vertical plane; paralleling one of the first two meridional furrows (fig. 155C). (See also figs. 153D;
154E relative to the third cleavage furrows of the bony ganoid fishes, Amia
calva and Lepisosteus (Lepidosteus) osseus.)


c. Equatorial Plane
b. The gastrula in relation to the primitive body plan


The equatorial plane of cleavage bisects the egg at right angles to the
c. Chart of blastula, gastrula, and primitive, body-form relationships (fig. 188)  
median axis and halfway between the animal and vegetal poles. It is never
ideally realized in the phylum Chordata, and the nearest approach to it is
found, possibly, in one of the fifth cleavage planes of the egg of Ambystoma
maculatum (fig. 149F) and the first cleavage plane of the egg of the higher
mammals (fig. MSA).


B. General processes involved in gastrulation


C. Morphogenetic movement of cells


284
1. Importance of cell movements during development and in gastrulation


2. Types of cell movement during gastrulation


CLEAVAGE (SEGMENTATION) AND BLASTULATION
a. Epiboly


b. Emboly


d. Latitudinal Plane
3. Description of the processes concerned with epiboly


The latitudinal plane of cleavage is similar to the equatorial, but it courses
4. Description of the processes involved in emboly
through the cytoplasm on either side of the equatorial plane. For example,
the third cleavage planes of the egg of Arnphioxus (fig. 1401) and of the frog
(figs. 141 E; 142F) are latitudinal planes of cleavage.


5. Some Fundamental Factors Involved in the Early
a. Involution and convergence
Cleavage of the Egg


a. Mechanisms Associated with Mitosis or Cell Division
b. Invagination


There are two mechanisms associated with cleavage or cell division:
c. Concrescence


( 1 ) that associated with the chromosomes and the achromatic (amphiastral) spindle, which results in the equal division of the chromosomes
d. Cell proliferation
and their distribution to the daughter nuclei, and


(2) the mechanism which enables the cytoplasm to divide.  
e. Polyinvagination


In ordinary cell division or mitosis these two mechanisms are integrated
f. Ingression
into one process. However, in embryonic development they are not always
so integrated. The following examples illustrate this fact: (1) In the early
development of insects, the chromatin materials divide without a corresponding division of the cytoplasm (fig. 62). (2) During the early cleavage phenomena of the elasmobranch fishes, the chromatin material divides before
corresponding cleavages of the cytoplasm appear (fig. 158A). (3) In the
later cleavage stages of teleost fishes, the peripheral cells of the blastoderm
fuse and form a continuous cytoplasm; within this cytoplasm the separate
nuclei continue to divide without corresponding cytoplasmic divisions and in
this way form the marginal syncytial periblast (fig. 159J, L, M).  


On the other hand, cytoplasmic division may occur without a corresponding nuclear division. This behavior has been illustrated in various ways but
g. Delamination
most emphatically by the work of Harvey (’36, ’38, ’40, ’51) which demonstrates that non-nucleate parts of the egg may divide for a period without
the presence of a nucleus. (See, particularly, Harvey, ’51, p. 1349.) Similarly, in the early development of the hen’s egg, a cytoplasmic furrow or
division occurs in the formation of the early segmentation cavity without
involving a nuclear division (fig. 156C, E). This type of activity on the
part of the cytoplasm illustrates the fact that the cytoplasm has a mechanism
for cell division independent of the nuclear mechanism. Lewis (’39) emphasizes the importance of the production of a superficial plasmagel “constriction
ring” which constricts the cytoplasm into two parts during cell division.  


b. Influence of Cytoplasmic Substance and Egg Organization upon
h. Divergence


Cleavage
i. Extension


1) Yolk. Since the time of Balfour, much consideration has been given to  
D. The organization center and its relation to the gastrulative process
the presence or absence of yolk as a factor controlling the rate and pattern


1. The organization center and the primary organizer


2. Divisions of the primary organizer


GENERAL CONSIDERATIONS
E. Chemodifferentiation and the gastrulative process


F. Gastrulation in various Chordata
1. Amphioxus


285
a. Orientation


b. Gastrulative movements


of cleavage. Undoubtedly, in many instances, the accumulation of yolk materials does impede or alter the cleavage furrows, although it does not suppress
1 ) Emboly
mitotic divisions of the nucleus as shown in the early cleavages in many
insects, ganoid fishes, etc. On the other hand, the study of cleavage phenomena as a whole brings out the fact that other intrinsic factors in the
cytoplasm and organization of the egg largely determine the rate and planes
of the cleavage furrows.


2) Organization of the Egg. An illustration of the dependence of the
2) Epiboly
pronuclei and of the position of the first cleavage amphiaster upon the general
organization of the cytoplasm of the egg is shown in the first cleavage spindle
in Amphioxus and Stye la. In the eggs of these species, the amphiaster of the
first cleavage always orients itself in such a way that the first cleavage plane
coincides with the median plane of the future embryonic body. The first
cleavage plane, consequently, divides the egg’s substances into two equal
parts, qualitatively and quantitatively. The movements of the pronuclei and
the first cleavage amphiaster are correlated and directed to this end.


Various theories have been offered in the past to account for the migrations
3) Antero-posterior extension of the gastrula and dorsal convergence of the  
. of the pronuclei at fertilization and for the position of the first cleavage
mesodermal cells
amphiaster. All of them, however, are concerned with the cytoplasm of the
egg or its movements, which in turn are correlated with the organization of
the egg. (See Wilson, E. B., ’25, p. 426.)


A second illustration of the dependence of the chromatin-amphiaster complex on conditions in the cytoplasm is afforded by experiments of Hans
4) Closure of the blastopore
Driesch in 1891 on the isolation of the blastomeres of cleaving eggs of the
sea urchin. He found that the first cleavage of the egg occurred from the
animal to the vegetal pole, resulting in two blastomeres. Now, if these blastomeres are shaken apart, the following cleavages in the isolated blastomeres
behave exactly as if the two blastomeres were still intaet, indicating a definite
progression of the cleavage planes. That is, there is a mosaic of cleavage
planes determined in the cytoplasm of the early egg.


A third example of the influence of egg organization upon cleavage is
c. Resume of cell movements and processes involved in gastrulation of Amphioxus
afforded by the egg of higher mammals. In this group, the first cleavage plane
divides the egg in many cases into a larger and a smaller blastomere. The
larger blastomere then begins to divide at a faster rate than the smaller
blastomere. This accelerated division is maintained in the daughter cells resulting from the larger blastomere. Here, then, is an egg whose yolk material
is at a minimum. Nevertheless, the blastomeres which result from the first
cleavage arc unequal in size, and the cellular descendants of one of these
blastomeres divide faster than the descendants of the other blastomere. Some
conditioning effect must be present in the egg’s cytoplasm which determines
the size of the blastomeres and the rate of the later cleavages. Many other
illustrations might be given from the studies on cell lineage. However, the
conclusion is inevitable that under normal conditions the cause of the cleavage


1 ) Emboly


2) Epiboly


286
2. Gastrulation in Amphibia with particular reference to the frog


a. Introduction


CLEAVAGE (SEGMENTATION) AND BLASTULATION
1) Orientation


2) Physiological changes which occur in the presumptive, organ-forming areas
of the late blastula and early gastrula as gastrulation progresses


pattern and the rate of blastomere formation is an internal one resident in
b. Gastrulation
the organization of the egg and the peculiar protoplasmic substances of the
various blastomeres. This apparent fact suggests strongly that the egg in its
development “is a builder which lays one stone here, another there, each of
which is placed with reference to future development” (F. R. Lillie, 1895,
p. 46).  


c. Influence of First Cleavage Amphiaster on Polyspermy
1) Emboly


In figure 138 is shown the behavior of the sperm nuclei during fertilization in the urodele, Triton. This figure demonstrates that the developing first
2) Epiboly
cleavage amphiaster suppresses the development of the accessory sperm nuclei.
Similar conditions appear to be present in the elasmobranch fishes, chick,
pigeon, etc.


d. Viscosity Changes During Cleavage
3) Embryo produced by the gastrulative processes


“The viscosity changes that occur in the sea-urchin egg are probably typical
4) Position occupied by the pre -chordal plate material
of mitosis in general. There is marked viscosity increase in early prophase,
then a decrease, and finally an increase just before the cell divides” (Heilbrunn,
’21 ). Similarly, Heilbrunn and W. L. Wilson (’48) in reference to the cleaving
egg of the annelid worm, Chaetopterus, found that during the metaphase of
the first cleavage the protoplasmic viscosity is low, but immediately preceding cell division protoplasmic viscosity increases markedly.


e. Cleavage Laws
c. Closure of the blastopore and formation of the neurenteric canal


Aside from the factors involved in cleavage described above, other rules
d. Summary of morphogenetic movements of cells during gastrulation in the frog
governing the behavior of cells during division have been formulated. These
and other Amphibia
statements represent tendencies only, and many exceptions exist. “The rules
of Sachs and Hertwig must not be pushed too far” (Wilson, E. B., ’25, p. 985 ) .


1) Sachs’ Rules:
1) Emboly


(a) Cells tend to divide into equal daughter cells.
2) Epiboly


(b) Each new cleavage furrow tends to bisect the previous one at right
3. Gastrulation in reptiles
angles.  


2) Hertwig’s Laws:
a. Orientation


(a) The typical position of the nucleus tends to lie in the center of the
b. Gastrulation
protoplasmic mass in which it exerts its influence.  


(b) The long axis of the mitotic spindle typically coincides with the long
4. Gastrulation in the chick
axis of the protoplasmic mass. In division, therefore, the long axis of
the protoplasmic mass tends to be cut transversely.


6. Relation of Early Cleavage Planes to the
a. Orientation
Antero-posterior Axis of the Embryo


In the protochordate, Styela, the first cleavage plane always divides the
b. Gastrulative changes
yellow and gray crescent material and other cytosomal substances into equal


1) Development of primitive streak as viewed from the surface of stained
blastoderms


2) Cell movements in the epiblast involved in primitive-streak formation as
indicated by carbon-particle marking and vital-staining experiments


GENERAL CONSIDERATIONS
3) Cell movements in the hypoblast and the importance of these movements
in primitive-streak formation


4) Primitive pit notochordal canal


287
5) Resume of morphogenetic movements of cells during gastrulation in the
chick


5. Gastrulation in mammals


right and left halves; it therefore achieves a sundering of the egg substances
a. Orientation
along the future median plane of the embryo. The second cleavage plane
occurs at right angles to the first (Conklin, ’05, a and b). This condition appears to be true of other ascidians, such as Ciona, Clavelina, etc. (Wilson,
E. B., ’25, p. 1012). The behavior of the early cleavage planes is similar in
Amphioxus (Conklin, ’32). Cleavage planes such as the foregoing, which
always divide the egg in a definite way have been described as “determinate
cleavage” (Conklin, 1897). Study figures 116, 132 and 167.  


The first cleavage plane in the eggs of some frogs (e.g., Rana fusca and
b. Gastrulation in the pig embryo  
Rana pipiens) shows a great tendency to bisect the gray crescent and thus
divide the embryo into right and left halves. However, unlike Styela, and
Amphioxus, the first plane is not definitely fixed; considerable deviation may
occur in a certain percentage of cases in any particular batch of eggs. In the
newt, Triturus viridescens, the first cleavage plane generally is at right angles
to the median plane of the future embryo (Jordan, 1893). In Necturus
maculosus the first cleavage plane may in some eggs coincide with the median
plane of the embryo, while the second cleavage plane may agree with this
plane in other eggs. In some eggs there is no correspondence between the first
two cleavage planes and the median plane of the embryo; however, the planes
always cut from the animal to the vegetal pole of the egg (Eycleshymer, ’04).


In the teleost fish, Fund ulus heteroclitus, in the greater percentage of cases,
c. Gastrulation in other mammals
the long axis of the embryo tends to coincide with either the first or second
cleavage planes (Oppenheimer, ’36). Other teleost fishes appear to be similar.
In the hen’s egg the first cleavage plane may or may not lie in the future
median plane of the embryo (Olsen, ’42).


In some species it appears that the unfertilized egg may possess bilateral
6. Gastrulation in teleost and elasmobranch fishes
symmetry. For example, in the frog, Rana fusca, the point of sperm entrance
evidently has an influence in orienting the plan of bilateral symmetry, and,
as a result, the gray crescent appears opposite the point of sperm contact with
the egg. However, in Rana esculenta and in Discoglossus pictus, two other
anuran species, there is no constant relationship between the point of sperm
entry and the plan of bilateral symmetry of the egg (Pasteels, ’37, ’38). In
the latter cases, unlike that of Rana fusca, the stimulus of sperm entry presumably does not influence the plan of bilateral symmetry which is determined
previous to sperm entrance.


It is to be noted that there is a strong tendency in many of the above species
a. Orientation
for the first cleavage amphiaster to orient itself in such a manner as to coincide
with the median plane of the embryo or to be at right angles to this plane.
This fact suggests that the first cleavage amphiaster is oriented in terms of
the egg’s organization. It further suggests that the copulation paths of the
respective pronuclei, as they move toward each other, together with the resulting cleavage path of the pronuclei (fig. 1 39), arc conditioned by the inherent
organization of the egg’s cytoplasm.  


b. Gastrulation in teleost fishes


1) Emboly


288
2) Epiboly


3) Summary of the gastrulative processes in teleost fishes


CLEAVAGE (SEGMENTATION) AND BLASTULATION
a) Emboly


b) Epiboly


B. Types of Cleavage in the Phylum Chordata
4) Developmental potencies of the germ ring of teleost fishes


Cleavage in the phylum Chordata often is classified as either holoblastic or
c. Gastrulation in elasmobranch fishes
meroblastic. These terms serve a general approach to the subject but fail to
portray the varieties and problems of cleavage which one finds within the
phylum. Under more careful scrutiny, three main categories of cleavage types
appear with typical holoblastic cleavage occupying one extreme and typical
meroblastic cleavage the other, while between these two are many examples
of atypical or transitional cleavage types. Moreover, the phenomena of cleavage are variable, and while we may list the typical cleavage of any one species
as holoblastic, transitional or meroblastic, under certain modifying circumstances the cleavage pattern may be caused to vary.


Holoblastic cleavage is characterized by the fact that the cleavage furrows
7. Intermediate types of gastrulative behavior
bisect the entire egg. In meroblastic cleavage, on the other hand, the disc of  
protoplasm at the animal pole only is affected, and the cleavage furrows cut
through this disc superficially or almost entirely. Superficial cleavage occurs
typically in certain invertebrate forms, particularly among the Insecta. However, in a sense, the very early cleavages in elasmobranch fishes, certain teleost
fishes, and in birds may be regarded as a kind of superficial cleavage.


1. Typical Holoblastic Cleavage
G. The late gastrula as a mosaic of specific, organ-forming territories


In typical holoblastic cleavage, the first cleavage plane bisects both poles
H. Autonomous theory of gastrulative movements
of the egg along the median egg axis, that is, the first plane of cleavage is
meridional. The second cleavage plane is similar but at right angles to the
first, thereby dividing the “germ” into four approximately equal blastomeres.
(See Sachs’ rule (a), p. 286.) The third cleavage plane in typical holoblastic
cleavage occurs at right angles to the median axis of the egg and the foregoing
two meridional planes. (See Sachs’ rule (b), p. 286.) As it does not cut along
the equatorial plane, but nearer the animal pole, it is described as a latitudinal
cleavage. Two meridional cleavage planes (see definition, p. 283) followed by
a latitudinal plane (see definition, p. 284) is the cleavage sequence characteristic of the first three cleavage planes of typical holoblastic cleavage. The
following chordate species exemplify typical holoblastic cleavage:


a, Amphioxus
I. Exogastrulation


In this cephalochordate there exists as typical a form of holoblastic cleavage
J. Pre-chordal plate and cephalic projection in various chordates
as is found anywhere in the phylum Chordata. The process of cleavage or
segmentation in Amphioxus hdiS been described in the studies of four different
men; as such, these descriptions form four of the classics of embryonic study.
These studies were made by Kowatewski in 1867; Hatschek in 1881, English
translation, 1893; Cerfontaine, ’06; and Conklin, ’32. With the exception
of certain slight errors of observation and interpretation, Hatschek’s work is
a masterpiece.


K. Blastoporal and primitive-streak comparisons


==Development of Primitive Body Form==
[[Book - Comparative Embryology of the Vertebrates 3-10|10. Tubulation and Extension of the Major Organ-forming Areas: Development of Primitive Body Form]]


A. Introduction


Fig. 139. Penetration path of the sperm, copulation paths of the pronuclei, the cleavage
1. Some of the developmental problems faced by the embryo after gastrulation
path of the pronuclei, and first cleavage spindle. (A) Conditions such as found in the
urodele, Triton. (B) Conditions such as found in the protochordate, Styelii. (C) Conditions such as found in the protochordate, Anipkioxus.


a. Tabulation


289
b. Increase in size and antero-posteri(*)r extension of the tubulated, major organforming areas


c. Regional modifications of the tubulated areas


2. Common, vertebrate, embryonic body form


3. Starting point for tabulation


4. Developmental processes which accomplish tabulation


a. Immediate processes


TYPES OF CLEAVAGE
b. Auxiliary processes


5. Blastocoelic space and body-form development


291
6. Primitive circulatory tubes or blood vessels


7. Extra-embryonic membranes


The first cleavage furrow cuts through the egg along the median axis of
B. Tabulation of the neural, epidermal, entodermal, and mesodermal, organ-forming  
the egg, starting at the postero-ventral side of the egg (fig. MOD, E). It,
areas in the vertebrate group
therefore, is a meridional plane of cleavage. The second cleavage plane cuts
at right angles to the first plane, producing four equal cells (fig. 140E, F).
The third cleavage involves four blastomeres. Its plane of cleavage is almost
equatorial but slightly displaced toward the animal pole, and therefore, more
truly described as latitudinal plane of cleavage. This cleavage plane divides
each of the four blastomeres into a smaller micromere at the animal pole
and a larger macromere at the vegetal pole. Eight blastomeres are thereby
produced (fig. 140G-I). In certain cases and at least in some varieties of
A mphioxus, the four micromeres may not be placed exactly above the macromeres, but may be rotated variously up to 45 degrees, forming a type of
spiral cleavage (fig. 140J). (See Wilson, E. B., 1893.) The fourth cleavage
planes are meridional, and all of the eight cells divide synchronously. The
result is sixteen cells, eight micromeres and eight macromeres (fig. MOJ, K).
The fifth planes of cleavage are latitudinal and simultaneous (fig. MOL). The


1. Neuralization or the tabulation of the neural plate area


. Fig. 140. Early cleavage and blastulation in A mphioxus. (K after Hatschek, 1893; all
a. Definition
others after Conklin, ‘32.) (A) Median section through egg in the plane of bilateral


symmetry, one hour after fertilization. Second polar body at animal pole; egg and sperm
b. Neuralizative processes in the Vertebrata
pronuclei in contact in cytoplasm containing little yolk. Approximate antero-posterior
axis shown by arrow. D. and V. signify dorsal and ventral aspects of future embryo.
MS. = mesodermal crescent. (B) Sperm and egg nuclei in contact surrounded by
astral rays. Sperm remnant, SR., shown at right. (C) First cleavage spindle in posteroventral half of the egg (see fig. 139C). Arrow shows median plane of future embryo
and also the median plane of the egg. Observe that the spindle is at right angles to this
plane of the egg. (D) Egg in late anaphase of first cleavage. Cleavage furrow deeper
at postero-ventral side of the egg. MS. = mesodermal crescent. (E) Two-cell stage.
Arrow shows median plane of embryo. MS. = mesodermal crescent now bisected into
two parts. (F) Four-cell stage at conclusion of second cleavage, IVi hours after fertilization. (G) Four-cell stage at beginning of third cleavage. Posterior cells, P., slightly
smaller than anterior cells. (H) Animal pole above, vegetal pole below. Cell at left is
posterior, the one at right anterior. Spindles show third or horizontal cleavage plane.
(I) Eight-cell stage, IV 2 hours after fertilization. Posterior cells, below at right, contain
most of mesodermal crescent. Arrow denotes antero-posterior axis of embryo. (J) Late
anaphase of fourth cleavage. (K) Sixteen-cell stage viewed laterally. There are eight
micromeres and eight macromeres. (L) Side view of 32-cell stage, VA hours after fertilization. Every nucleus in anaphase or metaphase of sixth cleavage. (M) Left side of
64-cell stage. Arrow denotes antero-posterior axis of embryo. (N) Blastula, 31A hours
after fertilization. Animal pole above, vegetal below. Entoderm cells at vegetative pole
are larger, are full of yolk, and are dividing. Blastocoel is large. (O) Eighth cleavage
period with more than 128 cells, 4 hours after fertilization. Antero-posterior axis of future
embryo shown by arrows. Polar body indicates animal pole of original egg. Dorsal and
ventral aspects indicated by D. and V., respectively. MS. = mesodermal crescent. (P)
Section of blastula, AV 2 hours after fertilization. Entoderm cells have nuclei shaded with
lines. (Q) Section of blastula, 5 Vi hours after fertilization. (R) Section of blastula,
6 hours after fertilization. Mesoderm cells lighter a^d on each side of entoderm cells.
Section nearly transverse to embryonic axis. (S) Section of blastula at stage of preceding but in a plane as in (Q). MS. = mesodermal crescent. (T) Pear-shaped, late
blastula. Pointed end is mesodermal; entoderm cells have cross-lined nuclei. D. and V.
indicate dorsal and ventral aspects of embryo. See also fig. 167.


1) Thickened keel method


2) Neural fold method


292
c. Closure of the blastopore in rounded gastrulae, such as that of the frog


d. Anterior and posterior neuropores; neurenteric canal


CLEAVAGE (SEGMENTATION) AND BLASTULATION
2. Epidermal tabulation


a. Development of the epidermal tube in Amphibia


plane nearest the animal pole divides each of the eight micromeres into an
b. Tabulation of the epidermal area in flat blastoderms
upper and a lower micromere, while the plane which furrows the eight macromeres divides each into upper and lower macromeres. Thirty-two cells are,
thus, the result of the fifth cleavage planes. The lowest of the macromeres
are larger and laden with yolk material (fig. MOL). The sixth cleavage planes
are synchronous and approximately meridional in direction in all of the 32
cells, resulting in 64 cells (fig. MOM). The blastocoelic cavity is a conspicuous area in the center of this cell mass and is filled with a jelly-like
substance (fig. MON). Study also figure 167.


When the eighth cleavage furrows occur, the blastocoel contained within
3. Formation of the primitive gut tube (enteric tabulation)  
the developing blastula is large (fig. MOP). As the blastula continues to
enlarge, the blastocoel increases in size, and the contained jelly-like substance
assumes a more fluid condition (fig. MOQ-S). The fully formed blastula is
piriform or “pear-shaped” (fig. MOT). (See Conklin, ’32.)  


The cleavage pattern of the urochordate, Styela partita, is somewhat similar
a. Regions of primitive gut tube or early metenteron
to that of Amphioxus, but considerable irregularity may exist after the first
three or four cleavages. In Styela the ooplasm of the egg contains differently
pigmented materials, and yellow and gray crescentic areas are visible at the
time of the first cleavage. (See fig. 132.) These different cytoplasmic areas
give origin to cells which have a definite and particular history in the embryo.
Observations devoted to the tracing of such cell histories are grouped under
the heading of “cell lineage.” Cell-lineage observations are more easily made
in the eggs of certain species because of definitely appearing cytoplasmic
areas, where colored pigments or other peculiarities associated with various
areas of the egg make possible a ready determination of subsequent cell
histories. The general organization of the egg of Amphioxus, regardless of the
fact that its cytoplasmic stuffs do not have the pigmentation possessed by the
egg of Styela, appears similar to that of the latter (cf. figs. MOA; 167A).
(See Conklin, ’32.)


b. Frog (Rana pipiens and R. sylvatica)
b. Formation of the primitive metenteron in the frog


The egg of the frog is telolecithal with a much larger quantity of yolk than
c. Formation of the tubular metenteron in flat blastoderms
is found in the egg of Amphioxus. The pattern of cleavage in the frog, therefore, is somewhat less ideally holoblastic than that of Amphioxus.


The first cleavage plane of the frog’s egg is meridional (figs. MIC; M2A-C) .
4. Tabulation (coelom formation) and other features involved in the early differentiation of the mesodermal areas
It occurs at about three to three and one-half hours after fertilization at ordinary room temperature in Rana pipiens. It begins at the animal pole and
travels downward through the nutritive or vegetal pole substance, bisecting
both poles of the egg. In the majority of eggs, it bisects the gray crescent. (See
p. 287.) The second cleavage plane divides each of the first two blastomeres
into two equal blastomeres; its plane of cleavage is similar to the first cleavage
plane but is oriented at right angles to the first plane (figs. MID; M2D-E).
The upper, animal pole end of each of the four blastomeres contains most


a. Early changes in the mesodermal areas


1) Epimere; formation of the somites


TYPES OF CLEAVAGE
2) Mesomere


3) Hypomere


293
b. Tabulation of the mesodermal areas


C. Notochordal area


of the dark pigment, while in the lower portion of each blastomere the yellowwhite yolk is concentrated. As a rule, the substance of the gray crescent is
D. Lateral constrictive movements
found in two of the blastomeres; the four blastomeres under the circumstances
are not qualitatively equal.  


The third or latitudinal cleavage plane is at right angles to both of the  
E. Tubulation of the neural, epidermal, entodermal, and mesodermal, organ-forming
foregoing and somewhat above the equator, dividing each of the four blastomeres into an animal pole micromere and a larger vegetal pole macromere
areas in Amphioxus


1. Comparison of the problems of tubulation in the embryo of Amphioxus with that
of the embryos in the subphylum Vertebrata


a. End-bud growth


Fig. 141. Normal development of Rana sylvatica. (A) Egg at fertilization. (B)
b. Position occupied by the notochord and mesoderm at the end of gastrulation  
Formation of gray crescent, sharply defined at one hour after sperm entrance. (C) First
cleavage furrow meridional. (D) Second cleavage furrow meridional. (E) Third cleavage furrows, latitudinal in position. Four micromeres above and four macromeres below.
(F) Fourth set of cleavage furrows, meridional in position, although some variation
may exist and vertical furrows may occur. (G-I) Later cleavage stages. Pigmented
pole cells become very small, and pigmented cells creep downward over vegetative pole
area. (J) Appearance of dorsal blastoporal lip. (K) Blastoporal lips spread laterally,
forming a broad, V-shaped structure. Pigmented cells proceed toward blastoporal lips.
(L) Yolk-plug stage of gastrulation. (After Pollister and Moore, ’37.)


2. Neuralization and the closure of the blastopore


3. Epidermal tubulation


Fig, 142. Early development of Rana pipiens. (A) Polar view of first cleavage. The
4. Tubulation of the entodermal area
animal pole is considerably flattened at this time and tension lines are visible extending
outward along either side of the furrow. (About V/2 hours after fertilization in the  
laboratory, room temperature 20 to 22® C.) (B) First cleavage furrow a little later.


(C) First furrow proceeds slowly though the yolk of vegetative (vegetal) pole. (D)
a. Segregation of the entoderm from the chordamesoderm and the formation of  
Second cleavage furow meridional and at right angles to the first furrow. (E) Four-cell
the primitive metenteric tube
stage, view from animal pole. Observe short “cross furrow” connecting first and second
cleavage planes. (F) Fourth cleavages meridional or nearly so. Taken from egg spawned
in nature. Considerable variation may exist. Some cleavages may be vertical and not
meridional. (G-I) Later blastula stage. (J) Stage just before appearance of dorsal
lip of blastopore. (K) Dorsal lip of blastopore. (L) Yolk-plug stage of gastrulation.


b. Formation of the mouth, anus, and other specialized structures of the metenteron


294
5. Tubulation of the mesoderm


6. Later differentiation of the myotomic (dorsal) area of the somite


7. Notochord


295
F. Early development of the rudiments of vertebrate paired appendages


G. The limb bud as an illustration of the field concept of development in relation to the
gastrula and the tubulated embryo


H. Cephalic flexion and general body bending and rotation in vertebrate embryos


Fig. 143. Stages in formation of the blastocoel in the cleaving egg of Rana pipiens
I. Influences which play a part in tubulation and organization of body form  
taken from stained sections. (A) Eight-cell stage; blastocoel appearing particularly
between micromeres. The macromeres form the floor of the developing blastocoel.
(B, C) Later stages of formation of the blastocoel. Blastocoel situated at animal pole.
Yolk-laden, vegetal pole cells form floor of the blastocoel while smaller, animal pole
cells form its sides and roof. (D) Blastocoel at beginning of gastrulation.


(figs. 141E; 142F). The fourth set of cleavages, both in Rana sylvatica and
J. Basic similarity of body-form development in the vertebrate group of chordate animals
Rana pipiens, in eggs that are spawned naturally, are oriented in a meridional
direction (figs. 141F; 142F). These furrows first involve only the animal
pole micromeres, but later meridionally directed furrows begin to develop in  
the yolk-laden macromeres (figs. 141F; 142F).


The cleavage of the various blastomeres of the egg to this point tends to
==Basic Features of Vertebrate Morphogenesis==
be synchronous, and is comparable to that of Amphioxus. However, from
[[Book - Comparative Embryology of the Vertebrates 3-11|11. Basic Features of Vertebrate Morphogenesis]]
this time on asynchronism is the rule and different eggs in a given lot manifest
various degrees of irregularity. Exceptional eggs may occur in which the next
two cleavage planes resemble the fourth and fifth series of planes in Amphioxus.
But, on the whole, the micromeres divide faster than do the macromeres and
thus give origin to many small, heavily pigmented, animal pole cells, while
the macromeres or vegetal pole cells are larger and fewer in number. The
smaller pigmented cells creep downward gradually in the direction of the  
larger vegetal pole cells (figs. 141G-I; 142G-K). The latter migration of the


A. Introduction


296
1. Purpose of This Chapter


2. Definitions


CLEAVAGE (SEGMENTATION) AND BLASTULATION
a. Morphogenesis and Related Terms


b. Primitive, Larval, and Definitive Body Forms (see fig. 255)


1) Primitive Body Form.


Fig. 144. Cleavage in the rabbit egg. (After Gregory, ’30.) (A) One-cell stage. (B)
2) Larval Body Form.


Two primary blastomeres, one larger than the other. (C) Eight-cell stage. (D) Sixteencell stage. (E) Morula stage of 32 cells. (F) External view of stage approximating
3) Definitive Body Form.  
that in (G). (G) Inner cell mass and blastocoelic cleft showing in embryo, about IV 2


hours after copulation. (H) Inner cell mass and blastocoelic space in embryo, approximately 90 hours after copulation. Entoderm cells have not yet appeared.  
3. Basic or Fundamental Tissues


pigment cells is marked toward the end of the blastular period and during
B. Transformation of the Primitive Body Tubes into the Fundamental
gastrulation. Cf. figs. 141 H~L; 142H-L.
or Basic Condition of the Various Organ Systems
Present in the Primitive Embryonic Body


The blastocoel within the mass of blastomeres of the cleaving egg of the
1. Processes Involved in Basic System Formation
frog forms somewhat differently from that in Amphioxus in that the cavity
arises nearer the animal pole. The smaller micromeres of the animal pole,
therefore, are more directly involved than the macromeres of the vegetal pole.
Beginning at the eight-cell stage, a spatial separation is present between the
four micromeres at the animal pole. The floor of this space or beginning
blastocoel is occupied by the yolk-laden macromeres (fig. 143 A, B). As
development proceeds, this eccentricity of position is maintained, and the


(a) extension and growth of the body tubes,


TYPES OF CLEAVAGE
(b) saccular outgrowths (evaginations) and ingrowths (invaginations)
from restricted areas of the tubes,


(c) cellular migrations away from the primitive tubes fo other tubes and
to the spaces between the tubes, and


297
(d) unequal growth of different areas along the tubes.




blastocoel or segmentation cavity becomes an enlarged space filled with fluid,
2. Fundamental Similarity of Early Organ Systems
displaced toward the animal pole (fig. 143B-D). The contained fluid of the
blastocoel of amphibia is alkaline, according to the work of Buytendijk and
Woerdeman (’27), having a pH of 8.4 to 8.6.


For general references regarding cleavage in the frog, see Morgan (1897);
C. Laws of von Baer
Pollister and Moore (’37); Rugh (’51); and Shumway (’40).  


c. Cyclostomata
D. Contributions of the Mesoderm to Primitive Body Formation and
Later Development


Cleavage in the eggs of the genera of the family, Petromyzonidae, resembles
1. Types of Mesodermal Cells
very closely that of the frog. Further description will not be included. However, in the marine cyclostomes or hagfishes, the cleavage phenomena are
strongly meroblastic. (See description of the marine cyclostomatous fish at
the end of this chapter.)


2. Atypical Types of Holoblastic Cleavage
2. Origin of the Mesoderm of the Head Region


A variety of cleavage types is found in the eggs of many vertebrate species
a. Head Mesoderm Derived from the Anterior Region of the Trunk
which do not follow the symmetrical, ideally holoblastic pattern exhibited in
the egg of Amphioxus or even in the egg of the frog. In all of these atypical
forms the entire egg ultimately is divided by the cleavage furrows with the
possible exception of the eggs of the bony ganoid fishes, Amia calva and
Lepisosteus osseus (and also in certain of the gymnophionan amphibia). In
the latter species the yolk material at the yolk-laden pole of the egg is invaded
by isolated nuclei which form a syncytium in the yolk material. Eventually
this yolk material is formed into definite cells and incorporated into the gut
area of the embryo.


a. Holoblastic Cleavage in the Egg of the Metatherian and Eutherian
b. Head Mesoderm Derived from the Pre-chordal Plate


Mammals
c. Head Mesoderm Contributed by Neural Crest Material


1) General Considerations. The eggs of metatherian and eutherian mammals are the most truly isolecithal of any in the phylum Chordata. They have
d. Head Mesoderm Originating from Post-otic Somites
also a cleavage pattern distinct from other chordate eggs. The first cleavage
plane in the higher mammalian egg very often divides the egg into a larger
and a slightly smaller blastomere (figs. 144B; 145 A, F; 147B, J). As shown
by the work of Heuser and Streeter (’41) in the pig, the smaller blastomere
is destined to give origin to the formative tissue of the embryo’s body, while
the larger blastomere gives rise to auxiliary tissue, otherwise known as the
nourishmenFobtaining or trophoblast tissue (fig. 145A-E). The smaller blastomere also contributes some cells to the trophoblast tissue. A similar condition of progressive specialization of the smaller and the larger blastomeres
of the two-cell stage, producing two classes of cells, the one mainly formative
and the other auxiliary or trophoblast, is present in the monkey (Heuser and
Streeter, ’41) and probably in other higher mammals as well.


If one compares the early history of these two blastomeres with the early
3. Origin of the Mesoderm of the Tail


4. Contributions of the Trunk Mesoderm to the  Developing Body


a. Early Differentiation of the Somites or Epimere


b. Early Differentiation of the Mesomere (Nephrotome)


Fig. 145. Early development of the pig. (A-E) Fate of the first two blastomeres.
c. Early Differentiation and Derivatives of the Hypomere
The langer blastomere of the two-cell stage gives rise to trophoblast tissue, whereas from
the smaller blastomere, formative cells and trophoblast cells arise. (After Heuser and  
Streeter, Carnegie Inst., Washington, Contrib. Embryol., 20:3.) (F) Section of two
cell stage. Specimen secured from oviduct of sow, killed two days, 3 Vi hours after ovulation. (G) Section of four-cell stage. Age is approximately IVz days. (H) Sixteen-cell
stage, drawn from unsectioned specimen, protjably 3 Vi days old. (I) Blastular stage.
Specimen secured from sow, days after copulation. (J-L) Stages showing the formation of the blastocoel. (J) About 4% days after copulation. (K) Six days, Wa
hours after copulation. (L) Six days, 20 hours after copulation. (M) Beginning disintegration of trophoblast cells over the inner cell mass and separation of entoderm cells
from the inner cell mass. (N) Trophoblast cells over inner cell mass almost absent,
entoderm forms a definite layer below inner cell mass. (O) Trophoblast cells almost
absent over the embryonic disc; entoderm layer continuous. (P-R) Stages shown in
(M), (N), (O) respectively, showing the whole blastocyst. In (Q) the entoderm cells
are shown migrating outward to line the cavity of the blastocyst.


5. Embryonic Mesenchyme and Its Derivatives


298


1. Neural Plate Area (Ectoderm)


TYPES OF CLEAVAGE
2. Epidermal Area (Ectoderm)


3. Entodermal Area


299
4. Notochordal Area


5. Mesodermal Areas


development of other vertebrate eggs, it is apparent that the nutritive (trophoblast) cells are located at one pole, while the formative cells of the embryo
6. Germ-cell Area
are found toward the opposite pole. The latter condition resembles the arrangement of formative cells and nutritive substances in teleost and elasmobranch fishes, in reptiles, birds, and prototherian mammals. This comparison
suggests, therefore, that the first cleavage plane in the higher mammals cuts
at right angles to the true median axis of the egg (cf. fig. 145A-E). If this
is so, the first cleavage furrow should be regarded as latitudinal and almost
equatorial, and the two blastomeres should theoretically be arranged as shown
in figure 145A.


The determination of the animal and vegetal poles of the egg in this group
F. Metamerism
of vertebrates is difficult by any other means than that suggested above. In
many lower chordate species the polar bodies act as indicators of the animal
pole, for they remain relatively fixed at this pole of the egg (e.g., Styela,
Amphioxus, etc.). But in higher mammals the polar bodies “are never stationary, and there is evidently much shifting” (Gregory, ’30, relative to the
rabbit), although in the two-cell stage, the polar bodies often appear between
the two blastomeres at one end. It appears in consequence that the fates of
the two blastomeres of the two-cell stage serves as a better criterion of egg
symmetry at this time than is afforded by the polocytes. According to this
view, the smaller blastomere should be regarded as indicating the animal
pole, while the larger blastomere signifies the vegetative pole (fig. MSA).  


With respect to the statements in the previous paragraph, it is well to
mention that Nicholas and Hall (’42) reported that two early embryos may
be produced by isolating the blastomeres of the two-cell stage in the rat, and
one embryo is produced as a result of experimental fusion of two fertilized
eggs. These experimental results suggest that the potencies of the two blastomeres are not so rigidly determined that two different kinds of development
result when the blastomeres are isolated. In normal development, however,
it may be that the innate potencies of the two blastomeres are not precisely
the same. The ability to regulate and thus compensate for lost substances
shown by many different types of early embryonic blastomeres, may explain
the production of two early embryos from the separated blastomeres of the
two-cell stage.


The second cleavage divides the larger blastomere into two cells, giving
G. Basic Homology of the Vertebrate Organ Systems
origin to three cells. Then the smaller blastomere divides, forming four cells.
Cleavage from this time on becomes irregular, and five-, six-, seven-, eight-,
etc., cell stages are formed.


Segmentation of the higher mammalian egg, therefore, is unique in its
1. Definition  
cleavage pattern. The synchrony so apparent in the egg of Amphioxus is
lacking. Irregularity and individuality is the rule, with the auxiliary or nutritive
pole cells dividing faster than those of the formative or animal pole cells.
 
Moreover, the blastomeres not only show their apparent independence of
 
 
 
300
 
 
CLEAVAGE (SEGMENTATION) AND BLASTULATION
 
 
each other through their irregularity in division but also by their tendency to
shift their position with respect to one another. One function of the zona
pellucida during the early cleavage period appears to be to hold “the blastomeres together” (Heuser and Streeter, ’29). From the 16-cell stage on, the
trophoblast or auxiliary cells begin to form the blastocoelic space, first by a
flattening process and later by the formation of a cleft among the cells (fig.
145D). The growing presence of the blastocoel consigns the formative or
inner cell-mass cells to one pole of the blastula (fig. 145J-L). A blastocoelic
space thus is formed which is surrounded largely by trophoblast or nutritive
cells (fig. 145K, L). The blastular stage of development of the mammalian
embryo is called the blastocyst.
 
2) Early Development of the Rabbit Egg. The following brief description
pertains to the early development of the rabbit egg up to the early blastocyst
condition.
 
a) Two-cell Stage. The two-cell stage is reached about 22 to 24 hours
after mating or 10 to 12 hours after fertilizcition. One cell has a tendency to
be slightly larger than the other (fig. 144B). (Cf. also figs. MSA, F; 146A;
147B, J.)
 
b) Four-cell Stage. This stage is present about 24 to 32 hours after
mating or 13 to 18 hours after fertilization. The larger cell divides first, giving
origin to three cells; the smaller cell then divides. (Cf. figs. MSB, C; M6B, C;
M7K, L. ) The mitotic spindles tend to assume positions at right angles to
each other during these cleavages.
 
c) Eight-cell Stage. Eight cells are found 32 to 41 hours after mating.
One member of the larger blastomeres of the four-cell stage divides, forming
a five-cell condition, followed by the division of the second larger cell, producing six cells. (Cf. figs. MSC; M7M.) After a short period, one of the
smaller cells segments, and thus, a total of seven blastomeres is formed. The
last cleavage is followed by the division of the other smaller cell, producing
eight blastomeres (fig-t- M4C; compare with fig. M7N). The mitotic spindles
of each of these cleavages form at right angles to one another, thus demonstrating an independence and asynchrony. The latter conditions are demonstrated further by the fact that the blastomeres shift their position continually
in relation to each other during these divisions.
 
d) Sixteen-cell Stage. The mitotic divisions increase in rate, and at
about 45 to 47 hours after mating the 16-cell stage is reached (fig. M4D).
The cells at the future trophoblast pole begin to flatten, and gradually certain
blastomeres are enclosed within. In the macaque monkey, 16 cells are present
at about 96 hours after fertilization.
 
e) Morula Stage. At about 65 to 70 hours after mating a solid mass of
cells is present. This condition is known as the morula (mulberry-like) stage
(fig. M4E, F). The trophoblast portion of the cell mass is more active in
cell division.
 
 
 
 
 
 
 
 
 
>-^rr
 
 
 
 
 
 
 
 
 
 
 
 
>#:
 
 
 
 
 
 
%\ ir:
 
 
.,rf>; ? %
 
 
 
 
 
 
\^l“5i
 
 
 
 
Fio. 146. Photomicrographs of cleavage in living monkey egp. (After Lewis and
Hartman, Carnegie Inst.. Washington, Contrib. Embryol., 24.) (Figures borrowed from
fig 33 Patten, ’48.) (A) Late two-cell stage. (B) Early three-cell stage. (C) Late
four-ccll stage. (D) Five-cell stage. (E) Six-cell stage. (F) Eight-cell stage; next
cleavage beginning.
 
 
 
Fig. 147. (See facing page for legend.)
 
 
 
 
 
TYPES OF CLEAVAGE
 
 
303
 
 
f) Early Blastocyst. A few hours later or about 70 to 75 hours after
mating, a well-defined cleft within the cells of the trophoblast pole becomes
evident (fig. 144G). (Cf. fig. 145D, J.) This cleft or cavity enlarges, and
the surrounding trophoblast cells lose their rounded shape and become considerably flattened. As the blastocoel gradually increases in size, the formative tissue or inner cell mass becomes displaced toward one end of the early
blastocyst, as indicated in fig. 144G, H. The blastocoelic space at this time
is filled with fluid, and the blastocyst as a whole completely fills the area
within the zona pellucida (fig. 144G, H). The pig embryo reaches a similar
condition in about 100 hours after fertilization, and that of the guinea pig in
140 hours.
 
During its passage down the Fallopian tube, the developing mass of cells
continues to be encased by the zona pellucida. The general increase in size
is slight. In the rabbit and in the opossum, as the cleaving egg passes down
the Fallopian tube, an albuminous coating is deposited around the outside
of the zona pellucida (figs. 144G, H; 147A). This albuminous layer forms
an accessory egg membrane or covering similar to the albuminous layers deposited around the egg by the oviducal cells in prototherian mammals, birds,
and reptiles. At about 80 to 96 hours after mating, the rabbit blastocyst enters
the uterus and gradually increases in size. Implantation of the mammalian
blastocyst upon the uterine wall will be considered later. (See Chap. 22.)
 
3) Types of Mammalian Blastocysts (Blastulae). The early blastocyst of
the rabbit described above is representative of the early condition of the
developing blastula of the eutherian (placental) mammal. However, in the
metatherian or marsupial mammals the early blastocyst does not possess a
prominent inner cell mass similar to that found in the eutherian mammals.
Comparing the early blastocysts of the higher mammals, we find, in general,
that there are three main types as follows:
 
( 1 ; In most of the Eutheria or placental mammals the inner cell mass
(embryonic knob) is a prominent mass of cells located at one pole of
the blastocyst during the earlier stages of blastocyst formation. (See
 
 
Fig. 147. Early development of the opossum egg. (A-H after Hartman, ’16; I-N
after McCrady, ’38.) (A) Unfertilized uterine egg, showing the first polar body; yolk
 
spherules (in black) within the cytoplasm; zona pellucida; albuminous layer; and the
outer shell membrane. (B) Two-cell stage. Observe yolk spherules discharged into the
cavity of the zona pellucida. (C) Section through three blastomeres of four-cell stage.
Observe yolk within and without the blastomeres. (D) Section through 16-celI cleavage
stage. Observe yolk within blastomeres and also in cavity of the zona between the
blastomeres. (E) Section through early blastocyst showing yolk and cytoplasmic fragments and an included nucleated cell within the blastocoel. (F-H) Early and later
blastocyst of the opossum, showing the formative tissue at one pole of the blastocyst.
(1) Surface view, fertilized egg. (J) Two-blastomere stage. (K) Cell A has divided
meridionally into A, and Aj. (L) Cell B has divided into B, and B.. (M) A, and Aj
have divided as indicated. (N) B, and B, divide next as indicated.
 
 
 
304
 
 
CLEAVAGE (SEGMENTATION) AND BLASTULATION
 
 
figs. 144G, H; 145J-L.) This condition is found in the monkey,
human, pig, rabbit, etc.
 
(2) On the other hand, in certain marsupials, such as the American opossum, Didelphys virginiana, and the Brazilian opossum, Didelphys
aurita, the inner cell mass is much less prominent during earlier stages
of the blastocyst. In these species it is indicated merely by a thickened
aggregation of cells at one pole of the blastocyst (fig. 147E— G).
 
(3) In the marsupial or native cat of Australia, Dasyurus viverrinus, cleavage results in an early blastocyst in the form of a hollow sphere of
rounded cells. As the blastocyst expands, the cells increase in number
and become flattened to form a thin layer of cells apposed against the
shell membrane without an apparent inner cell mass or embryonic
knob (fig. 148A-C).
 
A conspicuous feature of cleavage and early blastocyst formation in the
marsupials should be emphasized. For in this group, the early blastomeres
apparently use the framework of the zona pellucida as a support upon which
they arrange themselves. As a result, the blastocoelic space of the blastocyst
 
 
 
Fig. 148. Early blastular conditions of the marsupial cat of Australia, Dasyurus
viverrinus. (After Hill, ’10.) (A) Early blastula. (B) External view of blastocyst, 0.6
 
mm. in diameter. The cells are becoming flattened and finally reach the condition shown
in (C). (C) Section of wall of blastocyst, 2.4 mm. in diameter.
 
 
 
TYPES OF CLEAVAGE
 
 
305
 
 
forms directly by cell arrangement and not by the development of a cleft
within the trophoblast cells, as in the eutherian mammals. (See fig. 147C-E;
compare with figs. 144G; 145J.)
 
The descriptions of the mammalian blastocysts presented above pertain
only to the primary condition of the blastocyst. The changes involved in later
development, resulting in the formation of the secondary blastocyst, will be
described in the next chapter which deals specifically with blastulation.
 
(For more detailed descriptions of early cleavage in the metatherian and
eutherian mammals see: Hartman (T6) on the American opossum; Hill (T8)
on the opossum from Brazil; Hill (TO) on the Australian native cat, Dasyurus
viverrinus; Heuser and Streeter (’29), and Patten (’48) on the pig; Lewis and
Gregory (’29), Gregory (’30), and Pincus (’39) on the rabbit; Huber (T5)
on the rat; Lewis and Wright (’35) and Snell (’41 ) on the mouse; Lewis and
Hartman (’41) and Heuser and Streeter (’41) on the Rhesus monkey.)
 
b. Holoblastic Cleavage of the Transitional or Intermediate Type
 
Contrary to the conditions where small amounts of yolk or deutoplasm
are present in the egg of the higher mammal or in Amphioxus, the eggs of
the vertebrate species described below are heavily laden with yolk. As the
quantity of yolk present increases, the cleavage phenomena become less and
less typically holoblastic and begin to assume meroblastic characteristics.
Hence the designation transitional or intermediate cleavage.
 
1) Amhy stoma maculatum ( punctatum ), The newly spawned egg of
Ambystoma maculatum is nearly spherical and measures about 2 mm. in
diameter, although the egg size is somewhat variable. The animal pole contains within its median area a small depression, the “light spot” or “fovea.”
Within the fovea is a small pit harboring the first polar body. (A comparable
pit is shown in the frog’s egg, fig. 1 1 9C. ) After the second polar body is
formed, this pit may appear somewhat elongated, and the light spot disappears. Just before the first cleavage, the animal pole appears flattened similar
to the condition in the frog’s egg. The flattened area soon changes to an
elongated furrow which progresses gradually downward toward the opposite
pole (fig. 149 A, B). This cleavage furrow is meridional, dividing the egg into
two, nearly equal blastomeres. The second cleavage furrow is similar to the
first but at right angles to the first furrow (fig. 149C). However, considerable
variation may exist, and the second furrow may arise at various angles to the
first, dividing each of the first two blastomeres into two, slightly unequal,
daughter blastomeres. The third set of cleavages is latitudinal, and each
blastomere is divided into a smaller animal pole micromere, and a larger
vegetal pole macromere (fig. 149D). Later cleavages may not be synchronous.
 
The first three cleavages described above conform generally to the rules
of typical holoblastic cleavage. However, from this time on cleavage digresses
from the holoblastic pattern and begins to assume certain characteristics oj
 
 
 
306
 
 
CLEAVAGE (SEGMENTATION) AND BLASTULATION
 
 
 
Fig. 149, Early cleavage in Arnbystoma maculatum (punctatum). (After Eycleshymer,
J. Morphol., 10, and eggs in the laboratory.) (A, B) First cleavage furrow, meridional
plane. (C) Second cleavage furrow at right angles to first furrow, meridional plane.
 
(D) Third cleavage furrow, latitudinal, forming four micromeres and four macromeres.
 
(E) Fourth cleavage furrow; mixture of meridional and vertical planes of cleavage. (F)
Fifth cleavage furrows; mixture of latitudinal and vertical planes of cleavage. Observe
equatorial plane cutting the large macromeres. (G-I) Later cleavage stages.
 
meroblastic cleavage. For example, the fourth set of cleavages may be a
mixture of vertical and meridional furrows, as shown in figure 149E. The fifth
cleavages are a mixture of horizontal (i.e., latitudinal and equatorial, fig.
149F), vertical and meridional furrows. The sixth set of cleavages is made
up of vertical and horizontal cleavage planes of considerable variableness (fig.
149G). From this time on cleavage becomes most variable, with the animal
pole micromeres dividing much more rapidly than the yolk-laden macromeres
at the vegetal pole (figs. 149H, I).
 
The blastocoel makes its appearance at the eight-cell stage and appears
as a small space between the micromeres and the macromeres, the latter
forming the floor of the blastocoelic space. At the late blastula stage, the
blastocoel is roofed over by the smaller micromeres, and floored by the yolk
 
TYPES OF CLEAVAGE
 
 
307
 
 
VERTICAL FURROW
 
 
 
Fig. 150. Cleavage in the egg of Lepidosiren paradoxa. (After Kerr, ’09.) (A) Be
ginning of first cleavage, meridional in position. (B) Second cleavage planes, approximately meridional in position. (C) Third cleavage planes vertical in position, demonstrating a typical meroblastic pattern. (D) Early blastula. (E) Late blastula.
 
laden macromeres. The blastocoel is small in relation to the size of the egg
(Eycleshymer, 1895).
 
2 ) Lepidosiren paradoxa. The egg of the South American lungfish,
Lepidosiren paradoxa, measures about 6,5 to 7 mm. in diameter. Cleavage
of the egg is complete (i.e., holoblastic), and a relatively large blastocoel is
formed. As in Arnbystoma, the blastocoel is displaced toward the animal
pole. The floor of the blastocoel is formed by the large, yolk-laden macromeres.
 
The first two cleavage furrows are approximately meridional (fig. 150A, B).
These two furrows are followed by four vertical furrows, which, when completed, form eight blastomeres (fig. 150C). The latter cleavages are subject
to much variation. Although cleavage of the egg is complete, a distinct meroblastic pattern of cleavage is found, composed of two meridional furrows
followed by vertical furrowing (see Kerr, ’09).
 
3) ISecturus maculosus. In this species of amphibia the egg is large and
its contained yolk is greater than that of Arnbystoma. It measures about 5 to
6 mm. in diameter. The egg and its envelopes are attached individually by
the female beneath the flattened surface of a stone (Bishop, ’26).
 
Cleavage in this egg proceeds slowly. The first two cleavage furrows tend
to be meridional, but variations may occur in different eggs. Sometimes they
are more vertical than meridional (fig. 151A). (See Eycleshymer, ’04). The
 
 
308
 
 
CLEAVAGE (SEGMENTATION) AND BLASTULATION
 
 
 
 
 
Fig. 151. Cleavage in the egg of Necturus maculosus. (After Eycleshymer and Wilson,
’10.) (A) First two cleavage planes are meridional. (B) Third cleavage planes tend
to be vertical and meridional. (C) Fourth cleavage planes are vertical, meridional, and
irregular. (D~H) Following cleavage planes become irregular, offering a mixture of
modified latitudinal, vertical, and meridional varieties.
 
 
third cleavage furrows are irregularly vertical (fig. 15 IB), while the fourth are
latitudinal, cutting off four very irregular micromeres at the animal pole. Segmentation then becomes exceedingly irregular. (See Eycleshymer and Wilson,
*10). One characteristic of cleavage in Necturus is a torsion and twisting of
the cleavage grooves due to a shifting in the position of the blastomeres.
 
As shown in the figures, the first three cleavage planes assume a distinct
meroblastic pattern of two meridional furrows followed by vertical furrows.
The yolk material evidently impedes the progress of the furrows considerably.
 
4 ) Acipenser sturio. In the genus Acipenser are placed the cartilaginous
ganoid fishes. Cleavage in Acipenser sturio, the sturgeon, resembles that of
 
 
 
 
TYPES OF CLEAVAGE
 
 
309
 
 
 
Fig. 152. Cleavage in the egg of the sturgeon, Acipenser sturio, (After Dean, 1895.)
(A, B) First and second cleavage planes are approximately meridional. (C) Third
cleavage planes are vertical, usually parallel to first cleavage plane. (D) Fourth cleavage
planes are vertical, cutting off four central cells from the 12 marginal cells. (E, F)
Later cleavage stages.
 
Necturus, although the furrows in the yolk pole area are retarded more and
are definitely superficial. The third and fourth sets of cleavage furrows are
vertical and succeed in cutting off four central cells from twelve larger marginal cells (fig. 152). Cleavage in this form is more holoblastic in its essential
behavior than that in the egg of Amia and Lepisosteus described below
(Dean, 1895).
 
5) Amia calva. Amia calva is a species of bony ganoid fishes, and it represents one of the oldest living species among the fishes. Its early embryology
follows the ganoid habit, namely, its cleavages adhere to the meroblastic
pattern of the teleost fishes, with the added feature that the furrows eventually
pass distally toward the vegetal pole of the egg. A few yolk nuclei appear to
be formed during cleavage. These nuclei aid in dividing the yolk-filled cytoplasm into distinct cells. The latter gradually are added to the early blastomeres
and to the later entoderm cells of the developing embryo. In other words,
cleavage in this species is holoblastic, but it represents a transitional condition
between meroblastic and holoblastic types of cleavage.
 
The egg of Amia assumes an elongated form, averaging 2.2 by 2.8 mm.
The germinal disc is. a whitish cap in the freshly laid egg, reaching down over
the animal pole to about one third of the distance along the egg’s longer axis.
 
 
310
 
 
CLEAVAGE (SEGMENTATION) AND BLASTULATION
 
 
The vegetal pole is gray in color. The egg membrane is well developed, having
a zona radiata and a villous layer. Strands of the villous layer may attach the
egg to the stem of a water weed or other structure (fig. 153A).
 
The first cleavage plane is meridional and partly cleaves the protoplasmic
disc into two parts (fig. 153B). This cleavage furrow passes slowly toward
the vegetal pole of the egg. The second cleavage is similar to the first furrow
and at right angles to it (fig. 153C). The third cleavage is variable but, in
general, consists of two furrows passing in a vertical plane at right angles to
the first cleavage furrow (fig. 153D). The fourth set of cleavages is hori
 
 
SYNr. YTIAL NUCLEI SYNCYTIAL NUCLEI INVADE THE YOLK
 
 
Fig. 153. Cleavage in the egg of Amia calva. (After Dean, 1896.) (A) Egg mem
 
branes of Amia, showing the filamentous (villous) layer attaching the egg to the stem
of a water weed. (B) Second cleavage plane shown cutting through the protoplasmic
disc at one pole of the egg. Section made parallel to the first cleavage plane. (C) First
and second cleavage planes seen from above. (D) Third cleavage planes are vertical
in position as indicated. (E) Fourth cleavage, sectioned in a plane approximately parallel
to first (or second) cleavage. (F) Section through protoplasmic disc at eighth cleavage.
(G) Blastular stage. Blastocoel is indistinct and scattered between (?) blastomeres of
blastoderm. The description given by Whitman and Eycleshymer (1897) does not agree
in certain features with the above.
 
 
 
TYPES OF CLEAVAGE
 
 
311
 
 
 
Fig. 154. Early development of Lepisosteiis osseus. (After Dean, 1895.) (A) Un
cleaved egg, showing germinal disc. (B) First cleavage is trench-like, extending beyond
(i.e., laterally) to the margin of the germinal disc. (C) Transverse section of cleavage
furrows shown in (B). (D) Four-cell stage. (E) Third cleavage planes are vertical
 
as indicated. (F) Fourth cleavage planes also are vertical. (G) Germinal disc, set
tioned 25 hours after fertilization. Blastocoelic spaces dispersed.
 
zontal. While the latter is in progress the fifth cleavages, which are vertical,
begin. As a result of the fourth and fifth sets of cleavages, a mass of eight
central cells and twenty or more marginal cells arises. Horizontal (i.e., latitudinal) cleavages begin among the central cells at this time, and other cells
(see cell A, fig. 153F) appear to be budded off from the yolk floor from this
period on. The latter are contributed to the growing disc of cells above.
 
Four types of cleavage furrows now appear in the growing blastoderm as
follows :
 
( 1 ) cleavage among the central cells, increasing their number,
 
(2) cleavage among the marginal cells, contributing cells to the central
cells,
 
 
 
312
 
 
CLEAVAGE (SEGMENTATION) AND BLASTULATION
 
 
( 3 ) cleavage of the marginal cells, increasing the number of marginal cells
and contributing syncytial nuclei to the yolk floor,
 
(4) cleavage within the syncytial mass of the yolk floor, contributing cells
to the central cells, such as cell A, figure 153F.
 
Eventually a blastular condition is reached as a result of the foregoing
cleavages which does not possess an enlarged blastocoelic space; rather the
blastocoel is in the form of scattered spaces within a loosely aggregated cap
of cells (fig. 153G). This blastula might be regarded as a stereoblastula, i.e.,
solid blastula (Dean, 1896; Whitman and Eycleshymer, 1897).
 
6) Lepisosteus (Lepidosteus) osseiis. The early development of the
gar pike, Lepisosteus osseus, another bony ganoid fish, resembles that of Amia
described above. The disc of protoplasm which takes part in the early cleavages is a prominent mass located at one pole of the egg (fig. 154A). The first
two cleavage furrows appear to be meridional and partly cleave the protoplasmic cap of the egg, as indicated in figure 154B-D. The next cleavages
are vertical and somewhat parallel to one of the meridional furrows (fig.
152E). The fourth cleavages are vertical, cutting off four central cells from
the peripherally located marginal cells (fig. 152F). As in Amia, the marginal
cells contribute syncytial nuclei to the yolk bed below the protoplasmic cap,
and these in turn contribute definite cells to the growing blastodisc. The
blastula of Lepisosteus consists of a loosely aggregated cap of cells among
which are to be found indefinite blastocoelic spaces (fig. 152G). (See Dean,
1895.)
 
7) Gymnophionan Amphibia. Cleavage presumably is holoblastic, resulting in a disc of small micromeres at the animal pole, with large, irregular
macromeres, heavily yolk laden, located toward the vegetal pole (fig. 182A).
(See Svensson, ’38.) The latter cells become surrounded during gastrulation
by the smaller micromeres (Brauer, 1897). The blastula of the gymnophionan
amphibia essentially is solid and may be regarded as a stereoblastula.
 
3. Meroblastic Cleavage
 
The word meroblastic is an adjective which refers to a part of the germ;
that is, a part of the egg. In meroblastic cleavage only a small portion of the
egg becomes segmented and thus gives origin to the blastoderm. Most of the
yolk material remains in an uncleaved state and is encompassed eventually
by the growing tissues of the embryo. A large number of vertebrate eggs
utilize the meroblastic type of cleavage. Some examples of meroblastic cleavage are listed below.
 
 
a. Egg of the Common Fowl
 
{Note: As cleavage in reptiles resembles that of birds, a description of reptilian cleavage will not be given. The reader is referred to figure 231, con
 
 
TYPES OF CLEAVAGE
 
 
313
 
 
cerning the cleavage phenomena in the turtle. The information given below
is to be correlated also with the developing pigeon's egg.)
 
The germinal disc (blastodisc) of the hen’s egg at the time that cleavage
begins measures about 3 mm. in diameter. Its general relationship to the
egg as a whole is shown in figure 157A.
 
1) Early Cleavages. The first cleavage furrow makes its appearance at
about four and one-half to five hours after fertilization at the time when the
egg reaches the isthmus of the oviduct (figs. 155A; 157C, D). The first
 
 
 
Fig. 155. Cleavage in the chick blastoderm, surface views. (C after Olsen, ’42; the
rest after Patterson, ’10.) (A) First cleavage is approximately meridional. (B) Second
 
cleavage is at right angles to first. (C) Third cleavage planes are vertical as indicated
and approximately parallel to one of the other cleavage planes. Considerable inequality
may exist at this time. (This figure slightly modified from original.) (D) Seventeencell stage. Observe central and marginal cells. (E) Stage approximating 32-cell condition. (F) Surface view of 64-cell stage; 41 central and 23 marginal cells. (G) Surface
view of blastoderm in lower portions of oviduct; 31 marginal and 123 central cells. (H)
Later blastoderm, showing 34 marginal and 312 central cells.
 
 
 
 
TYPES OF CLEAVAGE
 
 
315
 
 
 
NUCLEUS OF PAND E R
BLASTODISC CELLS
 
 
ZONE OF JUNCTURE
 
(SYNCYTIAL GERM WALL)
 
 
 
 
 
CENTRAL PERIBLAST
 
PRIMITIVE 8LAST0C0EL
 
 
CELLULAR GERM WALL '
 
MARGINAL PERIBLAST
 
MARGIN OF OVERGROWTH
 
 
Fig. 156. Cleavage in chick blastoderm, sectional views. (After Patterson, ’10.) (A)
 
Median section through blastoderm approximately at right angles to furrow shown in
fig. 155A. (B) Section through blastoderm of about eight-cell stage. (C) Section
 
through blastoderm, showing 32 cells, also showing horizontal cytoplasmic cleft (segmentation cavity). (D) Median section through blastoderm similar to that shown in
fig. 155E, (E) Median section through blastoderm similar to that of fig. 155G. (F, G)
 
Diagrammatic views of developing avian blastoderms. (F) Diagrammatic section and
surface view of chick blastoderm shown in fig. 155G and fig. 156E. (G) Section of
 
chick blastoderm about time that egg is laid, depicting the primary blastocoel below
the blastoderm and syncytial tissue at the margins. Observe that the syncytial tissue
serves to implant the blastoderm upon the yolk substance.
 
 
furrow consists of a slight meridional incision near the center of the blastodisc,
cutting across the disc to an extent of about one half of the diameter of the
latter (fig. 155A). This furrow passes yolkward but does not reach the lower
portion of the disc where the cytoplasm is filled with coarse yolk granules
(fig. 156A). The second cleavage occurs about 20 minutes later and consists
of two furrows, one on either side of the first furrow and approximately at
right angles to the first furrow. These furrows may be regarded as meridional
(fig. 155B). Though both of the second furrows tend to meet the first furrow
at its midpoint, one of the second furrows may be displaced and, hence, may
not contact the corresponding furrow of the other side. The third set of
furrows is vertical, cutting across the second set of meridional furrows, and,
consequently, tends to parallel the first cleavage furrow (fig. 155C). The
fourth set of furrows is also vertical and, although not synchronous, it proceeds gradually to form eight central cells which are surrounded by twelve
 
 
 
 
316
 
 
CLEAVAGE (SEGMENTATION) AND BLASTULATION
 
 
marginal cells. l|[n figure 155D, five central cells are shown, while in figure
157E, eight central cells are present) The central cells do not have boundaries
below and, thus, are open toward the yolk. As a result, their protoplasm is
continuous with the protoplasm in the deeper-lying portions of the disc. The
marginal cells have boundaries only on two sides, and the cleavage furrows
which form the sides of the marginal cells continue slowly to extend in a
peripheral direction toward the margins of the disc (fig. 155D). The egg
is in this stage of development when it leaves the isthmus and enters the uterus
(fig. 157A, F).
 
Cleavage from this point on becomes very irregular, but three sets of furrows are evident:
 
(a) There are vertical furrows which extend peripherad toward the margin
of the blastodisc^^i'hese furrows meet at various angles the previously
established furrows which radiate toward the periphery of the blastodisc (in fig. 155E, see a., b., c.). A branching effect of the radiating
furrows, previously established, in this manner may be produced (in
fig. 155E, see c.)^
 
(b) Another set of vertical furrows is found which cut across the median
(inner) ends of the radiating furrows. The latter produce peripheral
boundaries for the centrally located cells (see fig. 155E, d., e., f. ). The
central cells thus increase in number as the blastodisc extends peripherally. As a result of this set of cleavage furrows, a condition of the
blastodisc is established in which there is a mass of central cells,
having peripheral boundaries, and an area of marginal cells which
lies more distally between the radiating furrows. It is to be observed
that the marginal cells lack peripheral boundaries (fig. 155E, F).
 
(c) A third and new kind of cleavage, cytoplasmic but not mitotic, now
occurs below the centrally placed cells, namely, a latitudinal or horizontal cleft which establishes a lower boundary for the centrally located cells with the subsequent appearance of a blastocoelic space filled
with fluid (fig. 156B, C).
 
Thus, at the 16- to 32-cell stages (fig. 155D, E) some of the more centrally located central cells have complete cellular boundaries (fig. 156C),
but central cells, located more peripherally, may not have the lower boundary.
The marginal cells also lack a lower boundary.
 
A little later, at the 60- to 100-cell stages (fig. 155F), the chick blastoderm
presents the following characteristics:
 
(a) There is a mass of centrally located cells. These cells lie immediately
above the horizontal cleft mentioned above (fig. 156C, D). They are
completely bounded by a surface membrane and represent distinct
cells. These cells continue to increase by mitotic division and, as early
as the 64-cell stage (fig. 155F), the centrally located cells are in the
 
 
 
 
Fig. 157. Chart showing ovary, oviducal, and pituitary relationships in passage of egg
from the ovary down the oviduct. Developing blastodisc shown in (B-G) in relation to
the oviducal journey. This chart shows an egg which has just been ovulated. Ordinarily,
however, this egg would not be ovulated until sometime after the egg shown in the uterus
has been laid.
 
 
 
318 CLEAVAGE (SEGMENTATION) AND BLASTULATION
 
 
 
BLASTODERM PERIBLAST
 
 
Fig. 158. Early cleavage phenomena in elasmobranch fishes. (A, B, E, F, G after
Ziegler, ’02, from Ruchert; C, D after Ziegler.) (A) Germ disc of Torpedo ocellata,
showing four cleavage nuclei, sperm nuclei, and beginning of first cleavage furrow. (B)
Stage of cleavage, possessing 16 cleavage nuclei. Four central cells and ten marginal
cells are evident from surface view. (C) Surface view of blastoderm of Scy Ilium
canicula with 64 cleavage nuclei. Twenty-nine central cells and seventeen marginal cells
are evident from surface view. (D) Later cleavage stage of 5. canicula with 145 cells
showing. (E) Transverse section of (B). (F) Transverse section of blastoderm of
 
T. ocellata with 64 cells. (G) Median section through blastoderm of T. ocellata at
the end of the cleavage period.
 
form of two layers situated immediately above the horizontal cleft or
segmentation cavity (fig. 156D).
 
(b) The horizontal cleft or segmentation cavity gradually widens and enlarges. It separates the central cells above from the uncleaved germinal
disc or central periblast below.
 
(c) At the margins of the central cells, these cleavages may be found: ( 1 )
Vertical cleavages occur which cut off more central cells from inner
ends of the marginal cells. As a result, there is an increase in the number of central cells around the periphery of the already-established cen
 
 
TYPES OF CLEAVAGE
 
 
319
 
 
tral mass of cells. (2) Vertical cleavages arise whose furrows extend
peripherad toward the margin of the disc. These furrows and previously
formed, similar furrows now approach the outer edge of the blastodisc (germinal disc). (See figs. 155H; 156D). (3) True latitudinal
or horizontal cleavages occur which serve to provide lower cell boundaries for the more peripherally located, central cells (see cell A, fig.
156E), and which also contribute nuclei without cell boundaries to
the disc substance in this immediate area (see cell B, fig. 156E). As
a result, the marginal or peripheral areas of the blastodisc around the
mass of completely formed, central cells are composed of: («) marginal cells which appear near the surface of the blastodisc, having
partial boundaries at the blastodisc surface, and {b) a deeper-lying
protoplasm, possessing nuclei without cell boundaries. This deeperlying, multinucleated, marginal protoplasm constitutes a syncytium
(fig. 156F).
 
2) Formation of the Periblast Tissue. As indicated above, the activities
of the blastoderm extend its margins peripherad. In so doing, some of the
mitotic divisions in the peripheral areas contribute nuclei which come to lie
in the deeper portions of the blastodisc. Some of these nuclei wander distally
and yolkward into the more peripherally located, uncleaved portions of the
protoplasm below the enlarging primary segmentation cavity or blastocoel.
A syncytial protoplasm containing isolated nuclei thus arises around the peripheral margin of the blastoderm in its deeper areas. This entire syncytial
protoplasm, composed of a continuous cytoplasm with many nuclei, is known
as periblast tissue. It is made up of two general areas: (1) the peripheral
periblast around the margin of the blastodisc and (2) a central periblast
below the primitive blastocoel (fig. 156G). This periblast tissue is a liaison
tissue which brings the yolk and the growing mass of cells of the blastodisc
into nutritive contact.
 
When this condition is reached, two kinds of embryonic tissues exist:
 
(a) the formative or embryonic tissue proper, composed of an aggregation of distinct cells. These cells constitute the cellular portion of the
blastoderm (see blastodisc cells, fig. 156G), and
 
(b) the peripheral and central periblast tissue (see fig. 156G). The latter
functions as a trophoblast tissue, and it is continuous with the segmented portilHi of the blastoderm around the peripheral areas of the
blastodisc. Centrally, however, it is separated from the segmented
area of the blastoderm by the primary blastocoelic cavity. The developmental condition at this time may be regarded as having reached
the primary blastular stage.
 
3) Morphological Characteristics of the Primary Blastula. This condition
of development is reached while the egg continues in the uterus (fig. 157G).
 
 
 
320
 
 
CLEAVAGE (SEGMENTATION) AND BLASTULATION
 
 
A transverse section through one of the diameteis of the primary blastula
presents the following features (fig. 156G):
 
(a) A central mass of cells of two or several cells in depth overlies the
blastocoelic space. This is the central or cellular portion of the
 
blastoderm.
 
(b) Underneath this central blastoderm is the primary segmentation cavity
or primary blastocoel.
 
(c) Below the primary blastocoel is the central syncytial periblast, which
continues downward to the yolk material; many yolk granules are
present in the layer of the central periblast near the yolk. Nuclei are
not present in the central area of the central periblast, but may be
present in its more peripheral portions.
 
(d) Around the peripheral areas of the central periblast and the cellular
portion of the blastoderm is the marginal periblast tissue which now
is called the germ wall. The germ-wall tissue contains much yolk material in the process of digestion and assimilation.
 
The central mass of cells or cellular blastoderm increases in cell number
and in size by the multiplication of its own cells and by the contribution of
marginal periblast tissue which gradually forms cells with boundaries from
its substance. The germ wall thus may be divided into two main zones: ( 1 )
an inner zone of distinct cells, which are dividing rapidly and, in consequence,
contribute cells to the peripheral portions of the growing cellular blastoderm
and (2) an outer peripheral zone, the syncytial germ wall (zone of junction).
The latter is in intimate contact with the yolk (fig. 156G). The central periblast tissue gradually disappears. At the outer boundary of the peripheral
periblast, there is an edge of blastodermic cells overlying the yolk. These
cells have complete boundaries and are known as the margin of overgrowth
(fig. 156G). A resume of the early development of the hen’s egg in relation
to the parts of the oviduct, pituitary control, laying, etc., is shown in figure 157.
 
4) Polyspermy and Fate of the Accessory Sperm Nuclei. The bird’s egg
is polyspermic and several sperm make their entrance at the time of fertilization (see fig. 157B). The supernumerary sperm stimulate abortive cleavage
phenomena in the peripheral area of the early blastodisc (fig. 155D). However, these cleavage furrows together with the extra sperm nuclei soon
disappear.
 
(References: Blount (’09); Lillie (’30); Olsen (’42); and Patterson (’10).)
 
For later stages in the development of the hen’s egg, see chapter 7.
 
b. Elasmobranch Fishes
 
1) Cleavage and Formation of the Early Blastula. Like the egg of the bird,
the egg of the elasmobranch fishes is strongly telolecithal, and a small disc
of protoplasm at one pole of the egg alone takes part in the cleavage phe
 
 
TYPES OF CLEAVAGE
 
 
321
 
 
nomena. Cleavage in the majority of these fishes simulates that of the bird,
but certain exceptional features are present. In some, as in Torpedo ocellata,
meroblastic cleavage is present in an extreme form. The zygotic nucleus divides and the two daughter nuclei divide again forming a syncytial state before
the appearance of the first cleavage furrow. The tendency of retardation or
suppression of the cytoplasmic mechanism of cleavage which occurs in the
bird blastoderm thus is carried to an extreme form in the early development
of some elasmobranch fishes.
 
The first cleavage furrow is meridional or nearly so (fig. 158A), and the
second furrow is similar and at right angles to the first furrow. The third set
of furrows is vertical and meets the previous furrows at various angles. The
fourth set of cleavages is vertical and synchronous, as is the preceding, and
gives origin to three or four central cells, which, on surface viewing, have
complete cell boundaries but below their cytoplasms are confluent with the
cytoplasm of the blastodisc (fig. 158B and E ). Around the periphery of these
central cells, are on the average ten marginal cells which have their cytoplasms
confluent below and peripherally with the general cytoplasm of the disc. The
fifth cleavage furrows are mixed. That is, in the central part of the disc the
cleavage furrows are latitudinal, as the mitotic spindles in this area form
perpendicular to the surface. As a result, distinct daughter cells are cut off
above, while the daughter cells below have cytoplasms confluent with the
general cytoplasm of the disc. A blastocoelic cavity appears between these
two sets of central cells. In the marginal areas the fifth set of cleavages is
vertical, cutting off more central cells and giving origin to more marginal cells.
The sixth set of cleavages is a mixture of vertical cleavages at the periphery
and latitudinal cleavages centrally; it produces a condition shown in figure
158F. In surface view, the blastoderm appears as in figure 158C, D.
 
From this time on cleavage becomes very irregular and a developmental
condition soon is produced which possesses a central blastoderm of many
cells with an enlarged blastocoelic cavity below (fig. 158G). A syncytial periblast tissue is present at the margins of the blastoderm which also extends
centrally below the blastocoelic space where it forms a central periblast (fig.
158G). In this manner, two kinds of cells are produced:
 
(a) a blastoderm of distinct cells which ultimately produces the embryo
and
 
(b) a surrounding trophoblast or periblast tissue which borders the yolk
substance peripherally and centrally. As in the chick, the periblast
tissue has nutritive (i.c., trophoblast) functions.
 
2) Problem of the Periblast Tissue in Elasmobranch Fishes. Two views
have been maintained, regarding the origin of the periblast nuclei in the
elasmobranch fishes. One view maintains that they arise from the accessory
sperm nuclei derived from polyspermy, for polyspermy is the rule here as it
 
 
 
322
 
 
CLEAVAGE (SEGMENTATION) AND BLASTULATION
 
 
is in reptiles and birds. In the latter groups, these accessory nuclei may divide
for a time but ultimately degenerate, playing no real part in ontogeny. In the
case of the elasmobranch fishes, the accessory nuclei tend to persist somewhat
longer, and accordingly, it is upon this evidence that some have maintained
that the periblast nuclei arise from them. Others hold that the sperm nuclei
degenerate as they do in reptiles and birds, and the periblast nuclei arise as
a result of the regular embryonic process. A third view concedes that both
these sources contribute nuclei.
 
In view of the origin of the periblast nuclei in teleost fishes, in the ganoid
fishes, Amia and Lepisosteus, and in reptiles and birds, and of the syncytial
tissue of the later mammalian trophoblast, it is probable that embryonic cells
and tissues and not accessory sperm nuclei are the progenitors of the periblast
tissue. This probability is suggested by figure 158F, G. Furthermore, later on
in the development of the elasmobranch fishes, the entoderm appears to contribute nuclei which wander into the periblast tissue which lies between the
entoderm and the yolk material (fig. 21 3K, L). In later stages the periblast
tissue is referred to as the yolk syncytium. In the yolk syncytium the periblast
nuclei gradually assume a much larger size.
 
For further details of the early development of the elasmobranch fishes,
consult Ziegler (’02) and Kerr (’19) and Chapter 7.
 
c. Teleost Fishes
 
1) Cleavage and Early Blastula Formation. During the fertilization process
of the egg in teleost fishes, the superficial cytoplasm of the egg migrates toward
the point of sperm entrance and hence a mound-like disc of protoplasm forms
at the pole of the egg where the sperm enters (figs. 122C; 123B, C). It is
this protoplasmic mass which takes part in cleavage (fig. 123E). The cleavage
planes in the teleost fishes manifest great regularity. The early cleavage furrows almost cut through the entire protoplasmic disc in most teleost eggs,
and a mere strand of cytoplasm is left near the yolk which is not cleaved
(fig. 159E).
 
In the sea bass, Serranus atrarius, the first two cleavage planes are meridional and at right angles to each other (fig. 159A); the third planes are vertical and parallel to the first plane. The result is a group of eight cells in two
rows (fig. 159B). The fourth cleavage furrows are vertical and parallel to
the long axis of the eight cells previously established. These furrows divide
each of the eight blastomeres into inner and outer daughter cells. The result
is 16 cells, arranged in parallel rows of four cells each (fig. 159C, D).
 
As the 16-cell condition is converted into 32 cells, the four inner cells
divide latitudinally, that is, the cleavage spindle forms perpendicular to the
surface, while the twelve surrounding cells divide vertically (fig. 159D, F, G).
From this time on latitudinal and vertical cleavages become mixed, and the
 
 
 
 
Fig. 159. Early development of the sea bass, Serraniis atrarius, and the trout, Salmo
fario. (A-M after Wilson, 1889 and 1891; N R after Kopsch, ’ll.) (A) Two-blastomere
 
stage, showing anaphase of next division. (B) Eight-blastomere stage (slightly modified). (C) Sixteen-cell blastoderm. (D) Sixteen-cell stage, showing anaphase nuclei
of next division. In the four centrally placed cells, the spindles are at right angles to the
surface, thus forming a latitudinal cleavage furrow in these cells. (E) Section through
center of four-blastomore stage. (F) Section through center of (D). Observe periblast
tissue. (G) Section showing change from 16-cell stage into 32 cells; see (D). (H)
 
Thirty-two to 64 cells. (I) Fate cleavage blastoderm. Observe marginal and central
periblast. (J) Multiplication of periblast nuclei around the margin of the blastoderm.
(K~M) Late blastoderm, showing marginal and central periblast tissue. (N-R) Cleavage of the blastodisc of the trout. Observe that periblast tissue is derived from the
blastodisc cytoplasm directly.
 
 
323
 
 
 
324
 
 
CLEAVAGE (SEGMENTATION) AND BLASTULATION
 
 
 
Fig. 160. Cleaving eggs of Platypus and Echidna. (After Flynn and Hill, ’39.) (A)
 
Egg, shell, and early cleavage in Ornithorhynchus. (B) Early cleavage in Echidna. See
fig. 16 ID.
 
synchronization of mitotic division is lost. In certain other teleost fishes, latitudinal cleavages begin as early as the 8-cell stage.
 
At the 32- to 64-cell stages in Serranus atrarius, the blastoderm presents a
cap-like mass of dividing cells overlying a forming blastocoel (fig. 159H, I).
Between the blastocoel and the yolk, there is a thin layer of protoplasm
connecting the edges of the cap. This thin protoplasmic layer is the forerunner
of the central periblast tissue; at this stage it contains no nuclei (fig. 159F, H).
 
2) Origin of the Periblast Tissue in Teleost Fishes. In the sea bass and
many other teleost fishes, some of the surrounding cells at the edge of the
blastoderm lose their cell boundaries and fuse together to form a common
syncytial tissue. The nuclei in this tissue continue to divide (fig. 159J) and
eventually migrate into the periblast tissue below the blastocoel (see arrow,
fig. 159L). The latter then becomes the central periblast, while the syncytial
tissue around the edges of the growing blastodisc forms the peripheral or
marginal periblast (fig. 159K-M).
 
In the trout, the early cleavage furrows of the blastodisc are incomplete,
and the periblast arises from the syncytial tissue established directly below
and at the sides of the protoplasmic cap (fig. 159N-R), This condition resembles the cleavage process in the elasmobranch fishes.
 
See Kerr (T9); Kopsch (Tl); and H. V. Wilson (1889).
 
d. Protolherian Mammalia
 
The Prototheria normally are placed in the class Mammalia along with
the Metatheria (marsupials) and Eutheria (true placental mammals). How
 
 
 
 
326
 
 
CLEAVAGE (SEGMENTATION) AND BLASTULATION
 
 
ever, the prototherian mammals are aberrant, highly specialized animals, whose
general anatomy and embryology delineates a group quite distinct from the
higher mammals. The duckbill or Platypus (Ornithorhynchus) is found only
in Australia. The other species belonging to this group is the spiny anteater
Echidna aculeata found in New Guinea, Tasmania, and Australia. The duckbill lays from one to three heavily yolk-laden eggs in an underground chamber
on a nest of weeds and grasses. The eggs have a leathery shell. The young are
hatched naked, and the mother holds them against her abdomen with her
tail, where they feed upon a milk-like substance which exudes from the milk
glands by means of pore-like openings. The Echidna lays two white, leathery
eggs about the size of the eggs of a sparrow which she places in a temporary
pouch or fold of skin on the ventral abdominal wall. They feed similarly to
the duckbill young.
 
The early cleavages of Echidna and Ornithorhynchus follow different cleavage patterns. (See Flynn and Hill, ’39, ’42.) The cleavage planes of the
Platypus are more regular and symmetrical and resemble to a degree the
pattern of early cleavage in teleost fishes (fig. 160A), whereas the early
cleavage planes in Echidna simulate to some degree those found in reptiles
(fig. 160B). In both species cleavage is meroblastic.
 
In Echidna the cleavage furrows cut almost all the way through the protoplasmic disc (fig. 161 E). The second cleavage in this species is at right angles
to the first, and divides the blastodisc into two larger and two smaller cells
(fig. 16 lA). The third cleavage furrows tend to parallel the first furrow,
forming eight cells (fig. 16 IB), while the fourth cleavages run parallel to
the second furrow, and 16 cells are formed (fig. 161C). The fifth cleavages
lack the constancy of the first four sets although they continue to be synchronous; they result in the formation of 32 cells (fig. 16 ID).
 
In transverse section, the cells of the 32-cell blastoderm appear as rounded
masses, each cell in its upper portion being free from the surrounding cells
but in its lower extremity intimately attached to the yolk substance (fig. 161F) .
Another feature of the early cleavages in Echidna is the tendency of the cells
to separate from each other; wide spaces consequently appear between the
blastomeres (fig. 161G). This tendency toward independence and isolationism
of the early blastomeres is characteristic of the higher mammals, as previously
observed. After the 32-cell stage, synchronization is lost and cleavage becomes
very irregular. A central mass of blastodermic cells eventually is formed,
surrounded by marginal cells, known as vitellocytes (fig. 175A).
 
As cleavage and development proceeds, the central blastomeres become
free from the underlying yolk, expand, and form a layer about two cells in
thickness (fig. 175B). The vitellocytes around the periphery of the blastoderm
eventually fuse to form a syncytium or multinucleated cytoplasmic mass intimately associated with the yolk (fig. 175B, C). This marginal mass of syncytial
tissue forms the marginal periblast. Within the central portion of the blasto
 
 
FORCE WHICH CAUSES THE BLASTOMERES TO ADHERE
 
 
327
 
 
derm itself two types of cells may be observed, namely, a superficial ectodermal cell and a more deeply situated, somewhat vacuolated, smaller entodermal cell (fig. 175B). (For later stages of blastulation, see chapter 7.)
 
e. Cleavage in the California Hagfish, Polistotrema (Bdellostoma) stouti
 
The California hagfish spawns an egg which is strongly telolecithal. The
germinal disc (blastodisc) is situated immediately below the egg membrane
at one end of the egg, adjacent to the micropyle and the anchor filaments
(fig. 162A). Cleavage begins in this disc, and the enlarging blastoderm slowly
creeps downward to envelop the massive yolk material. The freshly laid egg
measures about 29 mm. by 14 mm., including the shell. Without the shell,
the egg is about 22 mm, by 10 mm. and is rounded at each end (Dean, 1899).
 
The first two cleavage planes may be regarded as meridional (or vertical)
(fig. 162B). The third cleavage appears to be a mixture of vertical and horizontal (latitudinal) cleavages, with the former predominating (fig. 162D, E).
Cleavage from this time on becomes irregular, and a typical meroblastic blastoderm soon is attained with central and marginal cells (fig. 162F).
 
C. What is the Force Which Causes the Blastomeres to Adhere
Together During Early Cleavage?
 
A question naturally arises concerning the force which makes the blastomeres of most chordates adhere to one another during the early cleavage
 
 
 
Fig. 162. Egg and cleavage in the marine lamprey, Polistotrema (Bdellostoma) stouti.
After Dean, 1899.) (A) Animal pole end of the egg. (B) Surface view of blasto
dermic hillock, showing first cleavage furrow. (C) Same, second cleavage. (D) Third
cleavages. (E, F) Later cleavages, strongly irregular. (G) Egg with shell removed.
 
 
 
328
 
 
CLEAVAGE (SEGMENTATION) AND BLASTULATION
 
 
period. This subject was investigated in the amphibian blastula by Holtfreter,
’39. According to this investigator, blastomeres, when isolated by mechanical
means, appear to wander aimlessly about. When contact is made with other
blastomeres during this wandering process the cells stick or adhere together.
As a result, the mass of adhering cells gradually is formed which becomes
rounded into a ball-shaped structure. The results of this work suggest that
the force which draws the cells together is one of thigmotaxis or contact
affinity, aided by a surface stickiness of the cells. This force only becomes
influential when an isolated cell has made contact with another cell or cells.
 
On the other hand, the early blastomeres of the cleaving mammalian egg
are evidently held together also by the binding influence of the egg membrane
or zona pellucida. An adhering influence is not prominent until later cleavage
stages.
 
However, one must not be too ready to espouse a single, mechanical factor
as the main binding force which causes the blastomeres to adhere together,
to move in relation to each other, and to form a definite configuration. Factors
tending toward organization are at work during early and late cleavage as
well as in subsequent development. Relative to these matters, it is well to
cogitate upon the statement of Whitman (1893). “Comparative embryology
reminds us at every turn that the organism dominates cell-formation, using
for the same purpose one, several, or many cells, massing its material and
directing its movements, and shaping its organs, as if cells did not exist, or
as if they existed only in complete subordination to its will” (p. 653).
 
D. Progressive Cytoplasmic Inequality and Nuclear Equality of the
Cleavage Blastomeres
 
1. Cytoplasmic Inequality of the Early Blastomeres
 
In harmony with the differences in the location and activities of the various
blastomeres of the cleaving egg, it is apparent that a difference exists in the
ooplasmic substance within the various cells in many species. In the frog,
for example, the quantity of yolk substance present in the cells of the yolk
pole is much greater than that of the animal pole. Similarly in the four-cell
stage the substance of the gray crescent is located in two of the blastomeres,
while the other two blastomeres have little or none of this substance. Two
of these four cells, therefore, are qualitatively different from the other two.
In the ascidian, Styela partita, the presence of the yellow crescent, yolk substance, and gray crescent materials demonstrates that in the four- or eightcell stages there are qualitative differences in the ooplasmic substances which
enter into the composition of the respective blastomeres (Conklin, ’05, a
and b). Similar conditions may be demonstrated for Amphioxus although
pigmented materials are not present in the egg (fig. 167). (See Conklin, ’32,
’33.) As cleavage continues in the eggs of Styela and Amphioxus, a progres
 
 
 
Fig. 163. Developmental potencies (cell lineage) of isolated blastomeres of the cleaving sea-urchin egg, representing different levels along the egg axis (from Huxley and
DeBcer, ’34, after Horstadius). Observe the following: (1) Progressing from the animal
pole to the vegetative pole, the potency for developing the sensory cilia decreases from
animal pole cells 1 to animal pole cells 11. (2) The potency for developing motile cilia
increases from animal pole cell II to vegetative pole cell I. (3) The potency for gastrulation becomes greater from vegetative pole cell I to vegetative pole cell II. (4) In the
development of vegetative pole cell I, shown at the right of vegetative I, if the third
(equatorial) cleavage plane happens to be displaced near the animal pole, an isolated
vegetative cell I has more animal pole potencies and will develop apical cilia; if the
cleavage plane is displaced toward the vegetative pole, the vegetative pole cell I will
attempt to gastrulate. (5) The disc of vegetative cells II plus the micromeres produce a
gut so large it will not invaginate and hence forms an exogastrula.
 
 
329
 
 
 
330
 
 
CLEAVAGE (SEGMENTATION) AND BLASTULATION
 
 
sive difference in the cytoplasmic substances which enter into the various
blastomeres becomes evident.
 
That the presence or absence of a specific ooplasmic substance within the
blastomeres determines a difference in the developmental history of the cell
or cells has been shown experimentally for many animal species. For example,
in the amphibian embryo it has been demonstrated both by constriction of
the developing egg and its membranes with hair loops (Spemann, ’02, ’03)
and by placing a small glass rod in the cleavage furrow after the egg membranes have been removed (Ruud, ’25) that each of the blastomeres of the
two-cell stage will develop a complete embryo if the first cleavage plane bisects the gray crescent. If, on the other hand, the first cleavage plane is at
right angles to the median plane of the embryo, the blastomere which contains
the substance of the gray crescent will develop a complete embryo, whereas
the other one will give origin to a very imperfect form which does not gastrulate normally or produce a semblance of a normal embryo.
 
Similar experiments upon the egg of the newt, Triton palrnatus, indicate
that a marked difference in the “developmental potencies exists between the
dorsal and ventral sides of the egg within a few minutes from fertilization.
The formation of the gray crescent seems to be a secondary phenomenon
which makes this difference clearly visible in the eggs of some species”
(Fankhauser, ’48, p. 694).
 
In Amphioxus, similar evidence is obtained after the blastomeres have been
mechanically isolated. Typical embryos are developed always from the first
two blastomeres, for unlike the frog or newt, the first cleavage plane consistently furrows the median axis of the embryo. These twin embryos are half
the normal size (Wilson, E. B., 1893; Conklin, ’33). Right and left halves
of the four-cell stage also give rise to normal larvae. Moreover, blastulae also
develop from isolated blastomeres of the eight-cell stage, but the blastulae
which develop from the micromeres are smaller and have only one type of
cell, namely, ectoderm, and they never go further than the blastular stage.
On the other hand, those from the macromeres are larger and have entoderm,
mesoderm, as well as ectoderm, but they never progress further than the
gastrular stage of development (Conklin, ’33). Reference should be made to
figure 167B in this connection. It is to be observed that the macromeres contain
potential mesodermal, entodermal, and ectodermal ooplasm, whereas the micromeres lack the mesodermal and entodermal substances and contain only
ectodermal material.
 
In the protochordate, Styela, a somewhat different condition is found. If
the cleaving egg of this species, is separated at the two-cell stage into two
separate blastomeres, each blastomere develops only one half of an embryo
(Conklin, ’05b, ’06). That is, the right blastomere develops an embryo minus
the left half, while the left blastomere produces the opposite condition. There
is some tendency to develop or regulate into a complete embryo in that the
 
 
 
 
 
Fig. 164. Distribution of presumptive organ regions (cell lineage) during cleavage
in Ascaris. (After Durken: Experimental Analysis of Development, New York, W, W.
Norton, based upon figures by Boveri and zur Strassen.) (A) Two-cell stage, showing
primordial soma cell and first stem cell. (B) Two ectodermal cells, A and B. Soma cell,
Sj, is a mixture of mesoderm, stomodaeum, and entoderm; second stem cell, Pj, is a
mixture of mesoderm and germ-cell material. The symbolism used to designate the
various organ-forming substances is shown in (G). The progressive segregation into separate cells of the substances shown in cells S* and P 2 is given in (C-G). Cf. also fig. 6 IE.
 
 
331
 
 
 
332
 
 
CLEAVAGE (SEGMENTATION) AND BLASTULATION
 
 
POSITrON OF 2ND. MATURATION SPINDLE
I POINT OF SPERM PENETRATION
 
 
 
CLEAVAGE FURROW OF FIRST CLEAVAGE
 
ON PART OF NUCLEUS WHICH ENTERED "BRIDGE** IN C
 
 
Fig. 165. Drawings of cleavage of a partially constricted egg of Triturus viridescens,
illustrating delayed nucleation. (Slightly modified from Fankhauser, ’48.) (A) Shows
 
constricting loop, point of sperm entrance, and second maturation spindle. The constricted
portion to the right will contain the fusion nucleus. ^B) First cleavage furrow in right
half of egg. (C) Second cleavage. The nucleus in the “bridge” area has migrated into
the “bridge.” (D) Third cleavage. The nucleus in the bridge area has divided and produced cleavage furrow through the bridge cytoplasm as indicated. One of the daughter
nuclei of this cleavage is now in the constricted part of the egg at the left. (E) Fourth
cleavage — first division of left half. (F) Blastular stage — late blastula at right, middle
blastula at left.
 
ectoderm grows over the half of the embryo which failed to develop. Also,
the notochord rounds up into^ normally shaped notochord but is only half
the normal size. Essentially, however, these separated blastomeres develop
into “half embryos in which some cells have grown over from the uninjured
to the injured side, but in which absolutely no change has taken place in the
potency of the individual cells or of the different ooplasmic substances”
(Conklin, ’06). Similarly, at the four-cell stage isolation of anterior and posterior blastomeres gives origin to anterior and posterior half embryos respectively.
 
The developing sea-urchin egg has been used extensively for experimental
work in the study of isolated blastomeres. In figures 163 and 166A-D are
shown the different developmental possibilities which arise from isolated blastomeres of the early cleavage stages. Also, in cell-lineage studies on the developing egg of A scans, a difference in the developmental potencies of the
blastomeres is evident (fig. 164). (See also fig. 145A-D in respect to the
early development of the pig.)
 
The foregoing experiments and observations and others of a similar nature
suggest that, during the early cleavage stages of many different animal species,
a sorting-out process is at work which segregates into different blastomeres
 
 
CYTOPLASMIC INEQUALITY AND NUCLEAR EQUALITY
 
 
333
 
 
distinct ooplasmic substances which possess different developmental potencies. This segregation of different substances into separate blastomeric channels is one of the functions of cleavage. .
 
2. Nuclear Equality of the Early Blastomeres
 
Another question next arises: Is there a similar sorting out of nuclear substances during the cleavage period and do the nuclei in certain cells become
different from those of other cells? Or, do all of the nuclei retain an equality
during cleavage and development? Experimental evidence indicates a negative
answer to the former question and a positive one to the latter.
 
A precise and illuminating experiment demonstrating nuclear equality of
the early blastomeres may be performed by the hair-loop constriction method
(Spemann, ’28; Fankhauser, ’48). For example, the fertilized egg of the
newt, Triturus viridescens, may be constricted partially by a hair loop so that
the zygotic nucleus is confined to one side (fig. 165 A, B). The side possessing
the nucleus divides, but the other side does not divide (fig. 165B, C). By
releasing the ligature between the two sides at various stages of development
of the cleaving side, i.e., 2-, 4-, 8-, 16-, and 32-cell stages, a nucleus is permitted to “escape” into the cytoplasm of the uncleaved side (fig. 165C, E; in
D the escaped nucleus is seen in the blastomere to the left). By tightening
the loop again after the escaping nucleus has entered the uncleaved cytoplasm,
further nuclear “invasion” of the uncleaved part is blocked. If the original
constriction was made so that the plane of constriction coincides with the
plane of bilateral symmetry, i.e., if it constricts the gray crescent into two
halves, the result is two normal embryos. This occurs after the 2-, 4-, 8- and
16-cell stages of the cleaving half of the egg. Nuclei permitted to escape
when the cleaving side has reached the 32-cell stage do not produce normal
embryos in the uncleaved side, probably because of the changes which have
occurred in the meantime in the cytoplasm of the uncleaved side and not to
the qualitative differences in the nuclei at this stage.
 
Another type of experiment upon the early cleaving blastomeres which
demonstrates nuclear equality may be performed. It has been shown by
Pfliiger, Roux, and Driesch (Wilson, E. B., ’25, p. 1059) that a cleaving
egg pressed between two glass surfaces will divide parallel to the pressure
surfaces. That is, the mitotic spindle is moved into a position parallel to the
pressure surfaces. Under these circumstances, the spindle obeys the second
law of Hertwig, namely, that the mitotic spindle tends to coincide with the
long axis of the protoplasmic mass. Cleavage under pressure so applied, therefore, will result in a series of vertical cleavage planes. In the sea urchin
(fig. 166) if pressure is applied in the four-cell stage, the mitotic spindles
will form in a horizontal position, as shown in figure 166E, instead of in the
vertical position, as indicated in figure 166B, C, where no pressure is applied.
In other words, all of the nuclei shown in white in the upper blastomeres of
 
 
 
334
 
 
CLEAVAGE (SEGMENTATION) AND BLASTULATION
 
 
figure 166C will be displaced horizontally by the applied pressure, as shown
in figure 165F. If pressure is released at this stage, the mitotic spindle again
obeys Hertwig’s rule and forms in the long axis of the cytoplasm which is
now vertical in position. As a result, upper and lower cells are formed, as
in figure 166G. The original destiny of the nuclei in the cells producing ectoderm is shown in white circles; that for the cells destined to produce mesenchyme, entoderm, and ectoderm is shown in black (figs. 163, mesomeres;
166C, D). As shown in figure 166G, there is a mixture of these nuclei after
the pressure is released. Regardless of this redistribution of nuclei, development proceeds almost normally. Development thus appears to be governed
by the presence of special ooplasmic substances contained within the respective
blastomeres (figs. 163; 166A-D).
 
The evidence from the foregoing experiments suggests the conclusion that
the nuclei in the early blastomeres are qualitatively equal. Consequently, this
body of experimental evidence is antagonistic to the older view of Weismann,
who held that differences in the various parts of the developing organism
are to be attributed to “differential nuclear divisions” whereby different hereditary qualities (i.e., biophors) are dispersed to different cells. To quote
from Weismann (1893, p. 76):
 
Ontogeny depends on a gradual process of disintegration of the id of germplasm, which splits into smaller and smaller groups of determinants in the development of each individual, so that in place of a million different determinants, of
which we may suppose the id of the germ-plasm to be composed, each daughter-cell
in the next ontogenetic stage would only possess half a million, and each cell of
the following stage only a quarter of a million and so on. Finally, if we neglect
possible complications, only one kind of determinant remains in each cell, viz.,
that which has to control that particular cell or group of cells.
 
£. Quantitative and Qualitative Cleavages and Their Influence upon
Later Development
 
One of the earliest students of the problem of the developmental possibilities of isolated blastomeres was Hans Driesch (1891 and 1892). In these
publications, Driesch offered the results of experiments in which he shook
apart the early blastomeres of the sea urchin and studied their development.
Driesch found that the two blastomeres resulting from the first division continued to divide, and as though the other blastomeres were present. The
first division of the isolated blastomere was meridional, as if it had retained
contact with its mate of the two-cell stage. The next division was latitudinal,
also, as if it had retained contact with its original mate. Ultimately each isolated blastomere developed into swimming blastulae of half the normal size.
The four blastomeres of the four-cell stage were similarly isolated. Here,
also, each divides as if it were part of the whole, and free-swimming blastulae
develop. However, later development is imperfect or definitely abnormal.
 
 
 
QUANTITATIVE AND QUALITATIVE CLEAVAGES
 
 
335
 
 
Isolation of blastomeres in the eight-cell stage of development, in most cases,
results in abnormal development.
 
In Amphioxus, as mentioned previously, isolation of the first two blastomeres results in the production of twin embryos of half the normal size. In
the eight-cell stage in Amphioxus, the isolated smaller micromeres will develop blastulae of ectoderm only, whereas the macromeres will develop blastulae with developed entoderm, mesoderm, and ectoderm. In the four-cell
stage, if the two posterior blastomeres are separated from the two anterior
blastomeres, the former develop early embryos which have entoderm and
mesoderm together with ectoderm; the latter have notochord and neural plate
together with ectoderm and possibly a little of the mesoderm (Conklin, ’33).
Similarly, in the frog or in the newt, when the first cleavage plane bisects
the gray crescent, the isolation of the first two blastomeres results in the
 
 
 
Fig. 166. Nuclear equality in the sea-urchin egg. (A-D) Normal cleavage. White
nuclei and black nuclei theoretically so designed to show nuclei in animal and vegetal pole
cells respectively. (E) Four-cell stage flattened by pressure, showing position of spindles
for the third cleavage parallel to pressure surface. (F) Eight-cell stage under pressure.
Compare with (C), normal. (G) Horizontal cleavage resulting from release of pressure
after eight-cell stage. Note mixed distribution of nuclei. Later development normal, with
cytoplasmic, organ-forming substances determining development as in fig. 163. Thus it
appears that the nuclei are equal within the blastomeres, whereas the cytoplasm is unequally (i.e., qualitatively) distributed to the respective blastomeres, the particular type of
development of the blastomeres being dependent upon the cytoplasmic substance present.
 
Black cytoplasm ~ micromeres which form primary mesenchyme. Coarse dotting =
entoderm, secondary mesenchyme and coelomic material. White, light stipple, and vertical
lines = ectodermal cells.
 
 
 
336
 
 
CLEAVAGE (SEGMENTATION) AND BLASTULATION
 
 
formation of two normal embryos. However, if the first cleavage is at right
angles to the plane of bilateral symmetry of the egg, the blastomere containing
the gray crescent material will develop a normal embryo, but the other blastomere will not do so.
 
The above results from isolated blastomeres suggest the following; When
the division of the early egg is purely quantitative, so that the resulting blastomeres contain all of the cytoplasmic substances equally, as in the first one or
two cleavage planes in the sea urchin (fig. 166A, B) or the first cleavage in
the frog when it bisects the gray crescent, the isolation of the resulting blastomeres tends to produce complete embryos. Such blastomeres are known as
totipotent blastomeres. (See Chap. 8.) However, when cleavage is qualitative, such as the second cleavage of Amphioxus, the third cleavage of the
sea urchin (fig. 166C), or the first cleavage of the frog when it occurs at right
angles to the median axis of the embryo, the resulting development depends
upon the qualities (that is, ooplasmic substances) resident in the isolated
blastomeres.
 
 
Bibliography
 
 
Bishop, S. C. 1926. Notes on the habits
and development of the mud puppy,
Necturus maculosus (Rafinesque). New
York State Mus. Bull., Albany, New
York, May 1926, No. 268.
 
Blount, M. 1909. The early development
of the pigeon’s egg, with especial reference to polyspermy and the origin of the
periblast nuclei. J. Morphol. 20:1.
 
Brauer, A. 1897. Beitrage zur Kenntniss
der Entwicklungsgeschichte und der Anatomie der Gymnophionen. Zool. Jahrb.
10:389.
 
Buytendijk, F. J. J. and Woerdeman, M. W.
1927. Die physico-chemischen Erscheinungen wahrend der Eientwicklung. 1.
Die Messung der Wasserstoffionenkonzentration. Roux’ Arch. f. Entwick. d.
Organ. 112:387.
 
Cerfontaine, P. 1906. Recherches sur le
developpement de VAmphioxus. Arch,
biol., Paris. 22:229.
 
Cole, F. J. 1930. Early Theories of Sexual
Generation. Oxford University Press,
Inc., Clarendon Press, New York.
 
Conklin, E. G. 1897. The embryology of
Crepidula. J. Morphol. 13:1.
 
. 1905a. Mosaic development in
 
ascidian eggs. J. Exper. Zool. 2:145.
 
 
. 1905b. The organization and cell
lineage of the ascidian egg. J. Acad. Nat.
Sc., Philadelphia. 13:5.
 
. 1906. Does half of an ascidian
 
egg give rise to a whole larva? Arch. f.
Entwicklngsmech. der Organ. 21:727.
 
. 1932. The embryology of Amphi
oxus. J. Morphol. 54:69.
 
. 1933. The development of isolated
 
and partially separated blastomeres of
Amphioxus. J. Exper. Zool. 64:303.
 
Dean, B. 1895. The early development of
gar-pike and sturgeon. J. Morphol. 11:1.
 
. 1896. The early development of
 
Amia. Quart. J. Micr. Sc. 38:413.
 
. 1899. On the embryology of
 
BdeUostoma stouti. Festschrift von Carl
von Kupffer. Gustav Fischer, Jena.
 
Driesch, H. 1891. Entwicklungsmechanfsche Studien MI. Zeit. Wiss. Zool.
53:160.
 
. 1892. Entwicklungsmechanische
 
Studien III-VI. Zeit. Wiss. Zool. 55:1.
 
Eycleshymer, A. C. 1895. The early development of Amblystoma with observations on some other vertebrates. J.
Morphol. 10:343.
 
. 1904. Bilateral symmetry in the
 
egg of Necturus. Anat. Anz. 25:230.
 
 
 
BIBLIOGRAPHY
 
 
337
 
 
and Wilson, J. M. 1910. Normal
 
plates of the development of Necturus
maculosus. Entwicklungsgeschichte der
Wirbeltiere, Part 11. F. Keibel. Gustav
Fischer, Jena.
 
Fankhauser, G. 1948, The organization of
the amphibian egg during fertilization
and cleavage. Ann. New York Acad.
Sc. 49:684.
 
Flynn, T. T. and Hill, J. P. 1939. The
development of the Monotremata. Part
IV. Growth of the ovarian ovum, maturation, fertilisation, and early cleavage.
Trans. Zool. Soc., London, s.A. 24, Part
6:445.
 
and . 1942. The later stages
 
of cleavage and the formation of the
primary germ-layers in the Monotremata
(preliminary communication). Proc.
Zool. Soc., London, s.A. 111:233.
 
Gregory, P. W. 1930. The early embryology of the rabbit. Carnegie Inst., Wash.
Publ. 407. Contrib. Embryol. 21:141.
 
Harding, D. 1951. Initiation of cell division in the Arbacia egg by injury substances. Physiol. Zool. 24:54.
 
Hartman, C. G. 1916. Studies in the development of the opossum, Didelphys
virginiana L. I. History of the early
cleavage. II. Formation of the blastocyst.
J. Morphol. 27:1.
 
Harvey, E. B. 1936. Parthenogenetic merogony or cleavage without nuclei in
Arbacia punctulata. Biol. Bull. 71:101.
 
. 1938, Parthenogenetic merogony
 
or the development without nuclei of the
eggs of sea urchins from Naples. Biol.
Bull. 75:170.
 
. 1940. A comparison of the development of nucleate and non-nucleate
eggs of Arbacia punctulata. Biol. Bull.
79:166.
 
. 1951. Cleavage in centrifuged
 
eggs and in parthenogenetic merogones.
Ann. New York Acad. Sc. 51: Art. 8,
1336.
 
Hatschek, B. 1893. Amphioxus and Its
Development. The Macmillan Co., New
York.
 
Heilbrunn, L. V. 1921. Protoplasmic viscosity changes during mitosis. J. Exper.
Zool. 34:417.
 
and Wilson, W. L. 1948. Protoplasmic viscosity changes during mitosis
in the egg of Chaetopterus. Biol. Bull.
95:57.
 
 
Heuser, C. H. and Streeter, G. L. 1929.
Early stages in the development of pig
embryos from the period of initial cleavage to the time of the appearance of
limb-buds. Contrib. to Embryol. Carnegie Inst., Washington. Publ. No. 394.
20(109):1.
 
and . 1941. Development
 
of the macaque embryo. Contrib. to
Embryol. Carnegie Inst., Washington.
Publ. 538:17.
 
Hill, J. P. 1910. The early development
of the Marsupialia, with special reference to the native cat (Dasyurus viverrinus). Quart. J. Micr. Sc. 56:1.
 
. 1918. Some observations on the
 
early development of Didelphys aurita
(Contributions to the embryology of the
Marsupialia — V). Quart. J. Micr. Sc.
63:91.
 
Holtfreter, J. 1939. Studien zur Ermittlung
der Gestaltungsfaktoren in der Organentwicklung der Amphibien. I. Dynamischcs
Verhalten isolierter Furchungszellen und
Entwicklungsmechanik der Entodermorgane. Roux’ Arch. f. Entwick. d. Organ.
139:110.
 
Hooke, R. 1664. Micrographia. See page
114. Martyn and Allestry, London.
 
Huber, G. C. 1915. The development of
the albino rat, Mus norvegicus albinus. I.
From the pronuclear stage to the stage
of mesoderm anlage: end of the first to
the end of the ninth day. J. Morphol.
26:247.
 
Huxley, J. S. and De Beer, G. R. 1934. The
Elements of Experimental Embryology.
Cambridge University Press, London.
 
Jordan, E. O. 1893. The habits and development of the newt (Diemyctylus viridescens). J. Morphol. 8:269.
 
Kerr, J. G. 1909. Normal plates of the development of Lepidosiren paradoxa and
Protopterus annectens. Entwicklungsgeschichte der Wirbeltiere, Part 10. F.
Keibel. Gustav Fischer, Jena.
 
. 1919. Textbook of Embryology.
 
Vol. 2. The Macmillan Co., New York.
 
Kopsch, F. 1911. Die Entstehung des Dottersackentoblast und die Furchung bei
der Forelle (Salmo fario). Arch. f. mikr.
Anat. 78:618.
 
Kowalewski, A. 1867. Entwicklungsgeschichte des Amphioxus lanceolatus.
Mem. Acad. imp. d. sc. de St. Petersburg,
VU« Serie. 11: No. 4.
 
 
 
338
 
 
CLEAVAGE (SEGMENTATION) AND BLASTULATION
 
 
Lewis, W. H. 1939. The role of a superficial plasmagel layer in changes of form,
locomotion and division of cells in tissue
cultures. Arch. f. exper. Zellforsch. 23:1.
 
and Gregory, P. W. 1929. Cinematographs of living developing rabbit
eggs. Science. 69:226.
 
and Hartman, C. G. 1941. Tubal
 
ova of the Rhesus monkey. Contrib. to
Embryol. Carnegie Inst., Washington.
Publ. 538:9.
 
and Wright, E. S. 1935. On the
 
early development of the mouse egg.
Contrib. to Embryol. Carnegie Inst.,
Washington. Publ. No. 459. 25:113.
 
Lillie, F. R. 1895. The embryology of the
Vnionidae. A study in cell lineage. J.
Morphol. 10:1.
 
. 1930. The Development of the
 
Chick. 2d ed., Henry Holt & Co., New
York.
 
McCrady, E. 1938. The embryology of
the opossum. Am. Anat. Memoirs, 16,
The Wistar Institute of Anatomy and
Biology, Philadelphia.
 
Moore, J. A. 1941. Developmental rate
of hybrid frogs. J. Exper. Zool. 86:405.
 
. 1946. Studies in the development
 
of frog hybrids. I. Embryonic development in the cross Rana pipiens 9 x Rana
syhatica J. Exper. Zool. 101:173.
 
. 1947. Studies in the development
 
of frog hybrids. II. Competence of the
gastrula ectoderm of Rana pipiens 9 x
Rana sylvatica S hybrids. J. Exper. Zool.
105:349.
 
Morgan, T. H. 1897. Development of the
Frog’s Egg. An Introduction to Experimental Embryology. The Macmillan Co.,
New York.
 
Nelsen, O. E. 1948. Changes in the form
of the blastophore in blocked gastrulae
of Rana pipiens. Anat. Rec. 101:60.
 
. 1949. The cumulative effect of
 
oxygen-pressure in the blocking of gastrulation in the embryo of Rana pipiens.
Anat. Rec. 105:599.
 
Nicholas, J. S. and Hall, B. V. 1942. Experiments on developing rats. II. The development of isolated blastomeres and
fused eggs. J. Exper. Zool. 90:441.
 
Olsen, M. W. 1942. Maturation, fertilization and early cleavage in the hen’s egg.
J. Morphol. 70:513.
 
 
Oppenheimer, J. M. 1936. Processes of
localization in developing Fundulus. J.
Exper. Zool. 73:405.
 
Pasteels, J. 1937. Sur I’origine de la sym6trie bilaterale des amphibiens anoures.
Arch. Anat. Micr. 33:279.
 
. 1938. A propos du determinisme
 
de la symetrie bilaterale chez les amphibiens anoures. Conditions qui provoquent I’apparition du croissant gris.
Compt. rend. Soc. de biol. 129:59.
 
Patten, B. M. 1948. Embryology of the Pig.
3d ed.. The Blakiston Co., Philadelphia.
 
Patterson, J. T. 1910. Studies on the early
development of the hen’s egg. I. History
of the early cleavage and of the accessory cleavage. J. Morphol. 21:101.
 
Pincus, G. 1939. The comparative behavior of mammalian eggs in vivo and in
vitro. IV. The development of fertilized
and artificially activated rabbit eggs. J.
Exper. Zool. 82:85.
 
Pollister, A. W, and Moore, J. A. 1937.
Tables for the normal development of
Rana sylvatica. Anat. Rec. 68:489.
 
Rugh, R. 1951. The Frog, Its Reproduction and Development. The Blakiston
Co., Philadelphia.
 
Ruud, G. 1925. Die Entwicklung isolierter
Keimfragmente friihester Stadien von
Triton taeniatiis. Arch. f. Entwicklngsmech. d. Organ. 105:209.
 
Shumway, W. 1940. Cleavage and gastrulation in the frog. Anat. Rec. 78:143.
 
Snell, G. D. 1941. Chap. 1. The early embryology of the mouse in Biology of the
Laboratory Mouse, by staff of Roscoe
B. Jackson Memorial Laboratory. The
Blakiston Co., Philadelphia.
 
Spemann, H. 1902. Entwickelungsphysiologische Studien am Triton — Ei. II. Arch,
f. Entwicklngsmech. d. Organ. 15:448.
 
. 1903. Entwicklungsphysiologische
 
Studien am Triton — Ei. III. Arch. f.
Entwicklngsmech. d. Organ. 16:551.
 
. 1928. Die Entwicklung seitlicher
 
und dorso-ventraler Keimhiilften bei verzogerter Kernversorgung. Zeit. Wiss.
Zool. 132:105.
 
Svensson, G. S. O. 1938. Zur Kenntniss
der Furchung bei den Gymnophionen.
Acta zool. 19:191.
 
Weismann, A. 1893. The Germ-plasm: A
Theory of Heredity. English translation
by W. N. Parker and H. Ronnfeldt.
Walter Scott, Ltd., London.
 
 
 
BIBLIOGRAPHY
 
 
339
 
 
Whitman, C. O. 1893. The inadequacy
of the cell-theory of development. J.
Morphol. 8:639.
 
and Eycleshymer, A. C. 1897.
 
The egg of Amia and its cleavage. J.
Morphol. 12:309.
 
Wilson, E. B. 1893. Amphioxus, and
the mosaic theory of development. J.
Morphol. 8:579.
 
. 1925. The Cell in Development
 
and Heredity. 3d ed.. The Macmillan
Co., New York.
 
 
Wilson, H. V. 1889. The embryology of
the .sea bass (Serranus atrarius). Bull.
U. S. Fish Comm. 9:209.
 
Wilson, J. T. and Hill, J. P. 1908. Observations on the development of Ornithorhynchus. Philos. Tr. Roy. Soc., London,
s.B. 199:31.
 
Ziegler, H. E. 1902, Lehrbuch der vergleichenden Entwicklungsgeschichte der
niederen Wirbeltiere in systematischer
Reihenfolge und mit Beriicksichtigung
der experimentellen Embryologie bearbeitet. Gustav Fischer, Jena.
 
==The Chordate Blastula and Its Significance==
 
A. Introduction
 
1. Blastulae without auxiliary tissue
 
2. Blastulae with auxiliary or trophoblast tissue
 
3. Comparison of the two main blastular types
 
B. History of the concept of specific, organ-forming areas
 
C. Theory of epigenesis and the germ-layer concept of development
 
D. Introduction of the words ectoderm, mesoderm, endoderm
 
E. Importance of the blastular stage in Haeckel’s theory of “The Biogenetic Law of
Embryonic Recapitulation”
 
F. Importance of the blastular stage in embryonic development
 
G. Description of the various types of chordate blastulae with an outline of their organforming areas
 
1. Protochordate blastula
 
2. Amphibian blastula
 
3. Mature blastula in birds
 
4. Primary and secondary reptilian blastulae
 
5. Formation of the late mammalian blastocyst (blastula)
 
a. Prototherian mammal, Echidna
 
b. Metatherian mammal, Didelphys
 
c. Eutherian mammals
 
6. Blastulae of teleost and elasmobranch fishes
 
7. Blastulae of gymnophionan amphibia
 
 
A. Introduction
 
In the previous chapter it was observed thatftwo main types of blastulae
are formed in the chordate group: ^
 
(1) those blastulae without accessory or trophoblast tissue, e.g., Amphioxus, frog, etc. and
 
(2) those possessing such auxiliary tissue, e.g., elasmobranch and teleost
fishes, reptiles, birds, and mammals.
 
 
1. Blastulae Without Auxiliary Tissue
 
The blastulae which do not have the auxiliary tissues are rounded affairs
composed of a layer. of blastomeres surrounding a blastocoelic cavity (figs.  140T; 143C). The layer of blastomeres forms the blastoderm. The latter
may be one cell in thickness, as in Amphioxus (fig. MOT), or several cells
in thickness, as in the frog (fig. M3C). This hollow type of blastula often is
referred to as a coeloblastula or blastosphere. However, in the gymnophionan
amphibia, the blastula departs from this vesicular condition and appears
quite solid. The latter condition may be regarded as a stereoblastula, i.e., a
solid blastula. A somewhat comparable condition is present in the bony ganoid
fishes, Amia and Lepisosteus,
 
The main characteristic of the blastula which does not possess auxiliary
tissue is that the entire blastula is composed of formative cells, i.e., all the
cells enter directly into the formation of the embryo’s body.
 
2. Blastulae with Auxiliary or Trophoblast Tissue^
 
examination of those blastulae which possess auxiliary or trophoblast
tissues shows a less simple condition than the round blastulae mentioned above.
In the first plac^fi two types of cells are present, namely, formative cells which
enter into the "exposition of the embryonic body and auxiliary cells concerned mainly with trophoblast, or nutritional, functions. In the second place,
in the blastula which possesses auxiliary tissue, the latter often develops precociously, that is, in advance of the formative cells of the blastul^ As a
result, the arrangement of the formative cells into a configuration comparable
to that of those blastulae without trophoblast cells may be much retarded in
certain instances. This condition is true particularly of the mammalian blastula
(blastocyst).
 
Generally speak ingj(^the blastulae which possess auxiliary tissue consist in
their earlier stages of a disc or a mass of formative cells at the peripheral
margins of which are attached the non-formative, auxiliary cells (fig. 159,
blastoderm-formative cells, periblast-non-formative; also figs. M5K, L; M7G,
H). The blastocoelic space lies below this disc of cells. However, in mammals
the auxiliary or nourishment-getting tissue tends to circumscribe the blastocoel,
whereas the formative cells occupy a polar area (fig. MSG, H). Blastulae,
composed of a disc-shaped mass of cells overlying a blastocoelic space, have
been described in classical terms as discoblastulae.')
 
3. Comparison of the Two Main Blastular Types
 
If we compare these two types of blastulae in terms of structure, it is evident
that a comparison is not logical unless the essential or formative cells and
their arrangement are made the sole basis for the comparison, for only the
formative cells are common to both types of blastulae. To make the foregoing
statement more obvious, let us examine the essential structure of a typical
coeloblastula, such as found in Amphioxus, as it is defined by the presentday embryologist.
 
The studies by Conklin, ’32 and ’33, demonstrated that the fertilized egg
 
 
 
342
 
 
THE CHORDATE BLASTULA AND ITS SIGNIFICANCE
 
 
of Amphioxus possesses five major, presumptive, organ-forming areas (fig.
167A). These areas ultimately give origin to the ectodermal, mesodermal,
entodermal, notochordal, and neural tissues. In the eight-cell stage of cleavage,
the cytoplasmic substances concerned with these areas are distributed in such
a way that the blastomeres have different substances and, consequently, differ
qualitatively (fig. 167B). Specifically, the entoderm forms the ventral part
of the four ventral blastomeres; the ectoderm forms the upper or dorsal portion
of the four micromeres, while the mesodermal, notochordal, and neural substances lie in an intermediate zone between these two organ-forming areas,
particularly so in the blastomeres shown at the left in figure 167B. In figure
167C and D is shown a later arrangement of the presumptive, organ-forming
areas in the middle and late stages of blastular development. These figures
represent sections of the blastulae. Consequently, the organ-forming areas are
contained within cells which occupy definite regions of the blastula. In figure
167E-G are presented lateral, vegetal pole, and dorso-posterior pole views
of the mature blastula (fig. 167D), representing the organ-forming areas as
viewed from the outside of the blastula.
 
It is evident from this study by Conklin that the organization of the fertilized
egg of Amphioxus passes gradually but directly through the cleavage stages
into the organization of the mature blastula; also, that the latter, like the egg,
is composed of five, major, presumptive, organ-forming areas. It is evident
further that one of the important tasks of cleavage and blastulation is to develop and arrange these major, organ-forming areas into a particular pattern.
(Note: Later the mesodermal area divides in two, forming a total of six, presumptive, organ-forming areas.)
 
If we analyze the arrangement of these presumptive, organ-forming areas,
we see that the mature blastula is composed of a floor or hypoblast, made
up of potential, entoderm-forming substance, and a roof of potential ectoderm
with a zone of mesoderm and chordoneural cells which lie in the area between
these two general regions. In fact, the mesodermal and chordoneural materials
form the lower margins of the roof of the mature blastula (fig. 167D). Consequently, the mature blastula of Amphioxus may be pictured as a bilaminar
affair composed essentially of a hypoblast or lower layer of presumptive
entoderm, and an upper concave roof or epiblast containing presumptive
ectoderm, neural plate, notochord, and mesodermal cells. It is to be observed
further that the blastocoel is interposed between these two layers. This is the
basic structure of a typical coeloblastula. Furthermore, this blastula is composed entirely of formative tissue made up of certain definite, potential, organforming areas which later enter into the formation of the body of the embryo;
auxiliary or non-formative tissue has no part in its composition. All coeloblastulae conform to this general structure.
 
If we pass to the blastula of the early chick embryo, a striking similarity
may be observed in reference to the presumptive, organ-forming areas (fig.
 
 
 
ORGAN-FORMING AREAS
 
 
343
 
 
173). An upper, epiblast layer is present, composed of presumptive ectodermal, neural, notochordal, and mesodermal cells, while a hypoblast layer
of entodermal potency lies below. Between these two layers the blastocoelic
space is located. However, in the chick blastoderm, in addition to the formative
cells, a peripheral area of auxiliary or trophoblast (periblast) tissue is present.
 
B. History of the Concept of Specific, Organ-forming Areas
 
The idea that the mature egg or the early developing embryo possesses
certain definite areas having different qualities, each of which contributes
to the formation of a particular organic structure or of several structures,
finds its roots in the writings of Karl Ernst von Baer, 1828-1837. Von Baer’s
comparative thinking and comprehensive insight into embryology and its processes established the foundation for many of the results and conclusions that
have been achieved in this field during the past one hundred years.
 
Some forty years later, in 1874, Wilhelm His in his book, Unsere Korperform,
definitely put forth the organ-forming concept relative to the germ layers of
the chick, stating that “the germ-disc contains the organ-germs spread out
in a flat plate,” and he called this the principle of the organ-forming germregions (Wilson, ’25, p. 1041). Ray Lankester, in 1877, advanced views
supporting an early segregation from the fertilized egg of “already formed
and individualized” substances, as did C. O. Whitman (1878) in his classical
work on the leech, Clepsine. In this work. Whitman concludes that there is
definite evidence in favor of the preformation of organ-forming stuffs within
the egg. Other workers in embryology, such as Rabl, Van Beneden, etc., began
to formulate similar views (Wilson, ’25, pp. 1041-1042).
 
The ideology embodied within the statement of Ray Lankester referred
to above was the incentive for considerable research in that branch of embryological investigation known as “cell lineage.” To quote more fully from
Lankester’s statement in this connection, p. 410:
 
Though the substance of a cell may appear homogeneous under the most powerful
microscope, excepting for the fine granular matter suspended in it, it is quite possible, indeed certain, that it may contain, already formed and individualized, various
kinds of physiological molecules. The visible process of segregation is only the
sequel of a differentiation already established, and not visible.
 
The studies on cell lineage in many invertebrate forms, such as that of
Whitman (1878) on Clepsine, of Wilson (1892) on Nereis, of Boveri (1892)
and zur Strassen (1896; fig. 163B) on Ascaris, or the work of Horstadius
(’28, ’37; fig. 163A) on the sea urchin, serve to emphasize more forcefully
the implications of this statement. In these studies the developmental prospective fates of the various early cleavage blastomeres were carefully observed
and followed.
 
Much of the earlier work on cell lineage was devoted to invertebrate forms.
One of the first students to study the matter in the phylum Chordata was
 
 
 
344
 
 
THE CHORDATE BLASTULA AND ITS SIGNIFICANCE
 
 
E. G. Conklin who published in 1905 a classical contribution to chordate
embryology relative to cell lineage in the ascidian, Styela (Cynthia) partita.
This monumental work extended the principle of organ-forming, germinal
areas to the chordate embryo. However, the significance of the latter observations, relative to the chordate phylum as a whole, was not fully appreciated
until many years later when it was brought into prominence by the German
investigator, W. Vogt (’25, ’29).
 
Vogt began a series of studies which involved the staining of different parts
of the amphibian blastula with vital dyes and published his results in 1925
and 1929. The method employed by Vogt is as follows:
 
Various parts of the late amphibian blastula are stained with such vital
dyes as Nile-blue sulfate, Bismarck brown, or neutral red (fig. 168A). These
stains color the cells but do not kill them. When a certain area of the blastula
is stained in this manner, its behavior during later stages of development can
be observed by the following procedure: After staining a particular area, the
embryo is observed at various later periods, and the history of the stained
area is noted. When the embryo reaches a condition in which body form is
fully established, it is killed, fixed in suitable fluids, embedded in paraffin,
and sectioned. Or, the embryo may be dissected after fixation in a suitable
fluid. The cellular area of the embryo containing the stain thus may be detected and correlated with its original position in the blastula (cf. fig. 168A, B).
This procedure then is repeated for other areas of the blastula (fig. 168C-E).
Vogt thus was able to mark definite areas of the late blastula, to follow their
migration during gastrulation, and observe their later contribution to the formation of the embryonic body. Definite maps of the amphibian blastula in
relation to the future history of the respective blastular areas were in this
way established (fig. 169C).
 
This method has been used by other investigators in the study of similar
phenomena in other amphibian blastulae and in the blastulae and gastrulae
of other chordate embryos. Consequently, the principle of presumptive, organforming areas of the blastula has been established for all of the major chordate
groups other than the mammals. The latter group presents special technical
difficulties. However, due to the similarity of early mammalian development
with the development of other Chordata, it is quite safe to conclude that they
also possess similar, organ-forming areas in the late blastular and early gastrular stages.
 
The major, presumptive, organ-forming areas of the late chordate blastula
are as follows (figs. 167, 169, 1-73, 174, 179, 180, 181):
 
(1) There is an ectodermal area which forms normally the epidermal
layer of the skin;
 
(2) also, there is an ectodermal region which contributes to the formation
of the neural tube and nervous system;
 
 
 
EPIGENESIS AND THE GERM-LAYER CONCEPT
 
 
345
 
 
(3) a notochordal area is present which later gives origin to the primitive
axis;
 
(4) the future mesodermal tissue is represented by two areas, one on either
side of the notochordal area. In Amphioxus, however, this mesodermal
area is present as a single area, the ventral crescent, which divides
during gastrulation into two areas;
 
(5) the entodermal area, which gives origin to the future lining tissue of
the gut, occupies a position in the blastula either at or toward the vegetative pole;
 
(6) there is a possibility that another potential area, containing germinal
plasm, may be present and integrated with the presumptive entoderm
or mesoderm. This eventually may give origin to the primitive germ
cells;
 
(7) the pre-chordal plate region is associated with the notochordal area
in all chordates in which it has been identified and lies at the caudal
margin of the latter. In gastrulation it maintains this association. The
pre-chordal plate material is an area which gives origin to some of
the head mesoderm and possibly also to a portion of the roof of the
foregut. It acts potently in the organization of the head region. Accordingly, it may be regarded as a complex of entomesodermal cells,
at least in lower vertebrates.
 
C. Theory of Epigenesis and the Germ-layer Concept of Development
 
As the three classical germ layers take their origin from the blastular state
(see Chap. 9), it is well to pause momentarily to survey briefly the germ-layer
concept.
 
That the embryonic body is derived from definite tissue layers is an old
concept in embryology. Casper Friedrich Wolff (1733-94) recognized that
the early embryonic condition of the chick blastoderm possessed certain layers
of tissue. This fact was set forth in his Theoria Generationis, published in
1759, and in De forniatione intestinorum praecipue, published in 1769, devoted to the description of the intestinal tract and other parts of the chick
embryo. In these works Wolff presented the thesis that embryonic development of both plants and animals occurred by “a host of minute and always
visible elements that assimilated food, grew and multiplied, and thus gradually
in associated masses” produced the various structures which eventually become recognizable as “the heart, blood vessels, limbs, alimentary canal, kidneys, etc.” (The foregoing quotations are from Wheeler, 1898.) These statements contain the essence of Wolff’s theory of epigenesis. That is, that development is not a process of unfolding and growth in size of preformed structures;
rather, it is an indirect one, in which certain elements increase in number and
gradually become molded into the form of layers which later give rise to the
organ structures of the organism.
 
 
 
346
 
 
THE CHORDATE BLASTULA AND ITS SIGNIFICANCE
 
 
Two Other men contributed much to the layer theory of development,
namely, Heinrich Christian Pander (1794-1865) and Karl Ernst von Baer
(1792-1876). In 1817, Pander described the trilaminar or triploblast condition of the chick blastoderm, and von Baer, in his first volume (1828) and
second volume (1837) on comparative embryology of animals, delineated
four body layers. The four layers of von Baer’s scheme are derived from
Pander’s three layers by dividing the middle layer into two separate layers
of tissue. Von Baer is often referred to as the founder of comparative embryology for various reasons, one of which was that he recognized that the
layer concept described by Pander held true for many types of embryos,
vertebrate and invertebrate. The layer concept of development thus became
an accepted embryological principle.
 
While Pander and von Baer, especially the latter, formulated the germlayer concept as a structural fact for vertebrate embryology, to Kowalewski
(1846-1901) probably belongs the credit for setting forth the idea, in his
paper devoted to the early development of Amphioxus (1867), that a primary,
single-layered condition changes gradually into a double-layered condition.
The concept of a single-layered condition transforming into a double-layered
condition by an invaginative procedure soon became regarded as a fundamental embryological sequence of development.
 
Gradually a series of developmental steps eventually became crystallized
from the fact and speculation present during the latter half of the nineteenth
century as follows:
 
( 1 ) The blastula, typically a single-layered, hollow structure, becomes converted into
 
(2) the two-layered gastrula by a process of invagination of one wall or
delamination of cells from one wall of the blastula; then,
 
(3) by an outpouching of a part of the inner layer of the gastrula, or by
an ingression of cells from this layer, or from the outside ectoderm, a
third layer of cells, the mesoderm, comes to lie between the entoderm
and ectoderm; and finally,
 
(4) the inner layer of mesoderm eventually develops into a two-layered
structure with a coelomic cavity between the layers.
 
This developmental progression became accepted as the basic procedure
in the development of most Metazoa.
 
The original concept of the germ layers maintained that the layers were
specific. That is, entodermal tissue came only from entoderm, ectodermal
tissue from ectoderm, etc. However, experimental work on the early embryo
in which cells are transplanted from one potential layer to another has overthrown this concept ( Oppenheimer, ’40). The work on cell lineage and the
demonstration of the early presence of the presumptive, organ-forming areas
 
 
 
BIOGENETIC LAW OF EMBRYONIC RECAPITULATION
 
 
347
 
 
also have done much to overthrow the concept concerning the rigid specificity
of the three primary germ layers of entoderm, mesoderm, and ectoderm.
 
D. Introduction of the Words Ectoderm^ Mesoderm^ Endoderm
 
Various students of the Coelenterata, such as Huxley (1849), Haeckel
(1866) and Kleinenberg (1872), early recognized that the coelenterate body
was constructed of two layers, an outer and an inner layer. Soon the terms
ectoderm (outside skin) and endoderm (inside skin) were applied to the outer
and inner layers or membranes of the coelenterate body, and the word
mesoderm (middle skin) was used to refer to the middle layer which appeared in those embryos having three body layers. The more dynamic
embryological words epiblast, mesoblast, and hypoblast (entoblast) soon
came to be used in England by Balfour, Lankester, and others for the words
ectoderm, mesoderm, and endoderm, respectively. The word entoderm is used
in this text in preference to endoderm.
 
E. Importance of the Blastular Stage in Haeckel’s Theory of ^^The
Biogenetic Law of Embryonic Recapitulation”
 
In 1859, Charles Darwin (1809-82) published his work On the Origin
of Species by Means of Natural Selection, This theory set the scientific world
aflame with discussions for or against it.
 
In 1872 and 1874, E. Haeckel (1834-1919), an enthusiast of Darwin’s
evolutionary concept, associated the findings of Kowalewski regarding the
early, two-layered condition of invertebrate and vertebrate embryos together
with the adult, two-layered structure of the Coelenterata and published the
blastaea-gastraea theory and biogenetic principle of recapitulation. In these
publications he applied the term gastrula to the two-layered condition of the
embryo which Kowalewski has described as the next developmental step succeeding the blastula and put forward the idea that the gastrula was an embryonic form common to all metazoan animals.
 
In his reasoning (1874, translation, ’10, Chap. 8, Vol. I), Haeckel applied
the word blastaea to a “long-extinct common stem form of substantially the
same structure as the blastula.” This form, he concluded, resembled the
“permanent blastospheres” of primitive multicellular animals, such as the
colonial Protozoa. The body of the blastaea was a “simple hollow ball, filled
with fluid or structureless jelly with a wall composed of a single stratum of
homogeneous ciliated cells.”
 
The next phylogenetic stage, according to Haeckel, was the gastraea, a
permanent, free-swimming form which resembled the embryonic, two-layered,
gastrular stage described by Kowalewski. This was the simple stock form for
all of the Metazoa above the Protozoa and other Protista. Moreover, he
postulated that the gastrula represented an embryonic recapitulation of the
adult stage of the gastraea or the progenitor of all Metazoa.
 
 
 
348
 
 
THE CHORDATE BLASTULA AND ITS SIGNIFICANCE
 
 
The assumed importance of the blastula and gastrula thus became the
foundation for Haeckel’s biogenetic principle of recapitulation. Starting with
the postulation that the hypothetical blastaea and gastraea represented the
adult phylogenetic stages comparable to the embryonic blastula and gastrula,
respectively, Haeckel proceeded, step by step, to compress into the embryological stages of all higher forms the adult stages of the lower forms through
which the higher forms supposedly passed in reaching their present state
through evolutionary change. The two-chambered condition of the developing mammalian heart thus became a representation of the two-chambered,
adult heart of the fish, while the three-chambered condition recapitulated the
adult amphibian heart, etc. Again, the visceral arches of the embryonic pharyngeal regions of the mammal represented the gill-slit condition of the fish.
Ontogeny thus recapitulates phytogeny y and phytogeny of a higher species is
the result of the modification of the adult stages of lower species in the phylogenetic scale. The various steps in the embryological development of any
particular species, according to this reasoning, were caused by the evolutionary
history of the species; the conditions present in the adult stage of an earlier
phylogenetic ancestor became at once the cause for its existence in the embryological development of all higher forms. Embryology in this way became
chained to a repetition of phylogenetic links!
 
Many have been the supporters of the biogenetic law, and for a long time
it was one of the most popular theories of biology. A surprising supporter of
the recapitulation doctrine was Thomas Henry Huxley (1825-95). To quote
from Oppenheimer (’40): “One wonders how the promulgator of such a
distorted doctrine of cause and effect could have been championed by the
same Huxley who wrote: Tact I know and Law I know; but what is this
Necessity save an empty Shadow of my own mind’s throwing?’.”
 
The Haeckelian dogma that ontogeny recapitulates phytogeny fell into error
because it was formulated upon three false premises due to the fragmentary
knowledge of the period. These premises were:
 
( 1 ) That in evolution or phytogeny, recently acquired, hereditary characters were added to the hereditary characters already present in the
species;
 
(2) that the hereditary traits revealed themselves during embryonic development in the same sequence in which they were acquired in phytogeny;
and
 
(3) that Darwin’s concept of heredity, namely, pangenesis, essentially was
correct.
 
The theory of pangenesis assumed that the germ cells with their hereditary
factors were produced by the parental body or soma and that the contained
hereditary factors within the germ cells were produced by gemmules which
 
 
 
BIOGENETIC LAW OF EMBRYONIC RECAPITULATION
 
 
349
 
 
migrated from the various soma cells into the germ cells. This theory further
postulated the inheritance of acquired characters.
 
If these three assumptions are granted, then it is easy to understand Haeckel’s
contention that embryological development consists in the repetition of previous stages in phylogeny. For example, if we assume that the blastaea changed
into the gastraea by the addition of the features pertaining to the primitive
gut with its enteric lining, then the gastraea possessed the hereditary factors
of the blastaea plus the new enteric factors. These enteric features could
easily be added to the deric (outer-skin) factors of the blastaea, according
to Darwin’s theory of pangenesis. Furthermore, according to assumption (2)
above, in the embryonic development of the gastraea, the hereditary factors
of the blastaea would reveal themselves during development first and would
produce the blastaea form, to be followed by the appearance of the specific
enteric features of the gastraea. And so it proceeded in the phylogeny and
embryology of later forms. In this way the preceding stage in phylogeny became at once the cause of its appearance in the development of the next
phylogenetic stage.
 
•These assumptions, relative to heredity and its mechanism of transference,
were shown to be untenable by the birth of the Nageli-Roux-Weismann concept of the germ plasm (see Chaps. 3 and 5) and by the rebirth or rediscovery
of Mendelism during the latter part of the nineteenth century. Studies in embryology since the days of Weismann have demonstrated in many animal
species the essential correctness of Weismann’s assumption that the germ
plasm produces the soma during development, as well as the future germ
plasm, and thus have overthrown the pangenesis theory of Darwin. The assiduous study of Mendelian principles during the first twenty-five years of the
twentieth century have demonstrated that a fixed relation does not exist between the original character and the appearance of a new character as implied
in the Haeckelian law (Morgan, ’34, p. 148). Furthermore, that “in many
cases, perhaps in most, a new end character simply replaces the original one.
The embryo does not pass through the last stage of the original character
and then develop the new one — although this may happen at times — but the
new character takes the place of the original one” (Morgan, ’34, p. 148).
 
How then does one explain the resemblances of structure to be found
among the embryos at various stages of development in a large group of
animals such as the Chordata? Let us endeavor to seek an explanation.
 
In development, nature always proceeds from the general to the specific,
both in embryological development and in the development of phylogeny or
a variety of forms. The hereditary factors which determine these generalized
states or structural conditions apparently are retained, and specialized factors come into play after the generalized pattern is established. Generalized
or basic conditions, therefore, appear before the specialized ones. An example
of this generalized type of development is shown in the formation of the
 
 
 
350
 
 
THE CHORDATE BLASTULA AND ITS SIGNIFICANCE
 
 
blastula in chordate animals. Although many different specific types and shapes
of blastulae are present in the group as a whole, all of them can be resolved
into two basic groups. These groups, as mentioned in the beginning of this
chapter, are:
 
( 1 ) blastulae without auxiliary, nutritive tissue and
 
(2) blastulae with auxiliary tissue.
 
Moreover, if the auxiliary tissue of those blastulae which possess this tissue
is not considered, all mature chordate blastulae can be reduced to a fundamental condition which contains two basic layers, namely, hypoblast and epiblast layers. The epiblast possesses presumptive epidermal, neural, notochordal,
and mesodermal, organ-forming areas, while the hypoblast cells form the
presumptive entodermal area. The shapes and sizes of these blastulae will,
of course, vary greatly. Moreover, the hypoblast cells may be present in
various positions, such as a mass of cells at the caudal end of a disc-shaped
epiblast (teleost and elasmobranch fishes), an enlarged, thickened area or
pole of a hollow sphere (many Amphibia) y a single, relatively thin layer of
cells, forming part of the wall of a hollow sphere (Arnphioxus), a rounded,
disc-shaped mass of cells overlain by the thin, cup-shaped epiblast (Clavelina),
a thickened mass attached to the underside of the caudal end of the disc-shaped
epiblast (chick; certain reptiles), a thin layer of cells situated below the epiblast
layer (mammals), or a solid mass of cells, lying below a covering of epiblast
cells (gymnophionan Amphibia). Although many different morphological
shapes are to be found in the blastulae of the chordate group, the essential,
presumptive, organ-forming areas always are present, and all are organized
around the presumptive notochordal area.
 
But the question arises: Why is a generalized blastular pattern developed
instead of a series of separate, distinct patterns? For instance, why should the
notochordal area appear to occupy the center of the presumptive, organforming areas of all the chordate blastulae when this area persists as a prominent morphological entity only in the adult condition of lower chordates?
The answer appears to be this: The notochordal area at this particular stage
of development is not alone a morphological area, but it is also a physiological
instrument, an instrument which plays a part in a method or procedure of
development. The point of importance, therefore, in the late blastular stage
of development is not that the notochordal area is going to contribute to the
skeletal axis in the adult of the shark, but rather that it forms an integral part
of the biolgical mechanism which organizes the chordate embryo during the
period immediately following the blastular stage. Thus, if the notochordal
material can play an important role in the organization of the embryo and
in the induction of the neural tube in the fish or in the frog, it also can fulfill
a similar function in the developing chick or human embryo. Whatever it does
later in development depends upon the requirements of the species. To use
 
 
 
IMPORTANCE OF THE BLASTULAR STAGE
 
 
351
 
 
a naive analogy, nature does not build ten tracks to send ten trains with different destinies out of a station when she can use one track for all for at least
part of the way. So it is in development. A simple tubular heart appears in
all vertebrate embryos, followed by a simple, two-chambered* condition, not
because the two-chambered heart represents the recapitulated, two-chambered,
fish heart but rather because it, like the notochord, is a stage in a dynamic
developmental procedure of heart development in all vertebrates. As far as
the fish is concerned, when the common, two-chambered, rudimentary stage
of the heart is reached, nature shunts it off on a special track which develops
this simple, two-chambered condition into the highly muscular and efficient
two-chambered, adult heart adapted to the fish level of existence in its watery
environment. The three-chambered,* amphibian heart follows a similar pattern,
and it specializes at the three-chambered level because it fits into the amphibian
way of life. So it is with the embryonic pharyngeal area with its visceral and
aortal arches which resemble one another throughout the vertebrate group
during early embryonic development. The elaboration of a common, pharyngeal area with striking resemblances throughout the vertebrate group can
be explained more easily and rationally on the assumption that it represents
a common, physiologically important step in a developmental procedure.
 
This general view suggests the conclusion that ontogeny tends to use common developmental methods wherever and whenever these methods can be
utilized in the development of a large group of animals. Development or
ontogeny, therefore, recapitulates phylogenetic procedures and not adult morphological stages. One explanation for this conservation of effort may be
that, physiologically speaking, the number of essential methods, whereby a
specific end may be produced, probably is limited. Another explanation suggests that an efficient method never is discarded.
 
F. Importance of the Blastular Stage in Embryonic Development
 
Superficially in many forms, chordate and non-chordate, the blastula is a
hollow, rounded structure containing the blastocoelic space within. It is tempting to visualize this form as the basic, essential form of the blastula. However, the so-called blastular stage in reality presents many forms throughout
the animal kingdom, some solid, some round and hollow, and others in the
form of a flattened disc or even an elongated band. Regardless of their shape,
all blastulae have this in common/ they represent an association of presumptive organ-forming areas, areas which later move to new positions in
the forming body, increase in cellular mass, and eventually become molded
into definite structures. One of the main purposes of blastulation, therefore,
may be stated as the elaboration (or establishment) of the major, presumptive
organ-forming areas of the particular species and their arrangement in a
particular pattern which permits their ready manipulation during the next
 
* Exclusive of the sinus venosus.
 
 
 
352
 
 
THE CHORDATE BLASTULA AND ITS SIGNIFICANCE
 
 
step of development or gastrulation. jThc particular shape of the blastula has
its importance. However, this importance does not lie in the supposition that
it conforms to a primitive spherical type but rather that the various, presumptive, organ-forming areas are so arranged and so poised that the cell
movements so necessary to the next phase of development or gastrulation
may be properly executed for the particular species. In most species, the
formation of a blastocoelic space also is a necessary function of blastulation.
In some species, however, this space actually is not formed until the next
stage of development or gastrulation is in progress.
 
In summary, therefore, it may be stated that the importance of the blastula
does not reside in the supposed fact that it is a one-layered structure or
blastoderm having a particular shape. Rather, its importance emerges from
the fact that the blastoderm has certain, well-defined areas segregated within
it — areas which will give origin to future organ structures. Moreover, these
areas foreshadow the future germ layers of the body. In diploblastic Metazoa,
two germ layers are foreshadowed, while in triploblastic forms, three germ
layers are outlined. As far as the Chordata are concerned, the hypoblast is
the forerunner of the entoderm or the internal germ layer; whereas the
epiblast is composed potentially of two germ layers, namely, the epidermal,
neural plate areas which form the ectodermal layer and the chordamesodermal
or marginal zone cells which give origin to the middle germ layer.
 
In the following pages, the chordate blastula is described as a two-layered
structure composed of various, potential, organ-forming areas. This twolayered configuration, composed of a lower hypoblast and an upper epiblast,
is used to describe the chordate blastula for the dual purpose of comparison
and analysis of the essential structure of the various blastulae. The bilaminar
picture, it is believed, will enable the student to understand better the changes
which the embryo experiences during the gastrulative period.
 
G. Description of the Various Types of Chordate Blastulae with an
Outline of Their Organ-forming Areas
 
1. Protochordate Blastula
 
The following description pertains particularly to Amphioxus. With slight
modification it may be applied to other protochordates, such as Clavelina,
Ascidiella, Styela, etc.
 
As noted in the introduction to this chapter, the potential entodermal cells
of Amphioxus lie at the vegetal pole and form most of the floor or hypoblast
of the blastula (fig. 167D). The upper or animal pole cells form a roof of
presumptive epidermal, notochordal, mesodermal, and neural cells arched
above and around the entoderm. The latter complex of organ-forming cells
forms the epiblast. The blastocoelic cavity is large and insinuated between
the hypoblast and epiblast. The presumptive notochordal and mesodermal
 
 
 
 
Fig. 167. Presumptive organ-forming areas in the uncleaved egg and during cleavage and blastulation in Amphioxus. (Original diagram based upon data obtained from
Conklin, ’32, ’33.) (A) Uncleaved egg. (B) Eight-cell stage. (C) Early blastula in
 
section. (D) Late blastula in section. (E) Late blastula, external view from side.
(F) Late blastula, external, vegetal pole view. (G) Late blastula, external, dorsoposterior view. The localization of cytoplasmic materials in Styela partita is similar to
that of Amphioxus. Observe that the pointed end of the arrow defines the future cephalic
end of the embryo. The position of the polar body denotes the antero-ventral area, while
the position of the notochordal and neural plate material represents the antero-dorsal
region. The “tail end’’ of the arrow is the postero-ventral area of the embryo.
 
 
353
 
 
 
354 THE CHORDATE BLASTULA AND ITS SIGNIFICANCE
 
areas lie at the margins of the entodermal layer and surround it. As such,
some of the cells of these two, organ-forming areas may form part of the
floor of the bias tula. The presumptive, notochordal and neural plate cells lie
at the future dorsal lip of the blastopore and form the dorsal crescent, while
the mesodermal area occupies the ventral-lip region as the ventral crescent
(fig. 167F). In Amphioxus, the mature blastula is pear shaped, with the body
 
 
 
Fig. 168. Ultimate destiny within the developing body of presumptive organ-forming
areas of the late amphibian blastula, stained by means of vital dyes. (After Pastecls: J.
Exper. Zool., 89.) (A) Area of blastula, stained. (B) Destiny of cellular area, stained
 
in (A). (D, E) Ultimate destiny shown by broken lines of cellular areas, stained in
 
late blastula shown in (C). (E) Anterior trunk segment. (D) Posterior trunk segment.
 
 
 
 
 
Fig. 169. Presumptive organ-forming areas in the amphibian late blastula and beginning gastrula. (A, B) General epiblast and hypoblast areas of the early and late
blastular conditions, respectively. The hypoblast is composed mainly of entodermal or
gut-lining structures, whereas the epiblast is a composite of ectodermal (i.e., epidermal
and neural), mesodermal, and notochordal presumptive areas. Observe that the epiblast
gradually grows downward over the hypoblast as the late blastula is formed. (C) Beginning gastrula of the urodele, Triton. (Presumptive areas shown according to Vogt,
’29.) (D) Same as above, from vegetative pole. (Slightly modified from Vogt, ’29.)
 
(E) Lateral view of beginning gastrula of anuran amphibia. (F) Dorsal view of the
same. (E, F derived from description by Vogt, ’29, relative to Rami fusca and Bombinator; also Pasteels: J. Exper. Zool., 89, relative to Discogkmus.) Observe that an
antero-posterior progression of somites is indicated in C and D.
 
 
355
 
 
 
356
 
 
THE CHORDATE BLASTULA AND ITS SIGNIFICANCE
 
 
of the mesodermal crescent comprising much of the neck portion of the “pear”
(fig. 167E).
 
The blastula of Amphioxus thus may be regarded essentially as a bilaminar
structure (i.e., two-layered structure) in which the hypoblast forms the lower
layer while the epiblast forms the upper composite layer.
 
.^ 1 , Amphibian Blastula
 
In the amphibian type of blastula, a spherical condition exists similar to
that in Amphioxus (fig. 169). The future entoderm is located at the vegetative
(vegetal) pole, smaller in amount in the frog, Rana pipiens, and larger in
such forms as Necturus maculosus (fig. 169 A, B). The presumptive notochordal material occupies an area just anterior to and above the future dorsal
lip of the blastopore. The dorsal lip of the gastrula, when it develops, arises
within the entodermal area (fig. 169C-F). Extending laterally on either side
of the presumptive notochordal region is an area of presumptive mesoderm
(fig. 169C-F). Each of these two mesodermal areas tapers to a smaller dimension as it extends outward from the notochordal region. The presumptive
notochordal and mesodermal areas thus form a composite area or circular
marginal zone which surrounds the upper rim of the entodermal material.
 
Above the chordamesodermal zone are two areas. The presumptive neural
area is a crescent-hke region lying above or anterior to the presumptive
notochord-mesoderm complex. Anterior to the neural crescent and occupying
the remainder of the blastular surface, is the presumptive epidermal crescent
(fig. 169C-F).
 
In the various kinds of blastulae of this group, the yolk-laden, vegetal pole
cells actually form a mass which projects upward into the blastocoelic space
(fig. 169A, B). The irregularly rounded, presumptive entodermal, organforming area, therefore, is encapsulated partially by the other potential germinal
areas, particularly by the chordamesodermal zone (fig. 169B). In a sense,
this is true also of the protochordate group (fig. 167D).
 
The amphibian type of blastula includes those of the petromyzontoid
Cyclostomes, the ganoid fishes with the exception of bony ganoids, the dipnoan
fishes, and the Amphibia with the exception of the Gymnophiona, where a
kind of solid blastula is present.
 
It is to be observed that the amphibian and protochordate blastulae differ
in several details. In the first place, there is a greater quantity of yolk material
in the blastula of the Amphibia; hence the presumptive entodermal area or
hypoblast projects considerably into and encroaches upon the blastocoel.
Also, in Amphioxus, the presumptive notochordal area forms a distinct dorsal
crescent apart from the presumptive mesodermal or ventral crescent (fig.
167F), whereas, in the Amphibia, the notochordal material is sandwiched
in between the two wings of mesoderm, so that these two areas form one
composite marginal zone crescent (fig. 169D, E).
 
 
 
TYPES OF CHORDATE BLASTULAE
 
 
357
 
 
As in Amphioxus, the amphibian blastula may be resolved into a twolayered structure composed of a presumptive entodermal or hypoblast layer
and an upper, epiblast layer of presumptive epidermal, notochordal, mesodermal, and neural tissues. Each of these layers, unlike that of Amphioxus,
is several cells in thickness.
 
^3. Mature Blastula in Birds
 
Development of the hen’s egg proceeds rapidly in the oviduct (fig. 157B-G),
and at the time that the egg is laid, the blastodisc (blastula) presents the
following cellular conditions:
 
( 1 ) a central, cellular blastoderm above the primary blastocoel and
 
(2) a more peripheral portion, associated with the yolk material forming
the germ-wall tissue (fig. 156G).
 
The central blastoderm is free from the yolk substance and is known as
the area pellucida, whereas the germ-wall area with its adhering yolk material
forms the area opaca (fig. 170). Around its peripheral margin the area
pellucida is somewhat thicker, particularly so in that region which will form
the posterior end of the future embryo. In the latter area, the pellucid margin
may consist of a layer of three or even four cells in thickness (fig. 172A).
This thickened posterior portion of the early pellucid area forms the embryonic
shield (fig. 170). Anterior to the embryonic shield, the pellucid area is one
or two cells in thickness (figs. 171A; 172B).
 
Eventually the pellucid area becomes converted into a two-layered structure
with an upper or overlying layer, the primitive ectoderm or epiblast and a
lower underlying sheet of cells, the primitive entoderm or hypoblast (figs.
171 A; 172A). The space between these two layers forms the true or secondary
blastocoel. The cavity below the hypoblast is the primitive archenteric space.
At the caudal and lateral edges of the pellucid area, cells from the inner zone
of the germ wall appear to contribute to both hypoblast and epiblast.
 
The two-layered condition of the avian blastula shown in figure 171 A may
be regarded as a secondary or late blastula. At about the time that the secondary blastula is formed (or almost completely formed), the hen’s egg is
laid, and further development depends upon proper incubational conditions
outside the body of the hen. Shortly after the latter incubation period is
initiated, the primitive streak begins to make its appearance in the midcaudal
region of the blastoderm, as described in Chapter 9.
 
Much controversy has prevailed concerning the method of formation of
the entoderm and the two-layered condition in the avian blastoderm. Greatest
attention has been given to the origin of the entoderm in the eggs of the
pigeon, hen, and duck. The second layer is formed in the pigeon’s egg as it
passes down the oviduct, in the hen’s egg at about the time of laying, and
in the duck’s egg during the first hours of the external incubation period. The
 
 
 
 
Fig. 171. Origin of the hypoblast (entoderm) in the avian blastoderm. (A) Median,
antero-posterior section of chick blastoderm. Entoderm arises by delamination from
upper or epiblast layer; possibly also by cells that grow anteriad from thickened posterior
area. (Based upon data supplied by Peter, ’34, ’38, and Jacobson, ’38.) (B-D) For
mation of the hypoblast (entoderm) from epiblast by a process of delamination in the
duck embryo. (Based upon data supplied by Pasteels, ’45.)
 
 
unincubated chick blastoderm is about 3 mm. in diameter, that of the duck,
about 2 to 3 mm.
 
The most recent observations, relative to the formation of the second or
hypoblast layer, have been made upon the duck’s egg (Pasteels, ’45). In this
egg, Pasteels found that, at about nine hours after incubation is initiated, a
two-layered condition is definitely formed and that “the primary entoblast of
the duck is the result of a progressive delamination of the segmenting blastodisc
 
 
TYPES OF CHORDATE BLASTULAE
 
 
359
 
 
separating the superficial cells from the deeper ones” (fig. 171B-D). He
further suggests that “the bilaminar embryo of birds is to be homologized
with the blastula of the Amphibia, the cleft separating the two layers being
equivalent to the blastocoele” (p. 13). The formation of the hypoblast (primary entoderm ) by a process of delamination from the upper layer or epiblast
agrees with the observations by Peter (’38) on the developing chick and pigeon
blastoderm (fig. 172) and of Spratt (’46) on the chick. It also agrees with
some of the oldest observations, concerning the matter of entoderm formation,
going back to Ollacher in 1869, Kionka, 1894, and Assheton, 1896. Others,
such as Duval (1884, 1888) in the chick, and Patterson (’09) in the pigeon,
have ascribed the formation of the primary entoderm to a process of invagination and involution at the caudal margin of the blastoderm, while Jacobson
(’38) came to the conclusion that the entoderm of the pellucid area arose in
chick and sparrow embryos through a process of outgrowth of cells from the
primitive plate and from an archenteric canal produced by an inward bending of the epiblast and primitive plate tissue. The latter author believed that
the entoderm of the area opaca arose by delamination.
 
The hypoblast of the chick gives origin to most of the tissue which lines
the future gut, and, therefore, may be regarded as the potential entodermal
area. As in the amphibia and Amphioxus, the epiblast is composed of several, presumptive organ-forming areas (fig. 173A). (See Pasteels, ’36c;
Spratt, ’42, ’46.) At the caudal part of the epiblast is an extensive region
of presumptive mesoderm bisected by the midplane of the future embryonic
axis. Just anterior to this region and in the midplane is the relatively small,
presumptive notochordal area. Between the latter and the mesodermal area
is located the presumptive prechordal plate of mesodermal cells. Immediately in front of the notochordal region lies the presumptive neural area in
the form of a crescent with its crescentic arms extending in a lateral direc
 
 
Fig. 172. Delamination of hypoblast (entoderm) cells from upper or epiblast layer
in the chick blastoderm. (A) Posterior end of blastoderm (cf. fig. 17 lA). (B) Anterior
end of blastoderm.
 
 
 
 
360
 
 
THE CHORDATE BLASTULA AND ITS SIGNIFICANCE
 
 
 
Fig. 173. Presumptive organ-forming areas in the chick blastoderm. (A) Slightly
modified from Spratt, ’46. (B) Schematic section of early chick blastoderm passing
 
through antero-posterior median axis.
 
tion from the midline of the future embryonic axis. Anterior to the neural
crescent is the presumptive epidermal crescent. Within the area opaca is
found potential blood-vessel and blood-cell-forming tissue, as well as the
extensive extra-embryonic-tissue materials.
 
The above description of the presumptive organ-forming areas pertains to
the avian blastula just previous to the inward migrations of the notochordal,
pre-chordal plate, and mesodermal areas; that is, just previous to the appearance of the primitive streak and the gastrulative process.
 
4. Primary and Secondary Reptilian Blastulae
 
The primary blastula of turtle, snake, and lizard embryos is akin in essential features to that of birds. It consists of a central blastoderm or area
pellucida, overlying a primary blastocoelic cavity, and a more distally situated opaque blastoderm, together with an indefinite periblast syncytium. A
localized region of the central blastoderm, situated along the midline of the
future embryonic axis and eccentrically placed toward the caudal end, is
known as the embryonic shield.
 
A specialized, posterior portion of the embryonic shield, in which the upper
layer (epiblast) is not separated from the underlying cells (hypoblast), is
known as the primitive plate (fig. 174A-D). (Consult also Will, 1892, for
 
 
 
 
Fig. 174. Formation of hypoblast (entoderm) layer in certain reptiles; major presumptive organ-forming areas of reptilian blastoderm. (A) Section through blastoderm
of the turtle, Clemmys leprosa. This section passes through the primitive plate in the
region where the entoderm cells are rapidly budded off (invaginated?) from the surface
layer. It presumably passes through (E) in the area marked entoblast. It is difficult to
determine whether the entoderm cells are actually invaginated, according to the view of
Pasteels, or whether this area represents a region where cells are delaminated or budded
off in a rapid fashion from the overlying cells. (B) Similar to (A), diagrammatized
to show hypoblast cells in black. (C) Section through early blastoderm of the gecko,
Platydactylus. Epiblast cells are shown above, primitive entoderm cells below. (D) A
later stage showing primitive plate area with the appearance of a delamination or proliferation of entoderm (hypoblast) cells from the upper layer of cells. (E) Presumptive,
organ-forming areas of the turtle, Clemmys leprosa, before gastrulation. (F) Presumptive, organ-forming areas of the epiblast of turtle and other reptiles if the hypoblast is
budded off or separated from the underside of the epiblast without invagination. It is to
be observed that B and D represent modifications by the author.
 
 
361
 
 
 
362
 
 
THE CHORDATE BLASTULA AND ITS SIGNIFICANCE
 
 
accurate diagrams of the reptilian blastoderm.) Surrounding the primitive
plate, the central blastoderm is thinner and is but one (occasionally two cells)
cell in thickness (see margins of figs. 174A, C). As development proceeds,
a layer of cells appears to be delaminated or proliferated off from the undersurface of the primitive plate area (fig. 174C, D). This delamination gives
origin to a second layer of cells, the entoderm or hypoblast (Peter, ’34).
Some of these entodermal cells may arise by delamination from more peripheral areas of the central blastoderm outside the primitive plate area. In
the case of the turtle, Clemmys leprosa, Pasteels (’37a) believes that there
is an actual invagination of entodermal cells (fig. 174A-B). More study is
needed to substantiate this view.
 
Eventually, therefore, a secondary blastula arises which is composed of a
floor of entodermal cells, the hypoblast, closely associated with the yolk, and
an overlying layer or epiblast. The epiblast layer is formed of presumptive
epidermal, mesodermal, neural, and notochordal, organ-forming areas. The
essential arrangement of the presumptive organ-forming areas in the reptiles
is very similar to that described for the secondary avian blastula. The space
between the epiblast and hypoblast layers is the secondary blastocoelic space.
 
 
 
Fig. 175. Early blastoderms of the prototherian mammal, Echidna. (A) Early blastoderm showing central mass of cells: with peripherally placed vitellocytes. (B) Later
blastoderm. Central cells are expanding and the blastoderm is thinning out. Smaller
cells (in black) are migrating into surface layer. Vitellocytes have fused to form a
peripheral syncytial tissue. (C) Later blastoderm composed of a single layer of cells
of two kinds. The smaller cells in black represent potential entoderm cells. (D) Increase
of hypoblast cells and their migration into the archenteric space below to form a second
or hypoblast layer.
 
 
 
TYPES OF CHORDATE BLASTULAE
 
 
363
 
 
 
Fig. 176. Early development of blastoderm of the opossum. (Modified from Hartman,
*16.) (A) Blastocyst wall composed of one layer of cells from which entoderm cells
 
are migrating inward, (B-D) Later development of the formative portion of the blastoderm. Two layers of cells are present in the formative area, viz., an upper epiblast layer
and a lower hypoblast. Trophoblast cells are shown at the margins of the epiblast and
hypoblast layers.
 
Both hypoblast and epiblast are connected peripherally with the periblast
tissue.
 
5. Formation of the Late Mammalian Blastocyst (Blastula)
a. Prototherian Mammal, Echidna
 
In Echidna, according to Flynn and Hill (’39, ’42), a blastoderm somewhat comparable to that of reptiles and birds is produced. An early primary
blastular condition is first established, consisting of a mass of central cells
with specialized vitellocytes at its margin (fig. 175A). A little later, an extension of this blastoderm occurs, and a definite primary blastocoelic space
is formed below the blastoderm (fig. 175B). During this transformation,
small, deeper lying cells (shown in black, fig. 175B) move up to the surface
and become associated with the thinning blastoderm which essentially becomes
a single layer of cells (fig. 175C). The marginal vitellocytes in the meantime
fuse to form a germ-wall syncytium. This state of development may be regarded as the fully developed primary blastula. A little later, this primary
condition becomes converted into a two-layered, secondary blastula, as shown
in figure 175D by the secondary multiplication and migration inward of the
small cells to form a lower layer or hypoblast. The latter process may be
 
 
 
364
 
 
THE CHORDATE BLASTULA AND ITS SIGNIFICANCE
 
 
regarded as a kind of polyinvagination. In this manner the secondary blastula
is formed. It is composed of two layers of cells, the epiblast above and the
hypoblast below with the secondary blastocoelic space insinuated between
these two layers.
 
 
b. Metatherian Mammal, Didelphys
 
The opossum, Didelphys virginiana, possesses a hollow blastocyst akin to
the eutherian variety. (See Hartman, T6, T9; McCrady, ’38.) As observed
in the previous chapter, it is produced by a peculiar method. The early blastomeres do not adhere together to form a typical morula as in most other
forms; rather, they move outward and adhere to the zona pellucida and come
to line the inner aspect of this membrane. As cleavage continues, they eventually form a primary blastula with an enlarged blastocoel.
 
Following this primary phase of development, one pole of the blastocyst
begins to show increased mitotic activity, and this polar area gradually thickens
(fig. 176A). At this time certain cells detach themselves from the thickened
polar area of the blastocyst and move inward into the blastocoel (fig. 176A, B) .
 
 
 
 
Fig. 177. Schematic drawings of early pig development. (A) Early developing blastocyst. (B) Later blastocyst, showing two kinds of cells in the inner cell mass. (C)
Later blastocyst, showing disappearance of trophoblast cells overlying the inner cell mass.
(D) Later blastocyst. Two layers of formative cells are present as indicated with trophoblast tissue attached at the margins.
 
 
 
TYPES OF CHORDATE BLASTULAE
 
 
365
 
 
 
Fig. 178. Schematic drawings of the developing blastocyst of the monkey. (After
Hcuser and Streeter: Carnegie Inst,, Washington, Publ. 538. Contrib. to Embryol. No.
181.) (A, B) Early blastocysts showing formative and non-formative cells in the inner
 
cell mass. (C-E) Later arrangement of the formative cells into an upper epiblast and
lower hypoblast layer.
 
These cells form the mother entoderm cells, and by mitotic activity they give
origin to an entodermal layer which adheres to the underside of the thickened
polar area (fig. 176B, C). The polar area then thins out to form the expansive
condition shown in figure 176D. A bilaminar, disc-shaped area thus is formed
in this immediate region of the blastocyst, and it represents the area occupied
 
 
 
366
 
 
THE CHORDATE BLASTULA AND ITS SIGNIFICANCE
 
 
by the formative cells of the blastula. The edge of this disc of formative
cells is attached to the trophoblast or auxiliary cells (fig. 176D). Only the
formative cells give origin to the future embryonic body.
 
c. Eutherian Mammals
 
The eutherian mammals as a whole present a slightly different picture of
blastocyst development from that described above for marsupial species. These
differences may be outlined as follows:
 
( 1 ) During the earliest phases of blastocyst development in most eutherian
mammals, a distinct, inner cell mass is elaborated at the formative
or animal pole (fig. 177 A, B). This characteristic is marked in some
species (pig, rabbit, man, and monkey) and weaker in others (mink
and armadillo). It may be entirely absent in the early blastula of
the Madagascan insectivore, Hemicentetes semispinosus; however, in
the latter, a thickening corresponding to the inner cell mass later
 
 
 
Fig. 179. Presumptive organ-forming areas in the blastoderm of the shark embryo.
(A) Median section of the blastoderm of Torpedo ocellata. Hypoblast cells are shown
in black. Caudal portion of the blastoderm is shown at the right. Cf. (B). (This figure
partly modified from Ziegler, ’02 — see Chap. 6 for complete reference.) (B) Map of
the presumptive organ-forming areas of the blastoderm of the shark, Scyllium canicula.
 
 
TYPES OF CHORDATE BLASTULAE
 
 
367
 
 
E P I BLAST
 
N TO DERM OR
PRIMARY
' YPOBLAST
 
 
 
NEURAL ECTOOE RM
 
 
NOTOCHORD
 
 
ENTO DE RM
DORSAL BLASTOPORAL LIP
 
 
Fig. 180. Presumptive organ-forming areas of the teleost fish blastoderm. (A)
Median section through the late blastoderm of Fundulus heteroclitus just previous to
gastrulation. Somewhat schematized from the author’s sections. Presumptive entoderm
or hypoblast is shown exposed to the surface at the caudal end of the blastoderm and,
therefore, follows the conditions shown in (B). (B) Presumptive organ-forming areas
 
of the blastoderm of Fundulus heteroclitus. Arrows show the direction of cell movements during gastrulation. (Modified from diagram by Oppenheimer, ’36.)
 
appears. Within the inner cell mass, two types of cells are present,
namely, formative and trophoblast (figs. 177B; 178A).
 
(2) Unlike that of the marsupial mammal, an overlying layer of trophoblast cells, covering the layer of formative cells, always is present (fig.
177B). In some cases (rabbit, pig, and cat) they degenerate (the
cells of Rauber, fig. 177C), while in others (man, rat, and monkey)
the overlying cells remain and increase in number (fig. 178A-E).
 
(3) The entodermal cells arise by a separation (delamination) of cells
from the lower aspect of the inner cell mass (figs. 177C; 178A),
with the exception of the armadillo where their origin is similar to
that of marsupials. With these differences, the same essential goal
arrived at in the marsupial mammals is achieved, namely, a bilaminar,
formative area, the embryonic disc, composed of epiblast and hypoblast layers (figs. 177D; 178D, E), which ultimately gives origin to
the embryonic body. A bilaminar, extra-embryonic, trophoblast area,
consisting of extra-embryonic entoderm and ectoderm, also is formed
(figs. 177D; 178D, E). The secondary blastocoel originates between
the epiblast and hypoblast of the embryonic disc, while below the
hypoblast layer is the archenteric space (fig. 178E).
 
 
 
368
 
 
THE CHORDATE BLASTULA AND ITS SIGNIFICANCE
 
 
6. Blastulae of Teleost and Elasmobranch Fishes
 
In the teleost and elasmobranch fishes, the primary blastula is a flattened,
disc-shaped structure constructed during its earlier stages of an upper blastoderm layer of cells, the formative or strictly embryonic tissue, and a peripheral
and lower layer of trophoblast or periblast tissues; the latter is closely associated with the yolk substance (figs. 179A; 180A; 181 A). The primary blastocoelic space lies between the blastoderm and the periblast tissue.
 
That margin of the formative portion of the blastoderm which lies at the
future caudal end of the embryo is thickened considerably, and presumptive
entodermal material or primary hypoblast is associated with this area. Its relationship is variable, however. In some teleost fishes, such as the trout, the
entodermal cells are not exposed to the surface at the caudal portion of the
blastodisc (fig. 181 A; Pasteels, ’36a). In other teleosts, a considerable portion
of the entodermal cells may lie at the surface along the caudal margin of the
blastoderm (fig. 180A; Oppenheimer, ’36). In the elasmobranch fishes the
disposition of the entodermal material is not clear. A portion undoubtedly
lies exposed to the surface at the caudal margin of the disc (fig. 179 A, B;
Vandebroek, ’36), but some entodermal cells lie in the deeper regions of the
blastoderm (fig. 179A).
 
Turning now to a consideration of the other presumptive organ-forming
areas of the fish blastoderm, we find that the presumptive pre-chordal plate
material lies exposed on the surface in the median plane of the future embryo
immediately in front of the entoderm and near the caudal edge of the blastoderm. (It is to be observed that, in comparison, the pre-chordal plate lies well
forward within the area pellucida of the bird blastoderm.) This condition is
found in the shark, Scyllium, in Fundulus, and in the trout, Salrno (figs.
179B; 180B). However, in the trout it lies a little more posteriorly at the
caudal margin of the disc (fig. 181B). Anterior to the pre-chordal plate is
the presumptive notochordal material, and anterior to the latter is a rather
expansive region of presumptive neural cells. These three areas thus lie along
the future median plane of the embryo, but they exhibit a considerable variation in size and in the extent of area covered in Scyllium, Fundulus, and
Salrno (figs. 179, 180, 181).
 
Extending on either side of these presumptive organ-forming areas, is an
indefinite region of potential mesoderm. In Salrno, presumptive mesodermal
cells lie along the lateral and anterior portions of the blastoderm edge (fig.
18 IB). However, in Scyllium and in Fundulus, it is not as extensive (figs.
179B; 180B). In front of the presumptive neural organ-forming area is a
circular region, the presumptive epidermal area.
 
In their development thus far the three blastulae described above represent
a primary blastuiar condition, and the cavity between the blastodisc and the
underlying trophoblast or periblast tissue forms a primary blastocoel. This
condition presents certain resemblances to the early blastocyst in the higher
 
 
 
PERI BLA ST ^
 
 
ENTODERM OR PRIMARY
. . . HYPOBLAST
 
 
— MESODERM
 
 
1 . •• ■ >;.'.\ rr E P I OE R M AL
 
ECTODERM
 
 
 
 
 
-NEURAL ECTO DERM
 
 
-NOTO CHORD
 
 
— -—P RE-CHORDAL PLATE
 
DORSAL BLASTOPORAL LIP
 
 
Fig. 181. Presumptive organ-forming areas of the blastoderm of the trout, Salma
irideus. (A) Schematized section through blastoderm just previous to gastrulation. Presumptive entoderm (hypoblast) shown in black at caudal end of the blastoderm. Observe
that entoderm is not exposed to surface. Cf. (B). (B) Surface view of presumptive
 
organ-forming areas of the blastoderm just before gastrulation.
 
 
 
 
 
 
 
 
Fig. 182. Late blastoderms of Gymnophiona. (Modified from Brauer, 1897.) (A)
 
Late blastoderm of Hypogeophis alternans. Entoderm cells in black lie below. (B) Beginning gastrula of H. rostratus. Observe blastocoelic spaces in white between the entoderm cells.
 
 
369
 
 
370
 
 
THE CHORDATE BLASTULA AND ITS SIGNIFICANCE
 
 
mammals and the late blastula of birds. In both groups the trophoblast tissue
is attached to the edges of the formative tissue and extends below in such a
way that the formative cells and trophoblast tissue tend to form a hollow
vesicle. In both, the formative portion of the blastula is present as a disc or
mass of cells composed of presumptive, organ-forming cells closely associated
at its lateral margins with the trophoblast or food-getting tissue. A marked
distinction between the two groups, however, is present in that the future
entodermal cells in fishes are localized at the caudal margin of the disc,
whereas in mammals and birds they may be extensively spread along the
under margin of the disc. In reptiles the condition appears to be somewhat
similar to that in birds and mammals, with the exception possibly of the
turtles, where the future entoderm appears more localized and possibly may
be superficially exposed. Therefore, while great differences in particular features exist between the fishes and the higher vertebrates, the essential fundamental conditions of the early blastulae in teleost and in elasmobranch fishes
show striking resemblances to the early blastulae of reptiles, birds, and
mammals.
 
The blastulae of teleost fishes remain in this generalized condition until
about the time when the gastrulative processes begin. At that time the notochordal and mesodermal, cellular areas begin their migrations over the caudal
edge of the blastodisc to the blastococlic space below, where they ultimately
come to lie beneath the epidermal and neural areas. Associated with the migration of notochordal and mesodermal cells, an entodermal floor or secondary hypoblast is established below the notochordal and mesodermal cells
by the active migration of primary hypoblast cells in an antero-lateral direction.
In the elasmobranch fishes there is a similar cell movement from the caudal
disc margin, as found in teleost fishes, but, in addition, a delamination of
entodermal (and possibly mesodermal cells) occurs from the deeper lying
parts of the blastodisc.
 
7. Blastulae of Gymnophionan Amphibia
 
In the Gymnophiona, nature has consummated a blastular condition different from that in other Amphibia. It represents an intermediate condition
between the blastula of the frog and the blastodiscs of the teleost and elasmobranch fishes and of higher vertebrates (fig. 182). In harmony with the
frog blastula, for example, a specialized periblast or food-getting group of
cells is absent. On the other hand, the presumptive entoderm and the presumptive notochordal, mesodermal, neural, and epidermal cells form a compact
mass at one pole of the egg, as in teleosts, the ohick, and mammal. Similar
to the condition in the chick and mammal, the entodermal cells delaminate
(see Chap. 9) from the under surface of the blastodisc (Brauer, 1897).
 
 
 
Bibliography
 
 
Assheton, R. 1896. An experimental examination into the growth of the blastoderm of the chick. Proc. Roy. Soc., London, s.B. 60:349.
 
Baer, K. E., von. 1828-1837. Tiber Entwickelungsgeschichte der Thiere. Beobachtung und Reflexion. Borntrager,
Konigsberg.
 
Boveri, T. 1892. Tiber die Entstehung des
gegensatzes zwischen den Geschlcchtszellen und den somatischen Zellen bei ^scaris megalocephala, etc., in: Sitz. d.
gesellsch. d. Morph, u. Physiol. Miinchen. vol. 8.
 
Brauer, A. 1897. Beitriige zur Kenntniss
der Entwicklungsgeschichte und der
Aiiatomie der Gy mnophionen. Zool.
Jahrb. 10:389.
 
Conklin, E. G. 1905. The organization and
cell-lineage of the ascidian egg. J. Acad.
Nat. Sc., Philadelphia. 13:5.
 
. 1932. The embryology of Aniphi
oxus. J. Morphol. 54:69.
 
. 1933. The development of isolated
 
and partially separated blastomeres of
Amphioxus. J. Exper. Zool. 64:303.
 
Duval, M. 1884. De la formation du blastoderme dans I’oeuf d’oiseau. Ann. d.
Sc. Nat., Serie. 18:1.
 
. 1889. Atlas d’embryologic. G.
 
Masson, editeur. Librairie de I’academie
de medicine, Paris.
 
Flynn, T. T. and Hill, J. P. 1939. The
development of the Monotrematci. IV.
Growth of the ovarian ovum, maturation, fertilization and early cleavage.
Trans. Zool. Soc., London, s.A. 24: Part
6, 445.
 
and . 1942. The later stages
 
of cleavage and the formation of the
primary germ layers in the Monotremata (preliminary communication ) . Proc.
Zool. Soc., London, s.A. 111:233.
 
Haeckel, E. 1866. Generelle Morphologic.
Reimer, Berlin.
 
. 1872. Die Kalkschwamme. Eine
 
Monographie. Reimer, Berlin.
 
 
. 1874. Vols. 1 and 2 in the English translation, 1910. The Evolution of
Man, translated by J. McCabe. G. P.
Putnam’s Sons, New York.
 
Hartman, C. G. 1916. Studies in the development of the opossum, Didelphys
virginiana. 1. History of early cleavage.
II. Formation of the blastocyst. J.
Morphol. 27:1.
 
. 1919. III. Description of new material on maturation, cleavage and entoderm formation. IV. The bilaminar blastocyst. J. Morphol. 32:1.
 
Horstadiiis, S. 1928. Tjber die determination des Keimes bei Echinodermen. Acta
Zool. Stockholm. 9:1.
 
. 1937. Investigations as to the localization of the micromere-, the skeleton-, and the entoderm-forming material
in the unfertilized egg of Arbacia punctiilata. Biol. Bull. 73:295.
 
Huxley, T. H. 1849. On the anatomy and
affinities of the family of the Medusae,
Philos. Tr. Roy. Soc., London, s.B.
139:413.
 
. 1888. Anatomy of Invertebrated
 
Animals. D. Appleton & Co., New York.
 
Jacobson, W. 1938* The early development of the avian embryo. 1. Entoderm
formation. J. Morphol. 62:415.
 
Kionka, H. 1894. Die Furchung des
Huhnereies. Anat. Hefte. 3:428.
 
Kleinenberg, N. 1872. Hydra. Eine Monographic. Engelmann, Leipzig.
 
Kowalewski, A. 1867. Entwicklungsgeschichte des Amphioxus lanceolatus.
Mem. Acad, imp d. sc. de St. Petersburg, VIE Serie. 11: No. 4.
 
Lankester, R. 1877. Notes on the embryology and classification of the animal
kingdom. Quart. J. M. Sc. 17:399.
 
McCrady, E., Jr. 1938. The embryology of
the opossum. Am. Anat. Memoirs, 16,
The Wistar Institute of Anatomy and
Biology, Philadelphia.
 
 
371
 
 
 
372
 
 
THE CHORDATE BLASTULA AND ITS SIGNIFICANCE
 
 
Morgan, T. H. 1934. Embryology and
Genetics. Columbia University Press,
New York.
 
Gllacher, J. 1869. Untersuchungen iiber
die Furchung und Bliitterbildung im
Huhnerei. Inst. f. Exper. Path., Wien.
1:54.
 
Oppenheimer, J. M. 1936. Processes of
localization in developing Fundulus. J.
Exper. Zool. 73:405.
 
. 1940. The non-specificity of the
 
germ layers. Quart. Rev. Biol. 15:1.
 
Pander. H. C. 1817. Beitrage zur Entwickelungsgeschichte des Huhnchcns im
Eye. Wurzburg.
 
Pasteels, J. 1936a. Etude sur la gastrulation des vertebres meroblastiques. 1.
Teleosteens. Arch, biol., Paris. 47:205.
 
. 1937a. Etudes sur la gastrulation
 
des vertebres meroblastiques. II. Reptiles. Arch, biol., Paris. 48:105.
 
. 1937b. III. Oiseaux. Arch, biol.,
 
Paris. 48:381.
 
. 1945. On the formation of the
 
primary entoderm of the duck (Anas
domestica) and on the significance of the
bilaminar embryo in birds. Anat. Rcc.
93:5.
 
Patterson, J. T. 1909. Gastrulation in the
pigeon’s egg — a morphological and experimental study. J. Morphol. 20:65.
 
Peter, K. 1934. Die erste Entwicklung des
Chamiileons (Chamaeleon vulgaris) vergleichen mit der Eidechse (Ei, Keimbildung, Furchung, Entodermobildung).
Zeit. f. anat. u. Entwicklngesch. Abteil.
2, 102-103:11.
 
. 1938. Untersuchungen fiber die
 
Entwicklung des Dotter entoderms. 1.
Die Entwicklung des Entoderms beim
Hfihnchen. 2. Die Entwicklung des Entoderms bei der Taube. Zeit. mikr.-anat.
Forsch. 43:362 and 416.
 
Spratt, N. T., Jr. 1942, Location of organspecific regions and their relationship to
the development of the primitive .streak
in the early chick blastoderm. J. Exper.
Zool. 89:69.
 
 
. 1946. Formation of the primitive
 
streak in the explanted chick blastoderm marked with carbon particles. J.
Exper. Zool. 103:259.
 
Vandebroek, G. 1936. Les mouvements
morphogenetiques au cours de la gastrulation chez Scyllium canicula Cuv.
Arch, biol., Paris. 47:499.
 
Vogt, W. 1925. Gestaltungsanalyse am
Amphibienkeim mit ortlicher Vitalfarbung. Vorwort fiber Wege und Ziele. I.
Methodik und Wirkungsweise der ortlichen Vitalfarbung mit Agar als Farbtriiger. Arch. f. Entwicklngsmech. d.
Organ. 106:542.
 
. 1929. Gestaltungsanalyse, etc. II.
 
Teil. Gastrulation und Mesodermbildung
bei Urodelen und Anuren. Arch. f. Entwicklngsmech. d. Organ. 120:384.
 
Wheeler, W. M. 1898. Caspar Friedrich
Wolff and the Theoria Generationis.
Biological Lectures, Marine Biol. Lab.,
Woods Hole, Mass. Ginn & Co., Boston.
 
Whitman, C. O. 1878. The embryology
of Clepsine. Quart. J. M. Sc. 18:215.
 
Will, L. 1892. Beitrage zur Entwicklungsgeschichte der Reptilien. I. Die Anlage
der Keimblatter beim Gecko (Platydactylus facetanus Schreib). Zool. Jahrb.
6 : 1 .
 
Wilson, E. B. 1892. The cell lineage of
Nereis. J. Morphol. 6:361.
 
. 1898. Cell-Lineage and ancestral
 
reminiscence. Biological Lectures, Marine Biol. Lab., Woods Hole, Mass. Ginn
& Co., Boston.
 
. 1925. The Cell in Development
 
and Heredity. 3rd edit. The Macmillan
Co., New York.
 
Wolff, C. F. 1759. Theoria Generationis.
Halle.
 
. 1812. De formatione intestinorum
 
praecipe, etc. Published in Latin in
Vols. 12 and 13 of St. Petersburg Commentaries (Acad. Sci. Impt. Petropol.
1768“69) and translated by J. F. Meckel,
in Uber die Bildung des Darmkanals im
bebrfiteten Hfihnchen, Halle.
 
Zur Strassen, O. 1896. Embryonalentwickelung der Ascaris me galocephala. Arch.
f. Entwicklngsmech. 3:27, 133.
 
 
==Late Blastula in Relation to Certain Innate Physiological Conditions: Twinning==
 
A. Introduction
 
B. Problem of differentiation
 
1. Definition of differentiation; kinds of differentiation
 
2. Self-differentiation and dependent differentiation
 
C. Concept of potency in relation to differentiation
 
1. Definition of potency
 
2. Some terms used to describe different states of potency
 
a. Totipotency and harmonious totipotency
 
b. Determination and potency limitation
 
c. Prospective potency and prospective fate
 
d. Autonomous potency
c. Competence
 
D. The blastula in relation to twinning
 
1. Some definitions
 
a. Dizygotic or fraternal twins
 
b. Monozygotic or identical twins
 
c. Polyembryony •
 
2. Basis of true or identical twinning
 
3. Some experimentally produced, twinning conditions
 
E. Importance of the organization center of the late blastula
 
 
A. Introduction
 
In the preceding two chapters the blastula is defined as a morphological
entity composed of six, presumptive, organ-forming areas — areas which are
poised and ready for the next phase of development or gastrulation. However, the attainment of this morphological condition with its presumptive,
organ-forming areas is valid and fruitful in a developmental way only if it
has developed within certain physiological conditions which serve as a spark
to initiate gastrulation and carry it through to its completion.
 
The physiological conditions of the blastula are attained, as are its morphological characteristics, through a process of differentiation. Moreover,
during the development of the blastula, different areas acquire different abilities
to undergo physiological change and, hence, possess different abilities or
 
 
THE BLASTULA IN RELATION TO INNATE CONDITIONS
 
 
powers of differentiation. To state the matter differently, the various, presumptive, organ-forming areas of the blastula have acquired different abilities
not only in their power to produce specific organs of the future body of the
embryo, but also in that some presumptive areas possess this propensity in
a greater degree than do other areas. However, at this point, certain terms in
common usage relating to the problem of differentiation are defined in order
that a better understanding may be obtained concerning the ability to differentiate on the part of the presumptive, organ-forming areas of the late
blastula.
 
B. Problem of Differentiation
 
1. Definition of Differentiation; Kinds of Differentiation
 
The word differentiation is applied to that phase of development when a
cell, a group of cells, cell product experiences a change which results in a
persistent alteration of its activities. Under ordinary conditions an alteration
in structure or function is the only visible evidence that such a change has
occurred.
 
To illustrate these matters, let us recall the conditions involved in the
maturation of the egg. A subtle change occurs within the primitive oogonium
which causes it to enlarge and to grow. This growth results in an increase in
size and change in structure of both the cytoplasm and the nucleus. A little
later, as the egg approaches that condition which is called maturity, observable
morphological changes of the nucleus occur which accompany or initiate an
invisible change in behavior. These latter changes make the egg fertilizable.
Here we have illustrated, first of all, a subtle, invisible, biochemical change
in the oogonium which arouses the formation of visible morphological changes
in the oocyte and, secondly, a morphological change (i.e., nuclear maturation)
which accompanies an invisible physiological transformation.
 
Another illustration will prove profitable. Let us recall the development
of the mammary-gland tissue (fig. 58). Through the action of the lactogenic
(luteotrophic) hormone, LTH, the cells of the various acini of the fully
developed gland begin to secrete milk. The acini, it will be recalled, were
caused to differentiate as a result of the presence of progesterone. Similarly,
the various parts of the complicated duct system were stimulated to differentiate from a very rudimentary condition by the presence of estrogenic hormone.
Earlier in development, however, the particular area of the body from which
the duct rudiments ultimately arose was conditioned by a change which dictated the origin of the duct rudiments from the cells of this area and restricted
their origin from other areas.
 
In the foregoing history of the mammary gland, various types of differentiation are exemplified. The final elaboration of milk from the acinous cells
is effected by a change in the activity of the cells under the influence of LTH.
The type of change which brings about the functional activities of a structure
is called physiological differentiation. The morphological changes in the cells
 
 
 
PROBLEM OF DIFFERENTIATION
 
 
375
 
 
which result in the formation of the duct system and the acini are examples
of morphological differentiation. On the other hand, the invisible, subtle
change or changes which originally altered the respective cells of the nipple
area and, thereby, ordained or determined that the cells in this particular
locale should produce duct and nipple tissue is an example of biochemical
differentiation or chemodifferentiation. Chemodifterentiation, morphological
differentiation, and physiological differentiation, therefore, represent the three
types or levels of differentiation. Moreover, all of these differentiations stem
from a persistent change in the fundamental activities of cells or cell parts.
 
It should be observed further that chemodifferentiation represents the initial
step in the entire differentiation process, for it is this change which determines
or restricts the future possible activities and changes which the cell or cells
in a particular area may experience. Also, in many cases, differentiation appears to arise as a result of stimuli which are applied to the cell or cells externally. That is, internal changes within a cell may be called forth by an
environmental change applied to the cell from without.
 
In embryological thinking, therefore, the word differentiation implies a
process of becoming something new and different from an antecedent, lessdifferentiated condition. But beyond this, differentiation also connotes a certain suitableness or purposefulness of the structure which is differentiated.
Such a connotation, however, applies only to normal embryonic differentiation; abnormal growths and monstrosities of many kinds may fulfill the first
phase (i.e., of producing something new) of differentiation as defined in
the first sentence of this paragraph, but they do not satisfy the criteria of
purpose and of suitableness within the organized economy of the developing
body as a whole. It is important to keep the latter implications in mind, for
various structures may appear to be vestigial or aberrant during embryonic
development, nevertheless their presence may assume an important, purposeful status in the ultimate scheme which constructs the organization of the
developing body.
 
2. Self-differentiation and Dependent Differentiation
 
In the amphibian, very late blastula and beginning gastrula, the presumptive,
chordamesodermal area, when undisturbed and in its normal position in the
embryo, eventually differentiates into notochordal and mesodermal tissues.
This is true also when it is transplanted to other positions. That is, at this
period in the history of the chordamesodermal cells the ability resides within
the cells to differentiate into notochordal and mesodermal structures. Consequently, these cells are not dependent upon surrounding or external factors
to induce or call forth differentiation in these specific directions. Embryonic
cells in this condition are described as self-differentiating (Roux). Similarly,
the entodermal area with its potential subareas of liver, foregut, and intestine
develops by itself and this area does not rely upon stimuli from other con
 
 
376
 
 
THE BLASTULA IN RELATION TO INNATE CONDITIONS
 
 
tiguous cells to realize a specific potency. On the other hand, the presumptive,
neural plate region at this time is dependent upon the inducing influence of
the chordamesodermal cells during the process of gastrulation for its future
realization as neural tissue. This area has little inherent ability to differentiate
neural tissue and is described, therefore, as being in a state of dependent
differentiation (Roux). Furthermore, the presumptive skin ectoderm (i.e.,
epidermis), if left alone, will proceed to epidermize during gastrulation, but
foreign influences, such as transplantation, into the future neural plate area
may induce neural plate cells to form from the presumptive skin ectoderm
(fig. 183). The differentiation of neural cells from any of the ectodermal cells
of the late blastula thus is dependent upon special influencing factors applied
to the cells from without.
 
C. Concept of Potency in Relation to Differentiation
 
1. Definition of Potency
 
The word potency, as used in the field of embryology, refers to that property of a cell which enables it to undergo differentiation. From this viewpoint,
potency may be defined as the power or ability of a cell to give origin to a
specific kind of cell or structure or to various kinds of cells and structures.
 
It is questionable, in a fundamental sense, whether potency actually is
gained or lost during development. It may be that the expression of a given
kind of potency, resulting in the formation of a specific type of cell, is merely
the result of a restriction imposed upon other potentialities by certain modifying factors, while the total or latent potency remains relatively constant.
All types of differentiated cells, from this point of view, basically are totipotent;
that is, they possess the latent power to give origin to all the kinds of cells
and tissues of the particular animal species to which they belong.
 
The specific potencies which denote the normal development of particular
organs undoubtedly have their respective, although often quite devious, connections with the fertilized egg. However, one must concede the origin of
abnormal or acquired potency values due to the insinuation of special inductive or modifying factors which disturb the expression of normal potency
value. For example, tumors and other abnormal growths and tissue distortions
may be examples of such special potencies induced by special conditions which
upset the mechanism controlling normal potency expression.
 
2. Some Terms Used to Describe Different States of Potency
a. Totipotency and Harmonious Totipotency
 
The word totipotent, as applied to embryonic development, was introduced
into embryological theory by Wilhelm Roux, and it refers to the power or
ability of an early blastomere or blastomeres of a particular animal species
to give origin to the many different types of cells and structures characteristic
of the individual species. Speculation concerning the meaning of totipotency
 
 
 
POTENCY IN RELATION TO DIFFERENTIATION
 
 
377
 
 
of a single blastomere received encouragement from the discovery by Hans
Driesch, in 1891, that an isolated blastomere of the tWo- or four-cell stage
of the cleaving, sea-urchin’s egg could give origin to a “perfect larva.” Driesch
described this condition as constituting an equipotential state, while Roux
referred to it as a totipotential condition. As the word totipotential seems
more fitting and better suited to describe the condition than the word equipotential, which simply means equal potency, the word lotipotency is used
herein. The word omnipotent is sometimes used to describe the totipotent
condition; as it has connotations of supreme power, it will not be used.
 
The totipotent state is a concept which may be considered in different
ways. In many instances it has been used as described above, namely, as a
potency condition that has within it the ability to produce a perfect embryo
or individual. The word also has been used, however, to describe a condition
which is capable of giving origin to all or nearly all the cells and tissues of
the body in a haphazard way but which are not necessarily organized to produce
a normally formed body of the particular species. Therefore, as a basis for
clear thinking, it is well to define two kinds of totipotency, namely, totipotency
and harmonious totipotency. The former term is used to describe the ability
of a cell or cell group to give origin to all or nearly all the different cells and
tissues of the particular species to which it belongs, but it is lacking in the
ability to organize them into an harmonious organism. Harmonious totipotency, on the other hand, is used to denote a condition which has the above
ability to produce the various types of tissues of the species, but possesses,
in addition, the power to develop a perfectly organized body.
 
The fertilized egg or the naturally parthenogenetic egg constitutes an harmonious totipotential system. This condition is true also of isolated blastomeres of the two- or iour-blastomere stage of the sea-urchin development,
as mentioned above, of the two-cell state of Amphioxus, or of the first twoblastomere stage of the frog’s egg when the first cleavage plane bisects the
gray crescent. However, in the eight-cell stage in these forms, potency becomes more limited in the respective cells of the embryo. Restriction of
potency, therefore, is indicated by a restriction of power to develop into
a variety of cells and tissues, and potency restriction is a characteristic of
cleavage and the blastulative process (figs. 61; 163 A; 163B). When a stage
is reached in which the cells of a particular area are limited in potency value
to the expression of one type of cell or tissue, the condition is spoken of as
one of unipotency. A pluripotent state, on the other hand, is a condition in
which the potency is not so limited, and two or more types of tissues may
be derived from the cell or cells.
 
b. Determination and Potency Limitation
 
The limitation or restriction of potency, therefore, may form a part of the
process of differentiation; as such, it is a characteristic feature of embryonic
 
 
 
378
 
 
THE BLASTULA IN RELATION TO INNATE CONDITIONS
 
 
development. Potency limitation, however, is not always the result of the differentiation process. For instance, in the development of the oocyte in the
ovary, the building up of the various conditions, characteristic of the totipotent
state, is a feature of the differentiation of the oocyte.
 
The word determination is applied to those unknown and invisible changes
occurring within a cell or cells which effect a limitation or restriction of potency.
As a result of this potency limitation, differentiation becomes restricted to a
specific channel of development, denoting a particular kind of cell or structure.
Ultimately, by the activities of limiting influences upon the resulting blastomeres during cleavage, the totipotent condition of the mature egg becomes
dismembered and segregated into a patchwork or mosaic of general areas of
the blastula, each area having a generalized, presumptive, organ-forming potency. As we have already observed, in the mature chordate blastula there
are six of these major, presumptive organ-forming areas (five if we regard
the two mesodermal areas as one). By the application of other limiting influences during gastrulation or the next phase of development, each of these
general areas becomes divided into minor areas which are limited to a potency
value of a particular organ or part of an organ. The process which brings
about the determination of individual organs or parts of organs is called
individuation.
 
When potency limitation has reduced generalized and greater potency value
to the status of a general organ system (e.g., nervous system or digestive
system) with the determination (i.e., individuation) of particular organs
within such a system, the condition is described as one of rigid or irrevocable
determination. Such tissues, transplanted to other parts of the embryo favorable for their development, tend to remain limited to an expression of one
inherent potency value and do not give origin to different kinds of tissues or
organs. Thus, determined liver rudiment will differentiate into liver tissue,
stomach rudiment into stomach tissue, forebrain material into forebrain
tissue, etc.
 
In many instances determination within a group of cells is brought about
because of their position in the developing organism and not because of intrinsic, self-differentiating conditions within the cells. Because their position
foreordains their determination in the future, the condition is spoken of as
positional or presumptive determination. For example, in the late amphibian
blastula, the composite ectodermal area of the epiblast will become divided,
during the next phase of development, into epidermal and neural areas as a
result of the influences at work during gastrulation, especially the activities
of the chordamesodermal area. Therefore, one may regard these areas as
already determined, in a presumptive sense, even in the late blastula, although
their actual determination as definite epidermal and neural tissue will not
occur until later.
 
As stated in the preceding paragraphs, determination is the result of potency
 
 
 
POTENCY IN RELATION TO DIFFERENTIATION
 
 
379
 
 
limitation or inhibition. However, there is another aspect to determination,
namely, potency expression, which simply means potency release or development. Potency expression, probably, is due to an activating stimulus (Spemann,
’38). Consequently, the individuation of a particular organ structure within
a larger system of organs is the result of two synchronous processes:
 
( 1 ) inhibition of potency or potencies and
 
(2) release or calling forth of a specific kind of potency (Wiggles worth,
’48).
 
Associated with the phenomenon of potency inhibition or limitation is the
loss of power for regulation. Consequently, individuation and the loss of
regulative power appear to proceed synchronously in any group of cells.
 
c. Prospective Potency and Prospective Fate
 
Prospective fate is the end or destiny that a group of cells normally reaches
in its differentiation during its normal course of development in the embryo.
The presumptive epidermal area of the late blastula differentiates normally
into skin epidermis. This is its prospective fate. Its prospective potency, however, is greater, for under certain circumstances it may be induced, by transplantation to other areas of the late blastula, to form other tissue, e.g., neural
plate cells or mesodermal tissues.
 
d. Autonomous Potency
 
Autonomous potency is the inherent ability which a group of cells possesses
to differentiate into a definite structure or structures, e.g., notochord, stomach,
or liver rudiments of the late blastula of the frog.
 
Versatility of autonomous potency is the inherent ability which a group
of cells possesses to differentiate, when isolated under cultural conditions outside the embryo, into tissues not normally developed from the particular cell
group in normal development. In the amphibian late blastula this is true of
the notochordal and somitic areas of the chordamesodermal area, which may
give origin to skin or neural plate tissue under these artificially imposed
conditions.
 
 
e. Competence
 
Certain areas of the late amphibian blastula have the ability to differentiate into diverse structures under the stimulus of varied influence. Consequently, we say that these areas have competence for the production of this
or that structure. The word competence is used to denote all of the possible
reactions which a group of cells may produce under various sorts of stimulations. The entodermal area of the late amphibian blastula and early gastrula
has great power for self-differentiation but no competence, whereas the general, neural plate-epidermal area has competence but little power of self
 
 
380
 
 
THE BLASTULA IN RELATION TO INNATE CONDITIONS
 
 
differentiation (see p. 375). On the other hand, the notochord, mesodermal
area possesses both competence and the ability for self-differentiation.
 
Competence appears to be a function of a developmental time sequence.
That is, the time or period of development is all important, for a particular
area may possess competence only at a single, optimum period of development. The word competence is sometimes used to supersede the other terms
of potency or potentiality (Needham, ’42, p. 112).
 
D. The Blastula in Relation to Twinning
 
1. Some Definitions
 
a. Dizygotic or Fraternal Twins
 
Fraternal twins arise from the fertilization of two separate eggs in a species
which normally produces one egg in the reproductive cycle, as, for example,
in the human species. Essentially, fraternal twins are much the same as the
“siblings” of a human family (i.e., the members born as a result of separate
pregnancies) or the members of a litter of several young produced during
a single pregnancy in animals, such as cats, dogs, pigs, etc. Fraternal twins
are often called “false twins.”
 
b. Monozygotic or Identical Twins
 
This condition is known as “true twinning,” and it results from the development of two embryos from a single egg. Such twins presumably have an
identical genetic composition.
 
 
c. Polyembryony
 
Polyembrony is a condition in which several embryos normally arise from
one egg. It occurs regularly in armadillos (Dasypopidae) where one ovum
gives origin normally to four identical embryos (fig. 186).
 
2. Basis of True or Identical Twinning
 
The work of Driesch (1891) on the cleaving, sea-urchin egg and that of
Wilson (1893) on the isolated blastomeres of Amphioxus mentioned above
initiated the approach to a scientific understanding of monozygotic or identical
twinning. Numerous studies have been made in the intervening years on the
developing eggs of various animal species, vertebrate and invertebrate, and
from these studies has emerged the present concept concerning the matter of
twinning. True twinning appears to arise from four, requisite, fundamental,
morphological and physiological conditions. These conditions are as follows:
 
( 1 ) there must be a sufficient protoplasmic substrate;
 
(2) the substrate must contain all the organ-forming stuffs necessary to
assure totipotency, that is, to produce all the necessary organs;
 
 
 
THE BLASTULA IN RELATION TO TWINNING
 
 
381
 
 
(3) an organization center or the ability to develop such a center must
be present in order that the various organs may be integrated into
an harmonious whole; and
 
(4) the ability or faculty for regulation, that is, the power to rearrange
materials as well as to reproduce and compensate for the loss of substance, must be present.
 
3. Some Experimentally Produced, Twinning Conditions
 
The isolation of the first two blastomeres in the sea-urchin egg and in
Amphioxus with the production of complete embryos from each blastomere
 
 
 
Fig. 183. Early gastrula of darkly pigmented Triton taeniatus with a small piece of
presumptive ectoderm of T. cristatus lightly pigmented inserted into the presumptive,
neural plate area shown in (A). (B) Later stage of development. (C) Cross section of the
later embryo. I hc lighter eye region shown to the right was derived from the original
implant from T. cristatus, (After Spemann, ’38.)
 
 
 
Fig. 184. Demonstration that the presence of the organizer region or organization
center is necessary for development. (Redrawn from Spemann, ’38.) (A) Hair-loop
 
constriction isolates the organizer areas in the dorsal portion of the early gastrula. (B)
Later development of the dorsal portion isolated in (A). (C) Later development of
 
ventral portion of gastrula isolated in (A). (D) Constriction of organizer area of early
 
gastrula into two halves. (E) Result of constriction made in (D). Constrictions were
made at 2-cell stage.
 
 
 
382
 
 
THE BLASTULA IN RELATION TO INNATE CONDITIONS
 
 
CENTER OF ORGANIZATION
 
 
 
Fig. 185. Twinning in teleost fishes. (After Morgan, ’34; Embryology and Genetics,
Columbia University Press, pp. 102-104. A, B, C from Rauber; D from Stockard.) In
certain teleost fishes, especially in the trout, under certain environmental conditions,
two or more organization centers arise in the early gastrula. (A-C) These represent
such conditions. If they lie opposite each other as in (A), the resulting embryos often
appear as in (D). If they lie nearer each other as in (B) or (C), a two-headed monster
may be produced.
 
has been described in Chapter 6, In these cases all the conditions mentioned
above are fulfilled. However, in the case of the isolation of the first two blastomeres in Stye la described in Chapter 6, evidently conditions (1), (2), and
(3) are present in each blastomere when the two blastomeres are separated,
but (4) is absent and only half embryos result. That is, each blastomere has
been determined as either a right or left blastomere; with this determination
of potency, the power for regulation is lost. In the frog, if the first two blastomeres are separated when the first cleavage plane bisects the gray crescent,
all four conditions are present and two tadpoles result. If, however, the first
cleavage plane separates the gray-crescent material mainly into one blastomere
while the other gets little or none, the blastomere containing the gray-crescent
material will be able to satisfy all the requirements above, and it, consequently,
develops a normal embryo. However, the other blastomere lacks (2), (3),
and (4) and, as a result, forms a mere mass of cells. Again, animal pole
blastomeres, even when they contain the gray-crescent material, when separated entirely from the yolk blastomeres, fail to go beyond the late blastular
or beginning gastrular state (Vintemberger, ’36). Such animal pole blastomeres appear to lack requirements (I), (2), and possibly (3) above. Many
other illustrations of embryological experiments could be given, establishing
 
 
 
 
Fig. 186. Polyembryony or the development of multiple embryos in the armadillo,
Tatusia novemcincta. (After Patterson, ’13.) (A) Separate centers of organization in
 
the early blastocyst. (B) Later stage in development of multiple embryos. Each embryo
is connected with a common amniotic vesicle. (C) Section through organization centers
a and b in (A). The two centers of organization are indicated by thickenings at right
and left. (D) Later development of four embryos, the normal procedure from one
fertilized egg in this species.
 
 
383
 
 
 
384
 
 
THE BLASTULA IN RELATION TO INNATE CONDITIONS
 
 
the necessity for the presence of all the above conditions. Successful whole
embryos have resulted in the amphibia when the two-cell stage and beginning
gastrula is bisected in such a manner that each half contains half of the chordamesodermal field and yolk substance; that is, each will contain half of the
organization center (fig. 184).
 
Monozygotic twinning occurs occasionally under normal conditions in the
teleost fishes. In these cases, separate centers of organization arise in the
blastoderm, as shown in figure 185. When they arise on opposite sides of
the blastoderm, as shown in figure 185 A, twins arise which may later become
fused ventrally (fig. 185D). When the centers of organization arise as shown
in figure I85B, C, the embryos become fused laterally. Stockard (’21) found
that by arresting development in the trout or in the blastoderm of Fundutus
for a period of time during the late blastula, either by exposure to low temperatures or a lack of oxygen, twinning conditions were produced. The arrest
of development probably allows separate centers of organfzation to arise.
Normally, one center of organization makes its appearance in the late blastula
of these fishes, becomes dominant, and thus suppresses the tendency toward
totipotency in other parts of the blastoderm. However, in the cases of arrested
development, a physiological isolation of different areas of the blastoderm
evidently occurs, and two organization centers arise which forthwith proceed
to organize separate embryos in the single blastoderm. Conditions appear more
favorable for twinning in the trout blastoderm than in Fundulus. After the
late blastular period is past and gastrulation begins, i.e., after one organization
center definitely has been established, Stockard found that twinning could not
be produced.
 
In the Texas armadillo, Tatusia novemcincta, Patterson (’13) found that, in
the relatively late blastocyst (blastula), two centers of organization arise, and
that, a little later, each of these buds into two separate organization centers,
producing four organization centers in the blastula (fig. 186A-C). Each of
these centers organizes a separate embryo; hence, under normal conditions, four
embryos (polyembryony) are developed from each fertilized egg (fig. 186D).
 
It is interesting in connection with the experiments mentioned by Stockard
above, that the blastocyst (blastula) in Tatusia normally lies free in the uterus
for about three weeks before becoming implanted upon the uterus. It may
be that this free period of blastocystic existence results in a slowing down of
development, permitting the origin of separate organization centers. In harmony with this concept, Patterson (’13) failed to find mitotic conditions in
the blastoderms of the blastocysts during this period.
 
In the chick it is possible to produce twinning conditions by separating
the anterior end (Hensen’s node) of the early primitive streak into two parts
along the median axis of the developing embryo. Twins fused at the caudal
end may be produced under these conditions. In the duck egg, Wolff and Lutz
(’47) found that if the early blastoderm is cut through the primitive node
 
 
 
THE BLASTULA IN RELATION TO TWINNING
 
 
385
 
 
area (fig. 187A), two embryos are produced as in figure 187 A'. However, if
the primitive node and primitive streak are split antero-posteriorly, as indicated in figure 1876, two embryos, placed as in figure 187B', are produced.
 
It is evident, therefore, that in the production of monozygotic twins, condition (3) or the presence of the ability to produce an organization center
is of greatest importance. In the case of the separation of the two blastomeres
of the two-cell stage in Amphioxus or of the division of the dorsal lip of the
early gastrula of the amphibian by a hair loop, as shown in figure 184, a
mechanical division and separation of the ability to produce an organization
center in each blastomere (Amphioxus) or of the separation into two centers
of the organization center already produced (Amphibia) is achieved. Once
these centers are isolated, they act independently, producing twin conditions,
providing the substrate is competent. Similar conditions evidently are produced in the duck-embryo experiments of Wolff and Lutz referred to above.
 
In some teleost blastulae, e.g., Fundidus and Salmo, during the earlier period
of development, it has been found possible to separate the early blastoderm
into various groups of cells (Oppenheimer, ’47) or into quadrants (Luther,
’36), .and a condition of totipotency is established in each part. Totipotency
appears thus to be a generalized characteristic in certain teleost blastoderms
during the earlier phases of blastular development. Harmonious totipotency,
however, appears not to be achieved in any one part of the blastodisc of
these species during the early conditions of blastular formation. During the
 
 
 
Fig. 187. Isolation of the organization center in the early duck embryo. (From Dalcq,
’49, after Wolff and Lutz.) (A') Derived from blastoderm cut as in (A). (B') Derived
 
from blastoderm cut as in (B).
 
 
 
386
 
 
THE BLASTULA IN RELATION TO INNATE CONDITIONS
 
 
development of the late blastula, however, the posterior quadrant normally
acquires a dominant condition together with a faculty for producing harmonious totipotency. The other totipotent areas then become suppressed.
These basic conditions, therefore, serve to explain the experiments by Stockard
(’21) referred to above, where two organization centers tend to become dominant as a result of isolating physiological conditions which tend to interfere
with the processes working toward the development of but one center of organization. This probable explanation of the twinning conditions in the teleost
blastoderm suggests strongly that the separation and isolation of separate
organization centers is a fundamental condition necessary for the production
of monozygotic or true twinning.
 
It becomes apparent, therefore, that, in the development of the trout blastoderm (blastula), the development oj an area which possesses a dominant
organization center is an important aspect of blastulation. In other blastulae,
the seat or area of the organization center apparently is established at an
earlier period, as, for example, the gray crescent in the amphibian egg which
appears to be associated with the organization center during the late blastula
state. Similarly, in the teleost fish, Carassius, totipotency appears to be limited
to one part of the early blastula (Tung and Tung, ’43).
 
It also follows from the analysis in the foregoing paragraphs that in the
production of polyembryony in the armadillo or of spontaneous twinning in
forms, such as the trout (Salmo), a generalized totipotency throughout the
early blastoderm is a prerequisite condition. When a single dominant area
once assumes totipotency, it tends to suppress and control the surrounding
areas, probably because it succeeds in “monopolizing” certain, substrate,
“food” substances (Dalcq, ’49).
 
£. Importance of the Organization Center of the Late Blastula
 
It is also evident that one of the main functions of cleavage and blastulation
is the formation of a physiological, or organization, center which must be
present to dominate and direct the course of development during the next
stage of development. Consequently, the elaboration of a blastocoel with the
various, presumptive, organ-forming areas properly oriented in relation to
it is not enough. A definite physiological condition entrenched within the
so-called organization center must be present to arouse and direct the movement of the major, organ-forming areas during gastrulation.
 
 
 
Bibliography
 
 
Butler, E. 1935. The developmental capacities of regions of the unincubated
chick blastoderm as tested in chorioallantoic grafts. J. Exper. Zool. 70:357.
 
Daicq, A. M. 1949. The concept of physiological competition (Spiegelman) and
the interpretation of vertebrate morphogenesis. Experimental Cell Research,
Supplement 1 : p. 483. Academic Press,
Inc., New York.
 
Driesch, H. 1891. Entwicklungsmechanische Studien I-II. Zeit. Wiss. Zool.
53:160.
 
Holtfreter, J. 1938. Differenzierungspotenzen isolierter teile der Anuren-gastrula.
Roux’ Arch. f. Entwick. d. Organ.
138.657.
 
Luther, W. 1936. Potenzpriifungen an
isolierten Teilstiicken der Forellenkeimscheibe. Arch. f. Entwicklngsmech. d.
Organ. 135:359.
 
Morgan, T. H. 1934. Chap. IX in Embryology and Genetics. Columbia University Press, New York.
 
Needham, J. 1942. Biochemistry and Morphogenesis. Cambridge University Press,
London.
 
Oppenheimer, J. M. 1947. ‘Organization of
the teleost blastoderm. Quart. Rev. Biol.
22:105.
 
Patterson, J. T. 1913. Polyembryonic development in Tatusia novemcincta. J.
Morphol. 24:559.
 
 
Spemann, H. 1938. Embryonic Development and Induction. Yale University
Press, New Haven.
 
Stockard, C. R. 1921. Developmental rate
and structural expression: an experimental study of twins, “double monsters” and single deformities, and the
interaction among embyronic organs
during their origin and development.
Am. J. Anat. 28: 1 15.
 
Tung, T. C. and Tung, Y. F. Y. 1943.
Experimental studies on the development
of the goldfish. (Cited from Oppenheimer, ’47.) Proc. Clin. Physiol. Soc.
2 : 11 .
 
Vintemberger, P. 1936. Sur le developpement compare dcs micromeres dc I’oeuf
de Rana fusca divise enhuit (a) Apres
isolement (b) Apres transplantation sur
un socle de cellules vitellines. Compt.
rend. Soc. de Biol. 122:927.
 
Wigglesworth, V. B. 1948. The role of the
cell in determination. Symposia of the
Soc. for Exper. Biol. No. II. Academic
Press, Inc., New York.
 
Wilson. E. B. 1893. Amphioxiis and the
mosaic theory of development. J. Morphol. 8:579.
 
Wolff, E. and Lutz, H. 1947. Embryologie
experimentale — sur la production experimcntale dc jumeaux chez I’embryon
d’oiseau. Compt. rend. Acad. d. Sc.
224:1301.
 
 
387
 
 
 
==Gastrulation==
 
 
A. Some definitions and concepts
 
1. Gastrulation
 
2. Primitive vertebrate body plan in relation to the process of gastrulation
 
a. Fundamental body plan of the vertebrate animal
 
b. The gastrula in relation to the primitive body plan
 
c. Chart of blastula, gastrula, and primitive, body-form relationships (fig. 188)
 
B. General processes involved in gastrulation
 
C. Morphogenetic movement of cells
 
1. Importance of cell movements during development and in gastrulation
 
2. Types of cell movement during gastrulation
 
a. Epiboly
 
b. Emboly
 
3. Description of the processes concerned with epiboly
 
4. Description of the processes involved in emboly
 
a. Involution and convergence
 
b. Invagination
 
c. Concrescence
 
d. Cell proliferation
 
e. Polyinvagination
 
f. Ingression
 
g. Delamination
 
h. Divergence
 
i. Extension
 
D. The organization center and its relation to the gastrulative process
 
1. The organization center and the primary organizer
 
2. Divisions of the primary organizer
 
E. Chemodifferentiation and the gastrulative process
 
F. Gastrulation in various Chordata
1. Amphioxus
 
a. Orientation
 
b. Gastrulative movements
 
1 ) Emboly
 
2) Epiboly
 
3) Antero-posterior extension of the gastrula and dorsal convergence of the
mesodermal cells
 
4) Closure of the blastopore
 
c. Resume of cell movements and processes involved in gastrulation of Amphioxus
 
1 ) Emboly
 
2) Epiboly
 
 
388
 
 
 
 
GASTRULATION
 
 
389
 
 
2. Gastrulation in Amphibia with particular reference to the frog
 
a. Introduction
 
1) Orientation
 
2) Physiological changes which occur in the presumptive, organ-forming areas
of the late blastula and early gastrula as gastrulation progresses
 
b. Gastrulation
 
1) Emboly
 
2) Epiboly
 
3) Embryo produced by the gastrulative processes
 
4) Position occupied by the pre -chordal plate material
 
c. Closure of the blastopore and formation of the neurenteric canal
 
d. Summary of morphogenetic movements of cells during gastrulation in the frog
and other Amphibia
 
1) Emboly
 
2) Epiboly
 
3. Gastrulation in reptiles
 
a. Orientation
 
b. Gastrulation
 
4. Gastrulation in the chick
 
a. Orientation
 
b. Gastrulative changes
 
1) Development of primitive streak as viewed from the surface of stained
blastoderms
 
2) Cell movements in the epiblast involved in primitive-streak formation as
indicated by carbon-particle marking and vital-staining experiments
 
3) Cell movements in the hypoblast and the importance of these movements
in primitive-streak formation
 
4) Primitive pit notochordal canal
 
5) Resume of morphogenetic movements of cells during gastrulation in the
chick
 
5. Gastrulation in mammals
 
a. Orientation
 
b. Gastrulation in the pig embryo
 
c. Gastrulation in other mammals
 
6. Gastrulation in teleost and elasmobranch fishes
 
a. Orientation
 
b. Gastrulation in teleost fishes
 
1) Emboly
 
2) Epiboly
 
3) Summary of the gastrulative processes in teleost fishes
 
a) Emboly
 
b) Epiboly
 
4) Developmental potencies of the germ ring of teleost fishes
 
c. Gastrulation in elasmobranch fishes
 
7. Intermediate types of gastrulative behavior
 
G. The late gastrula as a mosaic of specific, organ-forming territories
 
H. Autonomous theory of gastrulative movements
 
I. Exogastrulation
 
J. Pre-chordal plate and cephalic projection in various chordates
 
K. Blastoporal and primitive-streak comparisons
 
 
 
390
 
 
GASTRULATION
 
 
A. Some Definitions and Concepts
 
1 . Gastrulation
 
(According to Haeckel, the word gastrula is the name given to “the important embryonic form” having “the two primary germ-layers,” and the word
gastrulation is applied to the process which produces the gastrula. Furthermore, “this ontogenetic process has a very great significance, and is the real
starting-point of the construction of the multicellular animal body” (1874, see
translation, ’10, p. 1 23 ji).^thers such as Lankester (1875) and Hubrecht
(’06) did much to establish the idea that gastrulation is a process during
which the. monolayered blastula is converted into a bilaminar or didermic
gastrulajjHaeckel emphasized invagination or the infolding of one portion of
the blastula as the primitive and essential process in this conversion, while
Lankester proposed delamination or the mass separation of cells as the primitive process^ While it was granted that invagination was the main process of
gastrulation in Amphioxus, in the Vertebrata, especially in reptiles, birds, and
mammals, delajpination was considered to be an essential process by many
embryologists. Some, however, maintained that the process of invagination
held true for all the Chordata other than the Mammalia, It may be mentioned
in passing that Lankester conferred the name “blastopore” upon the opening
into the interior of the blastoderm which results during gastrulation. The
words “blastopore” and “primitive mouth” soon were regarded as synonymous,
for in the Coelenterata, the blastopore eventually becomes the oral opening.
 
The definition of the gastrula as a didermic stage, following the monolayered blastula, is a simple concept, easy to visualize, and, hence, may have
some pedagogical value. However, it is not in accord with the facts unearthed
by many careful studies relative to cell lineage and it does not agree with the
results obtained by the Vogt method (see Chap. 7) applied to the process
of gastrulation in the vertebrate group.
 
One of the first to define gastrulation in a way which is more consonant
with the studies mentioned in the previous paragraph was Keibel (’01). He
defined gastrulation in the vertebrates (’01, p. 1111) as “the process by
which the entodermal, mesodermal and notochordal cells find their way into
the interior of the embryo.” It is to be observed that this definition embodies
the concept of migration of specific, organ-forming areas. We may restate the
concept involved in this definitioq^ in a way which includes invertebrates as
well as vertebrates as Gastrulation is the dynamic process during
 
which the major, presumptive \rgan-forming areas of the blastula (Chaps.
6 and 7) become rearranged and reorganized in a way which permits their
ready conversion into the body plan of the particular species. That is to say,
during the process of gastrulation, the presumptive organ-forming areas of
the blastula undergo axiation in terms of the body organization of the species.
In some animal species, this reorganization of the blastula into the structural
 
 
 
DEFINITIONS AND CONCEPTS
 
 
391
 
 
pattern of the gastrula results in the production of a two-layered form, for
example, as in Amphioxus; in others (actually in most metazoan species) it
brings about the formation of a three-layered condition^ It is apparent, therefore, as observed by Pasteels (’37b, p. 464), that “it is impossible to give a
general definition of the gastrula stage.” It is obvious, also, that one cannot
define gastrulation in terms of simple invagination, delamination, or the production of a two-layered condition. Many processes, involving intricate movements of cell groups, occur as outlined in the succeeding pages of this chapter.
 
Relative to the process of gastrulation and later development, emphasis
should be placed upon the importance of the blastocoel. The latter takes its
origin largely by the movement of groups of cells in relation to one another
during cleavage and blastulation. Therefore, we may enumerate the following events related to the blastocoel during the early phases of embryonic
development:
 
(1) The blastocoel is associated with those movements in the developing
blastula which produce the specific cellular configuration of the mature blastula;
 
(2) during gastrulation, it enables the various, presumptive organ-forming
areas of the blastula to be rearranged and to migrate into the particular
areas which permit their ready organization and axiation into the
scheme of the body form of the particular species; and
 
(3) in the period of development immediately following gastrulation, it
affords the initial space necessary for the tubulation of the major,
organ-forming areas.
 
The events mentioned in (3) will be described in Chapter 10.
 
2. Primitive Veiitebrate Body Plan in Relation to the
Process of Gastrulation
 
In the animal kingdom, each of the major animal groupings has a specific
body plan. In the phylum, Chordata, the cephalochordate, Amphioxus, and
the vertebrates possess such a plan. It is necessary at this point to review
briefly the rudiments of this primitive or basic body plan.
 
a. Fundamental Body Plan of the Vertebrate Animal
 
The vertebrate body essentially is a cylindrical structure with a head or
cephalic end, a middle trunk region, and a tail or caudal end. The dorsum
or dorsal region is the uppermost aspect, while the venter or belly lies below.
Also, the body as a whole may be slightly compressed laterally. Viewed in
transverse section, the body is composed basically of five hollow tubes, particularly in the trunk area. The epidermal tube forms the exterior and within
the latter are placed the neural, enteric, and two mesodermal tubes, all oriented
around the median skeletal axis or notochord as indicated in figures 188C
and 217G and N.
 
 
 
392
 
 
GASTRUtATION
 
 
b. The Gastrula in Relation to the Primitive Body Plan
 
If one watches a large transport plane preparing to take off at an airfield,
the following events may be observed:
 
( 1 ) The cargo and passengers are boarded, the engines are warmed, and
the plane is taxied toward the runway.
 
(2) Upon reaching the starting end of the runway, the engines are accelerated, and the plane is turned around and headed in the direction
of the take-off.
 
 
 
Fig. 188. Relationship between the presumptive organ-forming areas of the blastula
(diagram A) and the primitive tubular condition of the developing vertebrate body
(diagram C). The gastrula (diagram B) represents an intermediate stage. Consult chart
in text.
 
 
 
GENERAL PROCESSES
 
 
393
 
 
(3) The engines are further accelerated and the plane is moved down the
runway for the take-off into the airy regions.
 
Similarly, during cleavage and blastulation, the embryonic machine develops
a readiness, elaborates the major, organ-forming areas in their correct positions in the blastula, and taxies into position with its engines warming up, as
it were. Once in the position of the mature blastula, the various, major, presumptive organ-forming areas are turned around and reoriented by the gastrulafive processes, and thus, each major, organ-forming area of the gastrula is
placed in readiness for the final developmental surge which results in primitive
body formation. During the latter process the major, presumptive organforming areas in the vertebrate group are molded into the form of elongated
tubular structures with the exception of the notochordal area which forms an
elongated skeletal axis. (The latter phenomena are described in Chapter 10.)
 
c. Chart of Blastula, Gastrula, and Primitive Body-form Relationships
in the Vertebrate Group
(Fig. 188)
 
The major, presumptive organ-forming areas are designated by separate
numerals.
 
 
Blastula
 
 
Gastrula
 
 
Primitive Body Form
 
 
1. Epidermal crescent
 
2. Neural crescent
 
3. Entodermal area
 
4. Two «nesodermal
areas
 
5. Notochordal crescent
 
 
1. Part of ectodermal layer
 
2. Elongated neural plate a
part of ectoderm layer
 
3. Primitive archenteron in
rounded gastrulae, such as
frog; archenteric layer in
flattened gastrulae, such as
chick
 
4. Two mesodermal layers on
either side of notochord
 
 
5. Elongated band of cells
lying between mesodermal
layers
 
 
1. External epidermal tube
 
2. Dorsally placed neural tube
 
3. Primitive gut tube
 
 
4. Two primitive mesodermal
tubes; one along either side
of neural tube, notochord,
and gut tube; especially true
of trunk region
 
5. Rounded rod of cells lying
below neural tube and
above entodermal or gut
tube; these three structures
lie in the meson or median
plane of the body
 
 
B. General Processes Involved in Gastrulation
 
Gastrulation is a nicely integrated, dynamic process; one which is controlled
largely by intrinsic (i.e., autonomous) forces bound up in the specific, physicochemical conditions of the various, presumptive, organ-forming areas of the
late blastula and early gastrula. These internal forces in turn are correlated
 
 
 
 
 
 
394
 
 
GASTRULATION
 
 
with external conditions. One of the important intrinsic factors involves the
so-called organization center referred to in Chapter 7. However, before consideration is given to this center, we shall define some of the major processes
involved in gastrulation.
 
There are two words which have come into use in embryology relative to
the process of gastrulation, namely, epiboly and emboly. These words are
derived from the Greek, and in the original they denote motion, in fact, two
different kinds of motion. The word emboly is derived from a word meaning
to throw in or thrust in. In other words, it means insertion. The word epiboly,
on the other hand, denotes a throwing on or extending upon. These words,
therefore, have quite opposite meanings, but they aptly describe the general
movements which occur during gastrulation. If, for example, we consider
figure 169, these two words mean the following: All the presumptive organforming areas below line a-b in (C) during the process of gastrulation are
moved to the inside by the forces involved in emboly. On the other hand,
due to the forces concerned with epiboly, the presumptive organ-forming
materials above line a-b are extended upon or around the inwardly moving
cells.
 
Associated with the comprehensive molding processes of epiboly and emboly
are a series of subactivities. These activities may be classified under the following headings:
 
( 1 ) morphogenetic movement of cells,
 
(2) the organization center and its organizing influences, and
 
(3) chemodifferentiation.
 
C. Morphogenetic Movement of Cells
 
1.^ Importance of Cell Movements During Development
AND IN Gastrulation
 
The movement of cells from one place in the embryo to another to establish
a particular form or structure is a common embryological procedure. This
type of cell movement is described as a morphogenetic movement because it
results in the generation of a particular form or structural arrangement. It is
involved not only in the formation of the blastula where the movements are
slow, or in gastrulation where the cell migrations are dynamic and rapid, but
also in later development. (See Chap. 11.) In consequence, we may say that
cell migration is one of the basic procedures involved in tissue and organ
formation.
 
The actual factors — physical, chemical, physiological, and mechanical —
which effect cell movements are quite unknown. However, this lack of knowledge is not discouraging. In fact, it makes the problem more interesting, for
cells are living entities utilizing physicochemical and mechanical forces peculiar
 
 
 
MOVEMENT OF CELLS
 
 
395
 
 
to that condition which we call living. The living state is a problem which
awaits solution.
 
At the period when the process of blastulation comes to an end and the
process of gastrulation is initiated, there is an urge directed toward cell movement throughout the entire early gastrula. Needham (’42, p. 145) uses the
term “inner compulsion” to describe the tendency of the cells of the dorsal-Up
area to move inward (invaginate) at this time. Whatever it is called and
however it may be described, the important feature to remember is that this
tendency to move and the actual movement of the cells represent a living
process in which masses of cells move in accordance with the dictates of a
precise and guiding center of activity, known as the primary organizer or
organization center.
 
2. Types of Cell Movement During Gastrulation
 
The following types of cell movement are important aspects of the process
of gastrulation.
 
 
a. Epiboly
 
( 1 ) Extension along the antero-posterior axis of the future embryo.
 
(2) Peripheral expansion or divergence.
 
b. Emboly
 
(1) Involution.
 
(2) Invagination.
 
(3) Concrescence (probably does not occur).
 
(4) Convergence.
 
(5) Polyinvagination.
 
(6) Delamination.
 
(7) Divergence or expansion.
 
(8) Extension or elongation.
 
(9) Blastoporal constriction.
 
Note: While cell proliferation is not listed as a specific activity above, it is
an important aspect of gastrulation in many forms.
 
3. Description of the Processes Concerned with Epiboly
 
Epiboly or ectodermal expansion involves the movements of the presumptive epidermal and neural areas during the gastrulative process. The
general migration of these two areas is in the direction of the antero-posterior
axis of the future embryonic body in ail chordate embryos. In the rounded
blastula (e.g., frog, Amphioxus, etc.), the tendency to extend antero-posteriorly
produces an enveloping movement in the antero-posterior direction. As a
result, the presumptive epidermal and neural areas actually engulf and surround the inwardly moving presumptive notochordal, mesodermal, and ento
 
 
396
 
 
GASTRULATION
 
 
dermal areas. (Study fig. 190A-H.) In flattened blastulae the movements of
epiboly are concerned largely with antero-posterior extension, associated with
peripheral migration and expansion of the epidermal area. (See fig. 202.) The
latter movement of the presumptive epidermal area is pronounced in teleost
fishes, where the yolk is engulfed as a result of epidermal growth and expansion (figs. 210B; 21 ID).
 
The above-mentioned activities, together with cell proliferation, effect
spatial changes in the presumptive epidermal and neural areas as shown in
figures 189, 190, 191, 198, and the left portion of figure 202A-I. It is to be
observed that the epidermal crescent is greatly expanded, and the area covered is increased; also, that the neural crescent is changed into a shield-shaped
area, extended in an antero-posterior direction (figs. 192A; 2021).
 
4. Description of the Processes Involved in Emboly
 
While forces engaged in epiboly are rearranging the presumptive neural
and epidermal areas, the morphogenetic movements concerned with emboly
move the presumptive chordamesodermal and entodermal areas inward and
extend them along the antero-posterior axis of the forming embryo. This inward movement of cells is due to innate forces within various cell groups;
some apparently are autonomous (i.e., they arise from forces within a particular cell group), while others are dependent upon the movement of other
cell groups. ) (See p. 447.) We may classify the types of cell behavior during
this migration and rearrangement of the chordamcsoderm-entodermal areas
as follows:
 
 
a. Involution and Convergence
 
Involution is a process which is dependent largely upon the migration of
cells toward the blastoporal lip (e.g., frog, see heavy arrows, fig. 192) or
to the primitive streak (e.g., bird, see arrows, fig. 204C-E). The word involution, as used in gastrulation, denotes a “turning in” or inward rotation of
cells which have migrated to the blastoporal margin. In doing so, cells located
along the external margin of the blastoporal lip move over the lip to the inside
edge of the lip (see arrows, figs. 191C-E, H; 192B, C). The inturned or involuted cells thus are deposited on the inside of the embryo along the inner
margin of the blastopore. The actual migration of cells from the outside surface
of the blastula to the external margin of the blastoporal lip is called convergence. In the case of the primitive streak of the chick, the same essential
movements are present, namely, a convergence of cells to the primitive streak
and then an inward rotation of cells through the substance of the streak to
the inside (arrows, fig. 204; black arrows, fig. 202). If it were not for the
process of involution, the converging cells would tend to pile up along the
outer edges of the blastoporal lip or along the primitive streak. Involution
 
 
 
MOVEMENT OF CELLS
 
 
397
 
 
thus represents a small but extremely important step in the migration of cells
from the exterior to the interior during gastrulation.
 
b. Invagination
 
The phenomenon of invagination, as used in embryological development,
implies an infolding or insinking of a layer of cells, resulting in the formation
of a cavity surrounded by the infolded cells (figs. 189, 190, the entoderm).
Relative to gastrulation, this process has two aspects:
 
(1 ) mechanical or passive infolding of cells, and
 
( 2 ) active inward streaming or inpushing of cells into the blastocoelic space.
 
In lower vertebrates, the dorsal-lip area of the blastopore is prone to exhibit
the active form of invagination, whereas the entoderm of the lateral- and
ventral-lip regions of the blastopore tends to move in a passive manner. The
notochordal-canal, primitive-pit area of the primitive streak of higher vertebrates is concerned especially with the active phase of invagination.
 
c. Concrescence
 
This term is used in older descriptions of gastrulation. The word denotes
the movement of masses of cells toward each other, particularly in the region
of the blastopore, and implies the idea of fusion of cell groups from two
bilaterally situated areas. It probably does not occur, (However, see development of the feather in Chap. 12.)
 
d. Cell Proliferation
 
An increase in the number of cells is intimately concerned with the process
of gastrulation to the extent that gastrulation would be impeded without it,
in some species more than in others. Cell proliferation in Amphioxus, for
example, is intimately associated with the gastrulative process, whereas in the
frog it assumes a lesser importance.
 
e. Polyinvagination
 
Polyinvagination is a concept which implies that individual or small groups
of cells in different parts of the external layer of the blastula or blastodisc
invaginate or ingress into the segmentation (blastocoelic) cavity. That is,
there are several different and separate inward migrations of one or more cells.
This idea recently was repudiated by Pasteels (’45) relative to the formation
of the entodermal layer in the avian blastoderm. It applies, presumably, to the
ingression of cells during the formation of the two-layered blastula in the
prototherian mammal, Echidna (see p. 364).
 
/. Ingression
 
The word ingression is suitable for use in cases where a cell or small
groups of cells separate from other layers and migrate into the segmentation
 
 
 
398
 
 
GASTRULATION
 
 
cavity or into spaces or cavities developed within the developing body. In the
primitive-streak area of reptiles, birds, and mammals, for example, mesodermal cells detach themselves from the primitive streak and migrate into the
space between the epiblast and hypoblast. Also, in the formation of the twolayered embryo in the prototherian mammal. Echidna, the inward migration
of small entodermal cells to form the hypoblast may be regarded as cellular
ingrcssion (fig. 175D). Ingression and polyinvagination have similar meanings.
 
g. Delamination
 
The word delamination denotes a mass sunderance or separation of groups
of cells from other cell groups. The separation of notochordal, mesodermal,
and entodermal tissues from each other to form discrete cellular masses in
such forms as the teleost fish or the frog, after these materials have moved
to the inside during gastrulation, is an example of delamination (fig. 210E, F).
 
h. Divergence
 
This phenomenon is the opposite of convergence. For example, after cells
have involuted over the blastoporal lips during gastrulation, they migrate and
diverge to their future positions within the forming gastrula. This movement
particularly is true of the lateral plate and ventral mesoderm in the frog, or
of lateral plate and extra-embryonic mesoderm in the reptile, bird, or mammal
(fig. 192B, C, small arrows).
 
/. Extension
 
The elongation of the presumptive neural and epidermal areas externally
and of the notochordal, mesodermal, and entodermal materials after they have
moved inward beneath the neural plate and epidermal material are examples
of extension. The extension of cellular masses is a prominent factor in gastrulation in all Chordata from Amphioxus to the Mammalia. In fact, as a
result of this tendency to extend or elongate on the part of the various cellular
groups, the entire gastrula, in many instances, begins to elongate in the anteroposterior axis as gastrulation proceeds. The faculty for elongation and extension is a paramount influence in development of axiation in the gastrula and
later on in the development of primitive body form. The presumptive notochordal material possesses great autonomous powers for extension, and hence
during gastrulation it becomes extended into an elongated band of cells.l^"^
 
D. The Organization Center and Its Relation to the Gastrulative Process
 
1. The Organization Center and the Primary Organizer
 
Using a transplantation technic on the beginning gastrula of the newt, it was
shown by Spemann (T8) and Spemann and Mangold (’24) that the dorsallip region of the blastopore (that is, the chordamesoderm-entoderm cells in
this area), when transplanted to the epidermal area of another embryo of the
 
 
 
THE ORGANIZATION CENTER
 
 
399
 
 
same stage of development, is able to produce a secondary gastrulative process
and thus initiate the formation of a secondary embryo (fig. 193). Because
the dorsal-lip tissue was able thus to organize the development of a second
or twin embryo, Spemann and Mangold described the dorsal-lip region of the
beginning gastrula as an “organizer” of the gastrulative process. In its normal
position during gastrulation this area of cells has since been regarded as the
organization center of amphibian development. It is to be observed in this
connection that Lewis ( ’07 ) performed the same type of experiment but failed
to use an embryo of the same age as a host. As he used an older embryo, the
notochordal and mesodermal cells developed according to their presumptive
fate into notochordal and somitic tissue but failed to organize a new embryo.
 
More recent experiments upon early frog embryos by Vintemberger (’36)
and by Dalcq and Pasteels (’37), and upon early teleost fish embryos by other
investigators (Oppenheimer, ’36 and ’47) have demonstrated the necessity
and importance of yolk substance in the gastrulative process. This fact led
Dalcq and Pasteels (’37) to suggest a new concept of the organization center,
namely, that this center is dependent upon two factors: “the yolk and something normally bound to the gray crescent” (i.e., chordamesodermal area).
 
It was thought at first that the transplanted organizer material actually organized and produced the new embryo itself (Spemann, ’18, p. 477). But this
idea had to be modified in the light of the following experiment by Spemann
and Mangold (’24): Dorsal-lip material of unpigmented Triton cristatus was
transplanted to an embryo of T. taeniatus of the same age. The latter species
is pigmented. This experiment demonstrated that the neural plate tissue of
the secondary embryo was almost entirely derived from the host and not from
the transplanted tissue. Consequently, this experiment further suggested that
the organizer not only possessed the ability to organize but also to induce host
tissue to differentiate. Induction of neural plate cells from cells which ordinarily v'ould not produce neural plate tissue thus became a demonstrated fact.
 
The concept of an organizer in embryonic development had profound implications and stimulated many studies relating to its nature. Particularly,
intensive efforts were made regarding the kinds of cells, tissues, and other
substances which would effect induction of secondary neural tubes. The results of these experiments eventually showed that various types of tissues and
tissue substances, some alive, some dead, from many animal species, including
the invertebrates, were able to induce amphibian neural plate and tube formation. (See Spemann, ’38, Chap. X and XI; also see fig. 196A, B and compare
with fig. 193.) Moreover, microcautery, fuller’s earth, calcium carbonate, silica,
etc., have on occasion induced neural tube formation. However, the mere induction of neural tube development should not be confused with the organizing
action of normal, living, chordamesoderm-entoderm cells of the dorsal-lip
region of the beginning gastruIayThe latter’s activities are more comprehensive,
for the cells of the dorsal-lip area direct and organize the normal gastrulative
 
 
 
400
 
 
GASTRULATION
 
 
process as a whole and bring about the organization of the entire dorsal axial
system of notochord, neural tube, somites, etc. In this series of activities, neural
plate induction and neural tube formation merely are secondary events of a
general organization process.
 
A clear-cut distinction should be drawn, therefore, between the action of
the dorsal-lip organizer, in its normal position and capacity,’ and that of an
ordinary inductor which induces secondary neural tube development. The
characteristics of the primary organizer or organization center of the early
gastrula are:
 
(a) its ability for autonomous or self-differentiation (that is, it possesses the
ability to give origin to a considerable portion of the notochord, prechordal plate material, and axial mesoderm of the secondary embryo),
 
(b) its capacity for self-organizatiotu
 
(c) its power to induce changes within and to organize surrounding cells,
including the induction and early organization of the neural tube.
 
As a result of its comprehensive powers, it is well to look upon the organization center (primary organizer) as the area which determines the main features of axiation and organization of the vertebrate embryo. In other words,
it directs the conversion of the late blast ula into the axiated gastrular condition
— a condition from which the primitive vertebrate body is formed. Induction
is a tool-like process, utilized by this center of activity, through which it effects
changes in surrounding cells and thus influences organization and differentiation. Moreover, these surrounding cells, changed by the process of induction,
may in turn act as secondary inductor centers, with abilities to organize specific
subareas.
 
An example of the ability of a group of cells, changed by inductive influence,
to act as an inducing agent to cause further inductive processes is shown by
the following experiment performed by O. Mangold (’32). The right, presumptive, half brain of a neurula of Ambystoma mexicanum, the axolotl, was
removed and inserted into the blastocoel of a midgastrula of Triton taeniatus.
Eight days after the implant was made, a secondary anterior end of an embryo
was observed protruding from the anterior, ventral aspect of the host larva.
An analysis of this secondarily induced anterior portion of an embryo demonstrated the following:
 
( 1 ) The original implant had developed into a half brain with one eye and
one olfactory pit. However,
 
(2) it also had induced a more or less complete secondary larval head
with a complete brain, two eyes, with lenses, two olfactory pits, one
ganglion, four auditory vesicles, and one balancer. One of the eyes had
become intimately associated with the eye of the implant, both having
the same lens.
 
 
 
IHfc. URUAINIZAIION CtlN 1 tK
 
 
HUl
 
 
The series of inductive processes presumably occurred as follows: The implanted half brain induced from the epidermis of the host a secondary anterior
end of a neural plate; the latter developed into a brain which induced the
lenses, auditory vesicles, etc. from the host epidermis. Thus, the original
implant, through its ability to induce anterior neural plate formation from
the overlying epidermis, acted as a “head organizer.”
 
The transformation of the late blastula into the organized condition of the
late gastrula thus appears to be dependent upon a number of separate inductions, all integrated into one coordinated whole by the “formative stimulus”
of the primary organizer located in the pre -chordal plate area of entodermalmesodermal cells and adjacent chordamesodermal material of the early gastrula.
 
2. Divisions of the Primary Organizer
 
The primary organizer is divisible into two general inductor areas as follows:
 
(a) the pre-chordal plate of entomesodermal material, and
 
(b) the chordamesodermal cells which come to lie posterior to the prechordal plate area of the late gastrula.
 
The pre-chordal plate is a complex of entodermal and mesodermal cells
associated at the anterior end of the notochordal cells in the late gastrula. In
the beginning gastrula, however, it lies between the notochordal material and
the dorsal-lip inpushing of the entoderm in amphibia, and just caudal to the
notochordal area in teleosts, elasmobranch fishes, reptiles, and birds (figs.
169; 173 A; 179B; 180B). The chordamesodermal portion of the primary
organizer is composed of presumptive notochordal cells and that part of the
presumptive mesoderm destined to form the somites. The pre-chordal plate is
known as the head organizer, because of its ability to induce brain structures
and other activities in the head region. (The use of the phrase head organizer
as a synonymous term for pre-chordal plate is correct in part only, for a
portion of the anterior notochord and adjacent mesoderm normally is concerned also with the organization of the head.) On the other hand, the presumptive notochord with the adjacent somitic (somite) material is described
as the trunk or tail organizer (fig. 19 IG) because of its more limited inductive
power. For example, Spemann (’31) demonstrated that the head organizer
transplanted to another host embryo of the same age produced a secondary
head with eye and ear vesicles when placed at the normal head level of the
host. Also when placed at trunk level, it induced a complete secondary embryo
including the head structures. However, the trunk organizer is able to induce
head and trunk structures at the head level of the host; but in the trunk region
it induces only trunk and tail tissues. (See Holtfreter, ’48, pp. 18-19; Needham,
’42, pp. 271-272; Spemann, ’31, ’38. The student is referred also to Huxley
and De Beer, ’34, Chaps. 6 and 7; and Lewis, ’07.)
 
 
 
402
 
 
GASTRULATION
 
 
£. Chemodifferentiation and the Gastrulative Process
 
In the previous chapter it was observed that certain areas of the amphibian
blastula are foreordained to give origin to certain organ rudiments in the future
embryo because of their position and not because of their innate physiological
condition. This condition is true of the future neural plate ectoderm and epidermal ectoderm. During the conversion of the late blastula into the late gastrula, these areas become changed physiologically, and they no longer are
determined in a presumptive sense but have undergone changes which make
them self-differentiating. This change from a presumptively determined condition to a self-differentiating, fixed state is called determination and the
biochemical change which effects this alteration is known as chemodifferentiation (see Chap. 8).
 
Chemodifferentiation is an important phenomenon during gastrulation. As
a result of the physiological changes involved in chemodifferentiation, restrictive changes in potency are imposed upon many localized cellular areas
within the major, organ-forming areas. In consequence, various future organs
and parts of organs have their respective fates rigidly, and irrevocably determined at the end of gastrulation. The gastrula thus becomes a loose mosaic
of specific, organ-forming areas (figs. 194, 205). Consequently, the areas of
the beginning gastrula which possess competence (Chap. 8) become more and
more restricted as gastrulation proceeds. Chemodifferentiation apparently occurs largely through inductive (evocative) action.
 
F. Gastrulation in Various Chordata
 
1. Arnphioxus
a. Orientation
 
Consult figures 167, 189, and 190 and become familiar with the animalvegetal pole axis of the egg, the presumptive organ-forming areas, etc.
 
b. Gastrulative Movements
 
1) Emboly. As gastrulation begins, a marked increase in mitotic activity
occurs in the cells of the dorsal crescent, composed of presumptive notochordal and neural plate cells, and also in the cells of the ventral crescent
or future mesodermal tissue. The general ectodermal cells or future epidermis
also are active (figs. 167, 189, 190B). The entodermal cells, however, are
quiescent (Conklin, ’32). Accompanying this mitotic activity, the entodermal
plate gradually invaginates or folds inwardly into the blastocoel (figs. 189,
190). In doing so, the upper portion of the entodermal plate moves inward
more rapidly and pushes forward toward a point approximately halfway between the polar body (i.e., the original midanimal pole of the egg) and the
point which marks the anterior end of the future embryo (observe pointed
end of arrow, fig. 189). Shortly after the inward movement of the entodermal
 
 
 
GASTRULATION IN VARIOUS CHORDATA
 
 
403
 
 
plate is initiated, notochordal cells in the middorsal region of the blastopore
involute, move inward along with the entoderm, and come to occupy a position
in the middorsal area of the forming archenteron (fig. 190C-E). Similarly,
mesodermal cells in the upper or dorsal ends of the mesodermal crescent
gradually converge dorso-mediad and pass into the roof of the forming gastrocoel (archenteron) on either side of the median area occupied by the
notochordal cells (fig. 190F, G). Thus the roof of the gastrocoel is composed
of notochordal and mesodermal cells (fig. 195 A, B).
 
2) Epiboly. As the above events come to pass, the potential epidermal
and neural cells proliferate actively, and both areas gradually become extended
in an antero-posterior direction. In this way the neural ectoderm becomes
elongated into a median band which lies in the middorsal region of the gastrula
(figs. 190A-H; 247B-F), while the epidermal area covers the entire gastrula
externally with the exception of the neural area.
 
Thus, the general result of this proliferation, infolding, and involution of
the presumptive entodermal, notochordal, and mesodermal cells, together
with the extension and proliferation of the ectodermal cells is the production
of a rudimentary double-layered embryo or gastrula (figs. 189, 190). Ectodermal cells (epidermal and neural) form the external layer (fig. 190G).
The internal layer is composed of notochordal cells in the dorso-median area
with two narrow bands of mesodermal cells lying along either side of the
median notochordal band of cells while the remainder of the internal layer
is composed of entodermal cells (figs. 190G; 195 A, B). At the blastoporal
end of this primitive gastrula are to be found proliferating notochordal, mesodermal, entodermal, and ectodermal cells.
 
3) Antero-posterior Extension of the Gastrula and Dorsal Convergence of
the Mesodermal Cells, The processes associated with epiboly bring about an
antero-posterior extension of the ectodermal layer of cells. Similarly, the cells
which arc moved inward by embolismic forces are projected forward toward
the future cephalic end of the embryo and become extended along the median
embryonic axis. Epiboly and emboly, accompanied by rapid cell proliferation
at the blastoporal-lip area, thus effect an antero-posterior elongation of the
developing gastrula (figs. 189H; 190H).
 
As the gastrula is extended in the antero-posterior direction, a shift occurs
in the position of the mesodermal cells which form the ventral or mesodermal
crescent. The ventral crescent becomes divided ventrally into two halves, and
each half gradually moves dorsalward along the inner aspect of the lateral
blastoporal lips as gastrulation is accomplished. Each arm of the original
crescent in this manner converges dorso-mediad toward the median notochordal cells of the dorsal blastoporal lip, and a mass of mesodermal cells
comes to lie along either side of the notochordal cells. As a result of this
converging movement, entodermal cells of the blastoporal area converge dorso
 
 
GASTRULATION
 
 
mediad and come to occupy the ventral lip of the blastopore, together with
the externally placed, epidermal cells (fig. 190G, arrow). The blastopore as
a whole grows smaller and moves to a dorsal position during the latter changes
(fig. 247 A-C).
 
4) Closure of the Blastopore. See Chapter 10, neuralization in Amphioxus.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
DORSAL LIP OF BL
 
 
 
 
m
 
 
1 ^.
 
•^j
 
 
 
 
sM!
 
 
 
 
 
 
Fig. 189. Gastrulation in Amphioxus. (Modified from Conklin, ’32.) (A) Beginning
 
gastrula. (B) Observe that entodermal (hypoblast layer) is projected roughly in direction of future cephalic end of embryo. (C G) Observe continued projection of entoderm
toward cephalic end of future embryo. Note also position of polar body. In (F), (G),
and (H) the gastrula begins to elongate along the antero-posterior axis of the developing
embryo. (H) End of gastrular condition. Blastopore is closed by epidermal overgrowth,
and neurenteric canal is formed between archenteron and forming neural tube.
 
 
 
 
 
 
406
 
 
GASTRULATION
 
 
of gastrulation, although mitoses occur in other regions as well. During
later stages of gastrulation, the entire complex of cells around the
blastoporal region divides actively.
 
(c) Involution. Notochordal cells converge to the midregion of the dorsal
blastoporal lip and then turn inward (involute) over the lip area to
the inside.
 
(d) Extension. General elongation of the embryonic rudiment as a whole
occurs, including the neural plate area.
 
(e) Convergence. Mesodermal cells converge toward the middorsal area
of the blastopore. The path of this convergence is along the lateral
lips of the blastopore, particularly the inner aspects of the lips. This
movement is pronounced toward the end of gastrulation when each
half of the mesodermal crescent moves dorsad toward the middorsal
area of the blastopore. The mesoderm thus comes to lie on either
side of the notochordal material at the dorsal lip of the blastopore.
 
(f) Constriction of the blastopore. During later phases of gastrulation, the
blastopore grows smaller (fig. 247A-D), associated with a constriction
of the marginal region of the blastoporal opening, particularly of the
entodermal and epidermal layers. The movement of the mesoderm described in (e) above plays a part in this blastoporal change.
 
2) Epiboly. The caudal growth of the entire ectodermal layer of cells, epidermal and neural, and their antero-posterior extension is a prominent feature
of gastrulation in Amphioxus.
 
(Further changes in the late gastrula, together with the closing of the
blastopore, are described in Chapter 10. See tubulation of neural plate, etc.)
 
2. Gastrulation in Amphibia with Particular Reference
TO THE Frog
 
a. Introduction
 
1) Orientation. A line drawn from the middle region of the animal pole to
the midvegetal pole constitutes the median axis of the egg. In the anuran
Amphibia the embryonic axis corresponds approximately to the egg axis. That
is, the midanimal pole of the egg represents the future anterior or anterodorsal end of the embryo, while the midvegetal pole area denotes the posterior
region.
 
As indicated previously (Chap. 7), the very late blastula is composed of
presumptive organ-forming areas arranged around the blastocoelic space. The
yolk-laden, future entodermal cells of the gut or digestive tube form the
hypoblast and are concentrated at the vegetal pole. Presumptive notochordal
and mesodermal cells constitute a marginal zone of cells which surrounds
the upper region of the presumptive entodermal organ-forming area (fig.
169C-F). The presumptive notochordal area is in the form of a crescent.
 
 
 
GASTRULATION IN VARIOUS CHORDATA
 
 
407
 
 
whose midportion is located just above the future dorsal lip of the early gastrula, while the mesoderm lies to each side of the notochordal cells, extending
along the margin of the entoderm toward the corresponding mesodermal zone
of the other side (fig. 169D, F). The presumptive neural crescent occupies
a region just dorsal and anterior to the notochordal area. The remainder of
the animal pole is composed of presumptive epidermis. The presumptive
notochordal, neural plate, and epidermal areas are oriented along the general
direction of the future antero-posterior embryonic axis, the notochordal tissue
being the more posterior. Moreover, the midregion of the notochordal and
neural crescents at this time lies at the dorsal region of the future embryo
(fig. 194A). The presumptive entodermal area, on the other hand, does not
have the same orientation as that of the above areas. In contrast, its axiation
is at right angles to the future embryonic axis (fig. 194A). If one views a
very early gastrula of the anuran amphibian in such a way that the beginning
blastoporal lip is toward the right (fig. 194A), then:
 
( 1 ) The foregut material lies toward the right at the region of the forming
blastoporal lip;
 
(2) the stomach material is slightly to the left of this area; and
 
(3) the future intestinal area lies to the left and toward the vegetal pole.
 
Therefore, one aspect of the gastrulative processes is to bring the entodermal area into harmony with the future embryonic axis and, in doing so,
to align its specific, organ-forming subareas along the antero-posterior axis
of the embryo. In other words, the entodermal material must be revolved
about 90 degrees in a counterclockwise direction from the initial position
occupied at the beginning of gastrulation (compare fig. 194A, B).
 
2) Physiological Changes Which Occur in the Presumptive Organ-forming
Areas of the Late Blastula and Early Gastrula as Gastrulation Progresses.
A striking physiological change is consummated in the presumptive organforming areas of the epiblastic portion of the late blastula during the process
of gastrulation. This change has been demonstrated by transplantation experiments. For example, if presumptive epidermis of the very late blastula
and early gastrula is transplanted by means of a micropipette to the presumptive neural area and vice versa, the material which would have formed
epidermis will form neural tissue, and presumptive neural cells will form epidermis (fig. 196C, D). (See Spemann, M8, ’21; Mangold, ’28.)
 
\ The experiment pictured in figure 196 involves interchanges between two
presumptive areas within the same potential germ layer, i.e., ectoderm. However, Mangold (’23) demonstrated that presumptive epidermis transplanted
into the dorsal-lip area, i.e., into the presumptive mesodermal area, may invaginate and form mesodermal tissue. The converse of this experiment was
performed by Lopaschov (’35) who found that presumptive mesoderm from
the region of the blastoporal lip transplanted to the neural plate area of a
 
 
 
408
 
 
GASTRULATION
 
 
somewhat older embryo becomes, in some cases, normally incorporated in
the neural tube of the host. Similar interchanges of cells of the late blastula
have demonstrated that almost any part, other than the presumptive entoderm,
can be interchanged without disturbing the normal sequence of events. However, as gastrulation progresses, interchange from epidermal to neural areas
continues to be possible during the early phases of gastrulation (fig. 196C, D)
but not at the end of gastrulation. Similar changes occur also in the mesodermal
area. Pronounced physiological changes thus occur in the presumptive organforming aieas of the entire epiblastic region during gastrulation.
 
b. Gastrulation
 
1) Emboly. As gastrulation begins, a small, cleft-like invagination appears
in the entodermal material of the presumptive foregut area. This invagination
is an active inpushing of entodermal cells which fold inward and forward
toward the future cephalic end of the embryo (fig. 191B-E). The upper or
dorsal edge of the cleft-like depression visible at the external surface forms
the dorsal lip of the blastopore (fig. 19 IB). In this connection, study diagrams in figure 197. The pre-chordal plate cells are associated with the forming dorsal roof of the archenteron and, therefore, form a part of the invaginated material shortly after this process is initiated.
 
As the entodermal material migrates inward and the initial dorsal lip is
formed, notochordal cells move posteriad to the dorsal lip and involute to
the inside in close association with the pre-chordal plate cells. Also, the more
laterally situated, notochordal material converges toward the dorsal lip and
gradually passes to the inside, as gastrulation progresses, where it lies in the
mid-dorsal region of the embryo. (See arrows, figs. 188 A; 19 1C, D). Here it
begins to elongate antero-posteriorly (i.e., it becomes extended) and forms a
narrow band of cells below the forming neural plate (fig. 191C-G).
 
With the continuance of gastrulation, the entodermal material moves more
extensively inward (cf. fig. 191C-E) and the entodermal mass of yolk-laden
cells below the site of invagination begins to sink or rotate inwardly. The
dorsal blastoporal lip, therefore, widens considerably (fig. 197 A, B). In many
Amphibia the inner surface of the entoderm, as it progresses inward, forms
a cup-like structure which actually engulfs the blastocoelic fluid (fig. 191B-D).
It is not clear whether this cup-like form is produced by active inward migration of entodermal cells or whether it may be due in part, at least, to
constrictive forces at the blastoporal lip.
 
Synchronized with the events described above, the presumptive somitic
mesoderm, located externally along either side of the notochordal area of the
early gastrula, migrates (converges) toward the forming dorso-lateral lips of
the blastopore (fig. 197 A, B, broken arrows). Upon reaching the blastoporal
edge, the mesoderm moves over the lip (involutes) to the inside. However,
the mesoderm does not flow over the lip to the inside as a part of the entoderm
 
 
 
 
 
 
410
 
 
GASTRULATION
 
 
Coincident with the lateral extensions of the original dorsal lip of the blastopore to form the lateral lips, a more extensive convergence and involution of
presumptive mesoderm located in the lateral portions of the mesodermal crescent occurs (fig. 197 A, B). The latter mesoderm eventually forms the lateral
area of the hypomeric mesoderm of the future embryo (figs. 191G; 198B,
C). As the lateral lips of the blastopore continue to form in the ventral direction, they eventually reach a point where they turn inward toward the median
axis and thus form the ventral lip of the blastopore (fig. 197C). A rounded
blastopore, circumscribing the heavily, yolk-laden, entodermal cells, thus is
formed. Associated with the formation of the ventro-lateral and ventral blastoporal lips is the convergence and involution of the ventro-lateral and ventral
mesoderm of the gastrula (fig. 191D-F). Accompanying the inward migration
of the entoderm in the region of the dorso-lateral lip of the blastopore, there
is, presumably, an inward rotation of the entodermal mass which lies toward
the ventral blastoporal area. The result of this entodermal movement is the
production of a counterclockwise rotation of the entodermal, organ-forming
rudiments, as indicated in figure 194B, compared to their relative positions
at the beginning of gastrulation, shown in figure 194A. (This counterclockwise
rotation is present to a degree also in Amphioxus (fig. 190A-F). In this way,
the particular, organ-forming areas of the entoderm become arranged anteroposteriorly in a linear fashion along the embryonic axis. The foregut material
now is situated toward the anterior end of the developing embryo, while the
stomach, liver, small intestine, and hindgut regions are placed progressively
posteriad with the hindgut area near the closing blastopore (fig. 194B). The
yolk material lies for the most part within the ventral wall of this primitive
archenteron.
 
Associated with the axiation of the entodermal rudiments is the axiation
of the notochord-mesoderm complex. For example, the anterior segment of
the notochord and the pre-chordal plate (i.e., the head organizer) are located anteriorly in the gastrula, while the more posterior portions of the
notochord and adjacent mesoderm (i.e., the trunk organizer) are located in
the developing trunk region (fig. 19 IG). The mesoderm adjacent to the notochord eventually will form the somites or primitive mesodermal segments of
the embryo (figs. 19 IG; 217E; 224F). Experimentation, using the Vogt
method of staining with vital dyes, has demonstrated that the future, anterior,
presumptive somites lie closer to the blastoporal lips in the beginning gastrula,
whereas the more posterior, presumptive somites are situated at a greater distance from the blastoporal area. Because of this arrangement, the first or anterior pair of presumptive somites moves inward first, the second pair next, etc.
The mesoderm of the future somites in this way is arranged along the notochord
in an orderly sequence from the anterior to the posterior regions of the gastrula (fig. 169, somites 1, 2, 3, 4, etc.). Consequently, axiation and extension
of the somitic mesoderm occur along with the antero-posterior arrangement of
 
 
 
GASTRULATION IN VARIOUS CHORDATA
 
 
411
 
 
the notochordal material. A similar distribution is effected in other regions
of the mesoderm. Therefore, axiation and antero-posterior extension of the
entoderrnal, notochordal, and mesodermal cells are conspicuous results of the
activities which effect emboly.
 
2) Epiboly. The above description is concerned mainly with emboly, that
is, the inward migration of the notochord-mesoderm-entoderm-yolk complex.
Allied with these active events is the downward or caudal migration of the
blastoporal lips. This migration is illustrated in figure 191B-E. In this figure
it may be observed that, as the marginal zone cells of mesoderm and notochord
 
 
 
Fig. 192. Movements of the parts of the blastula during gastrulation in amphibia. (Cf.
fig. 191.) (A) Results of epiboly. (Cf. fig. 19 1 A T.) Epidermal and neural areas envelop
 
the other areas during gastrulation. (B) Movements of the areas of the blastula during
emboly, as seen from the vegetative pole. Heavy arrows, solid and broken, show the
converging movements during emboly; light arrows show the extension and divergence
of cells after involution at the blastoporal margin (cf. fig. 191A~F). (C) Similar to
 
(B), as seen from the left side.
 
 
SECONDARY EMBRYO
 
 
 
Fig. 193. Induction of a secondary embryo. (From Spemann, ’38.) (A) Host embryo
 
shown in this figure is Triton taeniatus. A median piece of the upper lip of the blastopore
of a young gastrula of T. cristatus of approximately the same age as the host was implanted into the ventro-lateral ectoderm of the host. The implanted tissue developed into
notochord, somites, etc.; the neural tube was induced from the host ectoderm. (B) Cross
section through embryo shown in (A).
 
 
412
 
 
GASTRULATION
 
 
 
Fig. 194. Developmental tendencies of entodermal area and their reorientation during
gastnilation. (A) Developmental tendencies of entodermal area of young anuran gastrula. (B) Counterclockwise rotation of approximately 90° of the entodermal area
during gastrulation.
 
 
together with the entoderm and yolk pass to the inside, the forces involved
in epiboly effect the expansion of the purely ectodermal portion of the cpiblast
which gradually comes to cover the entire external surface of the gastrula with
the exception of the immediate blastoporal area (study black and white areas
in fig. 191A-E). It may be observed further that the neural crescent now is
elongated along the antero-posterior, embryonic axis where it forms a shieldshaped region with the broad end of the shield located anteriorly (fig. 192A).
 
A study of figure 191 E and F shows that a rotation of the entire gastrula
occurs in the interim between E and F. This rotation is induced by the inward
movement of the entoderm and yolk, depicted in figure 191C-E, with a
subsequent shift in position of the heavy mass of yolk from the posterior pole
of the embryo to the embryo’s ventral or belly region. Most of the blastocoel
and its contained fluid is “engulfed” by the inward moving entoderm, as indicated in figure 191C-E, some of the blastocoelic lluid and blastocoelic space
passes over into the gastrocoel. The region of the entodermal yolk mass shown
to the left in figure 19 IE, therefore, is more dense and heavier than the area
shown to the right. The heavier region of the gastrula seeks the lower level;
hence the rotation of the entire gastrula, and the new position assumed in
figure 191F.
 
As the blastopore progressively grows smaller, it eventually assumes a small,
rounded appearance (fig. 197A-E), and the remnants of the presumptive
mesoderm pass over the lips of the blastopore before it closes. In doing so, the
presumptive tail mesoderm converges dorsally and becomes located inside
the dorso-lateral portion of the closed blastopore near the lateral aspects of the
posterior end of the folding neural plate.
 
A short while previous to blastoporal closure, the midregion of the neural
plate area begins to fold ventrad toward the notochord, while its margins are
 
 
 
GASTRULATION IN VARIOUS CHORDATA
 
 
413
 
 
elevated and projected dorso-mediad. The exact limits of the neural plate thus
become evident (fig. 197D).
 
3) Embryo Produced by the Gastrulative Processes. The general result of
epiboly and emboly in the Amphibia is the production of an embryo of three
germ layers with a rounded or oval shape. The potential skin ectoderm and
infolding, neural plate area form the external layer (fig. 192A). Underneath
this external layer are the following structural regions of the middle or mesodermal layer:
 
(a) Below the developing nerve tube is the elongated band of notochordal
cells;
 
(b) on either side of the notochord is the somitic (somite) mesoderm;
 
(c) lateral to the somitic area is the mesoderm of the future kidney system;
and
 
 
 
Fig. 195. Placement of the presumptive, organ-forming areas in an embryo of Amphioxus of about six to seven somites. (Modified from Conklin, ’32.) (A) Section
 
through anterior region. (J) Section through caudal end of embryo. (B-1) Successive
sections going posteriorly at different body levels between (A) and (J).
 
 
 
414
 
 
GASTRULATION
 
 
SECONDARY NE UR A L I Z A T I 0 N SECONDARY NEUR AL I Z AT ION
 
 
 
Fig. 196. Ectodermal potencies of the amphibian gastrula. (A and B from Spemann,
’38, after Fischer; C and D from Spemann, ’38, after Spemann, ’18.) (A) Induction of
 
a secondary neural plate in the axolotl gastrula by five per cent oleic acid, emulsified in
agar-agar. (B) Induction of secondary neural plate by nucleic acid from calf thymus.
(C) Formation of neural plate tissue from presumptive epidermal cells transplanted into
neural plate region. (D) Reverse transplant, presumptive neural plate becomes epidermal tissue.
 
(d) still more lateral and extending ventrally are the lateral plate and ventral
mesoderm (figs. 191F-I; 198A-C; 221).
 
The third or inner germ layer of entoderm is encased within the mesodermal
or middle germ layer. The entodermal layer is an oval-shaped structure containing a small archenteric cavity filled with fluid. Its ventral portion is heavily
laden with yolk substance. Also, the future trunk portion of the archenteric
roof is incomplete, the narrow notochordal band forming a part of its middorsal area (figs. 19 IF; 194B; 219D). Within each of these germ layers are
to be found restricted areas destined to be particular organs. Each layer may
be regarded, therefore, as a general mosaic of organ-forming tendencies.
 
4) Position Occupied by the Pre-chordal Plate Material. Another feature
of the late gastrula remains to be emphasized, namely, the pre-chordal plate
composed of entodermal and mesodermal cells integrated with the anterior
end of the notochord. During gastrulation the pre-chordal plate invaginates
with the entoderm and comes to occupy the roof of the foregut, just anterior
to the rod-like notochord (fig. 191D-F). In this position it lies below the
anterior part of the neural plate area; it functions strongly in the induction
and formation of the cephalic structures, including the brain as indicated above.
Because of this inductive ability, it is regarded as a principal part of the head
 
 
 
GASTRULATION IN VARIOUS CHORDATA
 
 
415
 
 
organizer (fig. 191E-G). Eventually pre-chordal plate cells contribute to the
pharyngeal area of the foregut and give origin to a portion of the head mesoderm, at least in many vertebrate species (Chap. 11, p. 523).
 
c. Closure of the Blastopore and Formation of the Neurenteric Canal
 
The closure of the blastopore and formation of the neurenteric canal is
described in Chapter 10, p. 471.
 
d. Summary of Morphogenetic Movements of Cells During Gastrulation
 
in the Frog and Other Amphibia
 
1) Emboly:
 
(a) Invagination. Invagination in the Amphibia appears to consist of two
phases: (1) an active infolding or forward migration of the future
foregut, stomach, etc., areas, and (2) an insinking and inward rotation
of future intestinal and heavily laden, yolk cells.
 
(b) Convergence. This activity is found in the presumptive, notochordal
and mesodermal cells as they move toward the blastoporal lips. A
dorsal convergence toward the dorsal, blastoporal-lip area is particularly true of the more laterally placed parts of the notochordal crescent
and to some extent also of the somitic and lateral plate mesoderm.
The tail mesoderm tends to converge toward the dorsal blastoporal
area when the blastopore nears closure.
 
(c) Involution. An inward rolling or rotation of cells over the blastoporal
lips to the inside is a conspicuous part of notochordal and mesodermal
cell migration.
 
(d) Divergence. After the mesodermal cells have migrated to the inside,
there is a particular tendency to diverge on the part of the lateral plate
and ventral mesoderm. The lateral plate mesoderm diverges laterally
and ventrally, while the ventral mesoderm diverges laterally in the
ventral or belly area of the gastrula.
 
(e) Extension. The phenomenon of extension or elongation is a characteristic feature of all gastrulative processes in the chordate group. Before arriving at the blastoporal lips, the converging notochordal and
mesodermal cells may undergo a stretching or extending movement.
That is, convergence and stretching are two prominent movements involved in the migration of the marginal zone or chordamesodermal
cells as they move toward the blastoporal lip. After these materials
have involuted to the inside, the chordal cells stretch antero-posteriorly
and become narrowed to a cuboidal band in the midline, and the
lateral plate mesoderm stretches anteriorly as it diverges laterally.
Antero-posterior extension of the somitic mesoderm also occurs.
 
(f ) Contractile tension or constriction. A considerable constriction or contraction around the edges of the blastopore occurs as gastrulation pro
 
 
416
 
 
GASTRULATION
 
 
gresses. This particularly is true when the blastopore gradually grows
smaller toward the end of the gastrulative process (Lewis, ’49).
 
2) Epiboly. Intimately associated with and aiding the above processes involved in emboly are the movements concerned with epiboly. These movements result from cell proliferation, associated with a marked antero-posterior
extension and expansion of the presumptive epidermal and neural plate areas.
These changes are integrated closely with the inward migration of cells of
the marginal zone (i.e., chordamesoderm ) , and the presumptive epidermal
and neural areas approach closer and closer to the blastoporal edge, until
finally, when mesodermal and notochordal cells have entirely involuted, ectodermal cells occupy the rim of the blastopore as it closes (figs. 192A; 220D).
 
 
 
Fig. 197. History of the blastopore and adjacent posterior areas of developing embryo
of the frog, Rana pipiens. (A) Dorsal lip of blastopore. Arrows show direction of
initial invagination to form the dorsal lip. (B) Dorso-lateral and lateral-lip portions of
the blastopore are added to original dorsal-lip area by convergence of mesodermal
cells (arrows) and their involution at the edge of the lip. Entodermal material is invaginating. (C) Blastopore is complete; yolk plug is showing. (D) Toward the end of
gastrulation. Blastopore is small; neural plate area becomes evident as neural folds begin
their elevation. (E) Neural folds are slightly elevated; blastopore is very small; size
of blastopore at this time is quite variable. (F) Blastopore has closed; neural folds are
well developed; neurenteric passageway between neural folds and dorsal evagination of
archenteric space into ttiil-bud area is indicated by N.C. (G) New caudal opening is
forming, aided by proctodaeal invagination, PR.; tail rudiment elevation is indicated.
(H) Proctodaeal opening and tail rudiment arc shown.
 
 
GASTRULATION IN VARIOUS CHORDATA
 
 
417
 
 
 
Fig. 198. Anterior extension (migration) of the mesoderm from the blastoporal-lip
area after involution at the lip in the urodele, Pleurodeles. (A-<^) Progressive inward
migration of the mantel of mesoderm, indicated by the white area stippled with coarse
dots. (A) Early gastrula. (B) Late gastrula. (C) Beginning neurula.
 
As a result, the presumptive epidermal and neural plate areas literally engulf
the inwardly moving cells.
 
3. Gastrulation in Reptiles
 
a. Orientation
 
The reptilian blastoderm, as gastrulation begins, is composed of an upper
epiblast and a lower hypoblast as indicated previously in Chapter 7 (fig.
174A-D). The formation of the hypoblast as a distinct layer proceeds in a
rapid fashion and immediately precedes the formation of a large notochordal
canal and subsequent cell migration inward. The two events of entodermal
layer (hypoblast) formation and the inward migration of notochordal and
mesodermal cells thus are closely and intimately correlated in reptiles. This
close relationship is true particularly of the turtle group. The upper layer or
epiblast of the reptilian blastoderm is a composite aggregation of presumptive
epidermal, neural, notochordal, and mesodermal cells (fig. 174E, F), arranged
in relation to the future, antero-posterior axis of the embryo. It is possible
that some entodermal material may be located superficially in the epiblast in
the turtle as gastrulation begins (Pasteels, ’37a).
 
b. Gastrulation
 
Immediately following the formation of the hypoblast, the gastrulative phenomena begin with a rather large inpushing or invagination involving the
notochordal, mesdoermal areas, particularly the pre-chordal plate and notochordal areas. This invagination extends downward and forward toward the
hypoblast along the antero-posterior embryonic axis, and it produces a pouchlike structure known variously as the notochordal canal, blastoporal canal, or
chordamesodermal canal (figs. 199A-C; 200A-C). The invaginated noto
 
418
 
 
GASTRULATION
 
 
 
Fig. 199, Surface views of blastoderm of the turtle, Chrysemys picta, during gastrulation. Darkened area in the center shows the embryonic shield, the region of the notochordal canal in the area of the primitive plate. (A) Young gastrula. External opening
of notochordal canal is wide. (B) Later gastrula. External opening of notochordal
canal is horseshoe-shaped; internal opening of canal is indicated by small crescentic light
area in front of external opening. (C) Very late gastrula. Notochord is indicated in
center; head fold is beginning at anterior extremity of blastoderm.
 
chordal canal reposes upon the entoderm, and both fuse in the region of contact (fig. 200C). The thin layer of cells in the area of fusion soon disappears,
leaving the antero-ventral end of the flattened notochordal canal exposed
to the archenteric space below. After some reorganization, the notochord
appears as a band, extending antero-posteriorly in the median line, associated
with the entoderm on either side (fig. 201B-G). However, at the extreme
anterior end of the gastrula, the notochordal material, together with the entoderm and to some extent the overlying ectoderm, presents a fused condition.
Within this area the pre-chordal plate or anterior portion of the head organizer
is located. In this general region of the embryo, foregut, brain, and other head
structures eventually arise (fig. 199C). The original, relatively large, notochordal invagination soon becomes a small canal which extends cranio-ventrally
 
 
 
GASTRULATION IN VARIOUS CHORDATA
 
 
419
 
 
from the upper or external opening to the archenteric space which lies below
the notochord and entoderm (fig. 200B, E).
 
Posterior to the opening of the notochordal canal is the thickened primitive
plate (primitive streak), composed of converged presumptive mesodermal
cells (fig. 199). This converged mass of cells involutes to the inside along the
lateral borders of the notochordal canal and also posterior to this opening.
However, most of the mesoderm of the future body of the embryo apparently
passes inward with the notochordal material during the formation of the notochordal canal, where it comes to lie on either side of the median notochordal
band between the ectoderm and the entoderm. These general relationships of
notochord, ectoderm, mesoderm, and entoderm are shown in figure 201A-H.
 
The extent to which the original notochordal inpushing is developed varies
in different reptilian species. In lizards and snakes its development is more
pronounced than in turtles (cf. fig. 200A, D).
 
During emboly, the presumptive neural plate and epidermal areas are
 
 
 
 
Fig. 200. Sagittal section of reptilian bIa.stoderms to show notochordal inpushing
(notochordal canal or pouch). (A) Section of early gastrulative procedure in Clernmys
leprosa. (After Pasteels, ’36b, slightly modified.) (B) Original from slide, Chrysemys
picta, showing condition after notochordal canal has broken through into archenteric
space. (C) Notochordal canal of the lizard, Platydactylus. (D) Later stage of (C).
(E) After notochordal canal has broken through into archenteric space. (OE, after
Will, 1892.)
 
 
 
420
 
 
GASTRULATION
 
 
A
 
 
B.
 
NEUR AL PLATE EPIDERMIS
 
 
NOTOCHORD
 
 
D.
 
INTERNAL OPENING OF
 
NOTOCHORDAL CANAL
 
 
NOTOCHORDAL CANAL
 
 
 
Fig. 201. Transverse sections of the late turtle gastrula as indicated by lines in fig. 199C.
 
elongated antero-posteriorly by the forces of epiboly. Meanwhile, the external
opening of the notochordal canal changes in shape and together with the
primitive plate moves caudally (fig. 199). As gastrulation draws to a close,
the neural plate area begins to fold inward, initiating the formation of the
neural tube.
 
 
 
 
 
 
4. Gastrulation in the Chick
a. Orientation
 
As described in Chapter 7, a twodayered blastoderm (blastula) composed
of an epiblast and a hypoblast is present, with the hypoblast more complete
at the posterior end of the blastoderm than at its extreme anterior and anterolateral margins (figs. 171A; 202A). The epiblast over the posterior half of
the blastoderm is composed of presumptive notochordal and mesodermal cells,
and anteriorly in the epiblast are found the presumptive epidermal and neural
areas (figs. 173 A; 202A).
 
 
b. Gastrulative Changes
 
1) Development of Primitive Streak as Viewed from the Surface of Stained
Blastoderms. The formation of the primitive streak is a progressive affair.
Figure 170 pictures a pre-streak blastoderm, and it is to be observed that the
entodermal layer below the epiblast is present as an irregular area in the
 
 
 
GASTRULATION IN VARIOUS CHORDATA
 
 
421
 
 
caudal region of the area pellucida. A median, sagittal section through a
comparable stage is shown diagrammatically in figure 171 A. Figure 203 A
illustrates an early beginning streak normally found eight hours after incubation of the egg is initiated, while figure 203B presents a medium streak, appearing after about 12 to 13 hours of incubation. In figure 203C, a definite
primitive streak appears in which the primitive groove, primitive pit, primitive
folds, and Hensen’s node (primitive knot) are outlined. This condition occurs
after about 18 to 19 hours of incubation. This may be regarded as the mature
streak. A later streak after about 19 to 22 hours of incubation is indicated
in figure 203 D. Observe that the head process or rudimentary notochord
extends anteriorly from Hensen’s node, while the mesoderm is a deeper-shaded
area emanating from the antero-lateral aspect of the streak. The clear proamnion region may be observed at the anterior end of the area pellucida. In the
proamnion area, mesoderm is not present at this time between the ectodermal
and entodermal layers.
 
2) Cell Movements in the Epiblast Involved in Primitive-streak Formation
as Indicated by Carbon-particle Marking and Vital-staining Experiments.
 
Recent experiments by Spratt (’46), using carbon particles as a marking device, have demonstrated that epiblast cells from the posterior half of the prestreak blastoderm gradually move posteriad and rnediad as gastrulation proceeds (figs. 202, 204, black arrows). Before the actual appearance of the
streak, mesodermal cells begin to appear between the epiblast and hypoblast
at the posterior margin of the area pellucida. (See fig. 202B, involuted mesoderm). As cellular convergence posteriorly toward the median line continues,
the primitive streak begins to form as a median thickening posteriorly in the
pellucid area (fig. 202C, observe posterior median area indicated in white).
The rudimentary primitive streak formed in this manner gradually advances
anteriorly toward the central region of the pellucid area of the blastoderm
(fig. 202D, E). In the thickened area of the developing primitive streak, shown
in white at the posterior median portion of the blastoderm in figure 202C,
there are about three to four cell layers of epiblast together with about the
same number of layers of mesoderm below. At its anterior end the streak is
thinner.
 
The anterior end of this early streak gradually grows forward as a result
of cell proliferation in situ and by cells added through convergence of cells
from antero-lateral areas (Spratt, ’46). Some of the cells at the anterior end
of the forming streak may involute or ingress from the epiblast into the space
between the hypoblast and epiblast and thus come to lie at the anterior end
of the forming streak, while other cells ingress laterally between these two
layers (fig. 202C-E, K-O).
 
As the streak differentiates anteriorly by addition of cells to its anterior
end, it also elongates posteriorly by cellular additions to its caudal end. The
carbon-marking experiments of Spratt demonstrated further that, during the
 
 
 
 
\22
 
 
GASTRULATION IN VARIOUS CHORDATA
 
 
423
 
 
formation of the streak up to about the condition present at 20 to 22 hours
of incubation (figs. 2021, K; 203D), almost the entire posterior half of the
pellucid area, consisting of presumptive pre-chordal plate, notochord, and
mesoderm, is brought into the streak and involuted to the inside between the
hypoblast and epiblast (figs. 202F-H; 204). This condition of development
is often referred to as the “head-process stage” (stage 5, Hamburger and Hamilton, ’51). At this stage the approximate, antero-posterior limits of the future
embryonic body of the chick, exclusive of the extra-embryonic tissue, are
shown by the general area beginning just anterior to the head process and extending for a short distance posterior to Hensen’s node (figs. 203D; 205D, E).
 
As indicated in figure 202, there are two parts to the primitive streak:
 
(1) the area of Hensen’s node and primitive pit concerned with invaginative movements of pre-chordal plate mesoderm and notochordal
cells and
 
(2) the body of the streak.
 
The former area appears to arise independently in the center of the pellucid
area^ while the body of the streak is formed at the median, caudal margin of
the pellucid area, from whence it grows anteriad to unite with Hensen’s node.
 
 
 
 
Fio. 202. Migration of cells during gastrulation in the chick. Drawing to the left of
the midline represents a surface view; to the right of the midline the epiblast layer has
been removed. (A-F) To the left of the midline based on data provided by Spratt, *46.
(J) Represents lateral, sectional view of (F)-(G), viewed from the left side. Arrows
indicate direction of cell migration. (K)* Indicates a left lateral view of (I), with the
epiblast cut away midsagitally throughout most of the left side of the blastoderm. (L-O)
Transverse sections of (K), as indicated on (K).
 
 
424
 
 
GASTRULATION
 
 
The body of the streak serves as the “door” through which migrating mesodermal cells other than the cells of the pre-chordal plate-notochordal area
pass from the epiblast layer downward to the space between the epiblast and
hypoblast.
 
Using the Vogt method of vital staining, Pasteels (’37b) was able to demonstrate morphogenetic movements of cells into the primitive streak area and
thence to the inside similar to that described by Spratt (fig. 202G-I).
 
The evidence derived from the carbon-particle-marking technic and that of
vital staining, therefore, strongly suggests that the primitive streak of the chick
forms as a result of:
 
(a) converging movements of the epiblast cells toward the median line of
the posterior half of the pellucid area and
 
(b) cell proliferation in situ within the streak.
 
3) Cell Movements in the Hypoblast and the Importance of Those Movements in Primitive-streak Formation. The hypoblast or entodermal layer of
the blastula appears to play a significant role relative to the formation of the
primitive streak in the bird. Various lines of evidence point to this conclusion.
For example, Waddington (’33) reported the results of experiments in which
he separated the epiblast from the hypoblast of early chick and duck embryos
in the early, primitive-streak stage. He then replaced the two layers so that
their longitudinal axes were diametrically reversed, that is, the anterior part
of the entoderm (hypoblast) lay under the posterior part of the epiblast, while
the posterior part of the entoderm lay below the anterior region of the epiblast.
The following results were obtained:
 
(1) The development of the original streak was suppressed; or
 
(2) a new, secondary, primitive streak was induced.
 
During later development, in some cases, the secondary streak disappeared;
in others, it persisted and a double monster was produced. In other instances
the primary primitive streak disappeared and the secondary streak persisted.
The general conclusion set forth by Waddington is as follows: the entoderm
does not induce the differentiation of a definite tissue, but rather, it induces
the form-building movements which lead to the development of the primitive
streak.
 
Certain experiments made by Spratt (’46) lend added evidence of the importance of the hypoblast in primitive-streak formation. In eight experiments
in which the hypoblast was removed before streak formation, six cases failed
to produce a streak, whereas in two instances a beginning streak was formed.
It may be that in the latter two cases, the induction of morphogenetic movements within the epiblast cells occurred previous to hypoblast removal. These
experiments are too few to permit a definite conclusion; however, they are
suggestive and serve to bolster the conclusion made by Waddington. In a
 
 
 
GASTRULATION IN VARIOUS CHORDATA
 
 
425
 
 
second set of experiments performed by Spratt, chick blastoderms in the prestreak and early-streak stages were inverted and marked with carbon particles.
The results showed that the hypoblast moves forward in the median line below
the epiblast layer. He also demonstrated that this forward movement of the
hypoblast “precedes the anterior differentiation of the primitive streak.” Spratt
further observed that: When the movement of the hypoblast deviated to the
left or to the right, the primitive streak similarly deviated. This evidence
“strongly suggests that the hypoblast influences the development of the primitive streak in the overlying epiblast” (Spratt, ’46).
 
 
 
 
PROAMNION
 
 
PMnpyONIC AREA
ir^^-PRIMITIVE
 
(HENSEN'S NODE)
'•PRIMITIVE PIT
 
 
 
 
A - E M
 
 
TIVE GROOVE
I T I V E FOLD
 
AREA OPA C A
 
AREA PELLUCIOi
MATURE PRIMITIV
 
 
 
Fig. 203. Surface-view drawings of photographs of developing primitive streak. (From
Hamburger and Hamilton, ’51, after Spratt.) (A) Initial streak, short, conical thickening at posterior end of blastoderm. (Hamburger and Hamilton, ’51, stage 2.) (B)
 
Intermediate streak. Thickened streak area approaches center of area pellucida. (Hamburger and Hamilton, ’51, stage 3.) (C) Definitive streak (average length, 1.88 mm.).
 
Primitive groove, primitive fold, primitive pit, and Hensen’s node are present. (Hamburger and Hamilton, ’51, stage 4.) (D) Head-process stage (19 to 22 hours of incu
bation). Notochord or head process visible as area of condensed mesoderm extending
anteriorly from Hensen’s node. Proamnion area is indicated in front portion of area
pellucida; head fold is not yet present. (Hamburger and Hamilton, ’51, stage 5.)
 
 
 
426
 
 
GASTRULATION
 
 
 
Fig. 204. Movements in the epiblast layer of the chick during gastrulation and
primitive-streak formation. (Modified slightly from Spratt, ’46.) (A) Pre-streak con
dition. Carbon particles are placed as indicated at a, b, c, d, e, f, and g. (B-G) Observe
migration of carbon particles. (C) Short streak. (E) Medium broad streak. (G) Long
streak. (See fig. 203C.)
 
 
4) Primitive Pit and Notochordal Canal* If one compares the notochordal
canal, formed during gastrulation in the reptilian blastoderm, with that of
the primitive pit in the chick, the conclusion is inevitable that the primitive
pit of the chick blastoderm represents an abortive notochordal canal. The
lizard, turtle, and chick thus represent three degrees of notochordal canal
development (figs. 200A, D; 202J). In certain birds, such as the duck, a
notochordal canal very similar to that of the turtle gastrula, is formed.
 
5) Resume of Morphogenetic Movements of Cells During Gastrulation in
the Chick. In view of the foregoing facts relative to primitive-streak formation, steps in the gastrulative procedure in birds may be described as follows:
 
(a) Shortly after the incubation period is initiated, hypoblast material at
the caudal end of the blastula starts to move in the median line toward
the future cephalic end of the embryo. This activity may be regarded
as a gastrulative streaming of the hypoblast. (This streaming movement probably represents the chick’s counterpart of the forward movement of the entodermal area in the dorsal-lip region of the frog
embryo. )
 
(b) After this movement of the hypoblast is inaugurated, cells from the
epiblast layer immediately overlying the moving hypoblast pass downward toward the hypoblast. That is, epiblast cells begin to involute
and come to lie between the epiblast and hypoblast; from this new
 
 
 
GASTRULATION IN VARIOUS CHORDATA
 
 
427
 
 
position the involuted cells migrate laterally and anteriorly between
the hypoblast and epiblast.
 
(c) In conjunction with the foregoing activities, epiblast cells (presumptive
mesoderm) from the posterior half of the epiblast of the pellucid area
migrate posteriad, converging from either side toward the median line
(fig. 204A-G).
 
(d) These converging cells begin to pile up in the posterior median edge
of the pellucid area (fig. 204C), where they produce a raphe-like
thickening which marks the beginning of the primitive streak (fig.
204C-G). The beginning streak first makes its appearance at about
seven to eight hours after the start of incubation in the egg of the
chick (fig. 204C).
 
(e) Once formed, the initial streak grows anteriad in the median line by:
(1) cell proliferation in situ, and by the addition of (2) converging
cells from the epiblast layer.
 
(f) Also, the primitive streak apparently grows posteriad by cell proliferation and the addition of converging cells.
 
(g-) When the migrating cells of the epiblast reach the primitive streak,
they involute and pass downward to the space between the epiblast
and hypoblast. From this new position they move laterad and anteriad
on either side of the midline, diverging to form a broad, middle layer
of mesodermal cells.
 
(h) As the primitive streak grows anteriad in the epiblast, it eventually
approaches the presumptive pre-chordal plate and presumptive notochordal areas.
 
(i) The pre-chordal plate and notochordal cells then invaginate to form
the primitive pit; the latter represents a shallow or vestigial notochordal
canal, a structure strongly developed in reptiles and some birds, and
occasionally in mammals.
 
(j) Notochordal cells from the notochordal crescent converge to the pit
area and probably pass downward in the walls of the pit, whence
they ingress and move forward in the median line (fig. 202 A-G, J, K).
The definitive primitive streak is formed after about 18 to 19 hours
of incubation. At about 20 to 22 hours of incubation, the prospective,
notochordal material (e.g., the head process) has already invaginated.
At this time it represents a mass of cells in the median line intimately
associated with the neural plate ectoderm above the pre-chordal plate
cells and the entoderm below (fig. 2021, K). As the primitive streak
recedes posteriad (see p. 431 ), the notochordal material gradually separates from the surrounding, pre-chordal plate cells and also from the
neural plate material. Eventually the notochordal cells become a distinct median mass which elongates rapidly (i.e., undergoes extension)
as the nodal area and the primitive streak recede caudally (Spratt, ’47).
 
 
 
 
Fig. 205. (See facing page for legend.)
 
 
428
 
 
GASTRULATION IN VARIOUS CHORDATA
 
 
429
 
 
(k) Somitic mesoderm (i.e., the mesoderm of the future somites) apparently passes inward between the epiblast and hypoblast from the anterolateral portions of the primitive streak. It migrates forward and becomes extended along either side of the notochordal cells during the
period of primitive-streak recession. The nephric and lateral plate
mesoderm involutes along the middle portions of the streak, and this
mesoderm becomes extended antcro-posteriorly. The hypomeric or
lateral plate mesoderm also diverges laterally. The extra-embryonic
mesoderm moves inward along the postero-lateral portions of the
streak; it migrates laterally and anteriorly (fig. 2021, extra-embryonic
mesoderm).
 
 
 
Fig. 205. Three-germ-layered blastoderm or late gastrula of chick, showing the mosaic
distribution of developmental tendencies. (A-C after Rawles, ’36; D and E after Rudnick,
’44, from various sources.) (A-C) The lines transversely placed across embryo are at
levels of 0.3 mm. and 0.7 mm. from the center one, considered as 0.0 mm. (A) Ectodermal or external layer: neural plate area is indicated in black, epidermal area in white.
(B) Mesodermal or middle germ layer. (C) Entodermal or inner germ layer. (D)
Ectodermal layer shown on left, mesodermal and entodermal on right. (E) Superficial
or ectodermal layer shown at left, deeper layer, at right. {Note: These diagrams should
be considered only in a suggestive way; final knowledge relative to exact limits of potencies, especially in the mesodermal layer, should be more thoroughly explored.)
 
 
 
 
Fig. 206. Recession of the primitive streak of the chick and growth of the embryo in
front of Hensen’s node. Marked cell groups represented by heavy dots; dashes opposite
these are reference marks placed on the plasma clot to permit orientation. This diagram
based upon 6 different types of carbon-marking experiments with the generalized results
6-15 hours following explantation. In type I, the stippled area is invaginated as indicated.
Observe especially type VI, the history of the three areas marked by the three heavy
dots placed on the blastoderm at the head-process stage. It is to be observed that the
embryo as a whole arises from the area in front of Hensen’s node. (After Spratt, ’47.)
 
 
430
 
 
 
 
GASTRULATION IN VARIOUS CHORDATA
 
 
431
 
 
(1) While the above activities take place, the area pellucida becomes
elongated posteriorly. The entire pellucid area thus becomes piriform,
i.e., pear-shaped (figs. 202F-I; 203C).
 
(m) This change in shape of the pellucid area is associated primarily with
the activities involved in epiboly which accompany the embolic activities observed above. Epiboly brings about the elongation of the presumptive neural crescent, converting it into an elongated band of cells.
It also effects the expansion and antero-posterior extension of the
overlying presumptive, neural plate and epidermal cells. The latter
behavior is intimately associated with the antero-posterior extension
of the notochordal and mesodermal cellular areas mentioned in (j)
and (k) on pp. 427, 428.
 
(n) Most of the gastrulative processes in the chick are completed at about
20 to 22 hours after incubation starts. At this time the blastoderm is
in the head-process stage. The so-called head process or “notochordal
process” represents the rudimentary notochord which projects forward
from the primitive streak. (See (j) on p. 427.) At this time the various,
specific, organ-forming areas appear to be well established (figs. 2021;
205A-E). (See Rawles, ’36; Rudnick, ’44.) From this time on the
primitive streak regresses caudally, as the embryo and embryonic tissues develop in front of it. The caudal regression of the streak is shown
in figure 206. Spratt (’47) concludes that as the streak regresses, it
becomes shortened by transformation of its caudal end into both embryonic and extra-embryonic ectoderm and mesoderm. Finally, the
anterior end of the streak, that is, the primitive knot or Hensen’s node
together with possibly some condensation of adjacent streak tissue
(Rudnick, ’44), forms the end bud. The latter, according to Homdahl
(’26) gives origin to the posterior portion of the embryo caudal to
somite 27 and to the tail. The remains of the end bud come to a final
resting place at the end of the tail.
 
5. Gastrulation in Mammals
a. Orientation
 
In the mammals, the formative area of the blastocyst (blastula) is located
at one pole and is known as the embryonic or germ disc. It consists of a lower
hypoblast and an upper epiblast. This embryonic disc is connected to the
non-formative or trophoblast cells around its edges (figs. 176, 177, 178).
In some species the embryonic disc is superficial and uncovered by trophoblast
cells (pig, cat, rabbit, opossum), while in others, it is sequestered beneath a
covering of trophoblast (human, monkey, rat). (See figs. 177, 178.)
 
 
 
 
 
GASTRULATION IN VARIOUS CHORDATA
 
 
433
 
 
b. Gastrulation in the Pig Embryo
 
In the pig embryo, two centers of activity are concerned with the formation of the primitive streak, namely, a caudal area of mesodermal proliferation
which forms the body of the primitive streak and an anterior primitive knot
or Hensen’s node. The similarity of behavior of these two portions of the
primitive streak in the chick and pig suggests strongly that their formation
by a convergence of superficial epiblast cells occurs in the pig as it does in
the chick. Hensen’s node, originally described by Hensen (1876) in the rabbit
and guinea pig, is a thickened area of the epiblast in the midline near the
middle of the embryonic disc. As in the chick, the body of the primitive streak
takes its origin at the caudal end of the embryonic disc, where the first appearance of the streak is indicated by a thickening of the epiblast (fig. 209 A, B).
From this thickened region, cells are budded off between the epiblast and
hypoblast, where they migrate distad as indicated by the lightly stippled areas
in figure 208. The streak ultimately elongates, continuing to give origin to cells
between the hypoblast and epiblast. Eventually, the anterior neck region of
the body of the streak merges with fdensen’s node (fig. 208E, F). From the
anterior aspects of the primitive (Hensen’s) node, cells are proliferated off
between the epiblast and hypoblast, and a depression or pit, the primitive pit,
appears just caudal to the node.
 
The proliferation of cells from the nodal area deposits a median band of
cells which merges anteriorly with the hypoblast below. More caudally, the
hypoblast becomes attached to either side of the median band of cells (fig.
209C). The median band of nodal cells thus forms part of two regions, viz.,
an anterior, pre-chordal plate region, where the nodal cells are merged with
hypoblast (entoderm), and an elongated notochordal band or rod of cells
extending backward between the hypoblast cells (fig. 209C) to Hensen’s node,
where it unites with the hypoblast posteriorly (fig. 209D). Unlike the condition in the chick, the notochordal rod, other than in the pre-chordal plate
area, is exposed to the archenteric space below (fig. 209C). It simulates
strongly that of the reptilian blastoderm as gastrulation draws to a close.
 
In the meantime, mesodermal cells from the primitive streak migrate forward between the hypoblast and epiblast along either side of the notochord
in the form of two wing-like areas (figs. 208H, I; 209C). Other mesodermal cells migrate posteriad and laterad. Consequently, one is able to distinguish two main groups of mesodermal cells:
 
( 1 ) formative or embryonic mesoderm, which remains within the confines
of the embryonic or germinal disc and
 
(2) distal ly placed non-formative or extra-embryonic mesoderm.
 
The former will give origin to the mesoderm of the embryonic body, while
from the latter arises the mesoderm of the extra-embryonic tissues.
 
In conclusion, therefore, we may assume that, during gastrulation in the
 
 
 
 
Fig. 208. Development of primitive streak, notochord, and mesodermal migration in
the pig. (After Streeter, ’27.) (A) Primitive .streak represented as thickened area at
 
caudal end of embryonic (germ) disc. Migrating mesoderm shown in heavy stipple.
(B-E) Later stages of streak development. Observe mass migration of mesoderm. The
mesoderm outside the germ disc is extra-embryonic mesoderm. (F) Forward growing,
primitive streak makes contact with Hensen’s node. (G-I) Observe elongation of
notochord accompanied by recession of primitive streak shown in (I). Observe in (I)
that an embryo with three pairs of somites has formed anterior to Hensen’s node. Compare with Spratt’s observation on developing chick, fig. 206, type VI.
 
 
434
 
 
 
GASTRULATION IN VARIOUS CHORDATA
 
 
435
 
 
 
Fig. 209. Longitudinal and transverse sections of the early embryonic (germ) disc of
the pig. (C and D after Streeter, ’27.) (A) Early, pre-streak, germ disc, showing caudal
 
thickening of epiblast layer. (B) Early streak germ disc, showing thickened caudal edge
of disc and beginning migration of mesodermal cells (see fig. 208A). (C) Transverse
 
section through late gastrula, showing three germ layers. Observe that entoderm is
attached to either side of median notochordal rod. (D) Longitudinal section through
pre-somite, pig blastoderm, showing the relation of notochord to Hensen’s node, entoderm,
and pre-chordal plate.
 
pig embryo, emboly and epiboly are comparable and quite similar to these
activities in the chick.
 
 
c. Gastrulation in Other Mammals
 
Though the origin of notochordal and pre-chordal plate cells in the pig simulates the origin of these cells in the chick, their origin in certain mammals,
such as the mole (Heape, 1883) and the human (fig. 207), resembles the
condition found in reptiles, particularly in the lizards, where an enlarged
notochordal pouch or canal is elaborated by an invaginative process. Consequently, in reptiles, birds, and mammals, two main types of presumptive prechordal plate-notochordal relationships occur as follows:
 
( 1 ) In one group an enlarged notochordal canal or pouch is formed which
pushes anteriad in the midline between the hypoblast and epiblast;
and
 
(2) in others an abortive notochordal canal or primitive pit is developed,
and the notochordal cells are invaginated and proliferated from the
 
 
 
436
 
 
GASTRULATION
 
 
thickened anterior aspect of the pit, that is, from the primitive knot
or primitive node (Hensen’s node).
 
Another peculiarity of the gastrulative procedure is found in the human
embryo. In the latter, precocious mesoderm is elaborated during blastulation
presumably from the trophoblast. Later this mesoderm becomes aggregated
on the inner aspect of the trophoblast layer, where it forms the internal layer
of the trophoblast. This precocious mesoderm gives origin to much of the
extra-embryonic mesoderm. However, in the majority of mammals, embryonic
and extra-embryonic mesoderm arise from the primitive streak as in the chick.
 
6. Gastrulation in Teleost and Elasmobranch Fishes
a. Orientation
 
Gastrulation in teleost and elasmobranch fishes shows certain similarities,
particularly in the fact that in both groups the migrating cells use principally
the dorsal-lip area of the blastopore as the gateway from the superficial layer
to the deeper region inside and below the superficial layer. The lateral and
ventral lips are used to some degree in teleosts, but the main point toward
which the migrating cells move is the region of the dorsal lip of the blastopore.
 
As previously described (Chap. 7), the late blastular condition or blastodisc of elasmobranch and teleost fishes consists of an upper layer of formative
tissue, or blastodisc (embryonic disc) and a lower layer of trophoblast or
periblast tissue. The latter is associated closely with the yolk (figs. 179A;
180A; 181 A; 210A). In teleost fishes much of the presumptive entodermal,
organ-forming area (the so-called primary hypoblast) is represented by cells
which lie in the lower region of the caudal portion of the blastodisc (figs. 1 80A;
181 A; 2 IOC). The exact orientation of the hypoblast appears to vary with
the species. In Fundulus, a considerable amount of the presumptive entoderm
appears on the surface at the caudal margin of the blastodisc (fig. 180A, B).
(See Oppenheimer, ’36.) However, in the trout, Salmo, presumptive entoderm
lies in the lower areas of the thickened caudal portion of the disc, and the prechordal plate of presumptive entomesoderm alone is exposed (fig. 181A, B).
(See Pasteels, ’36.) The position of the presumptive entoderm in the shark,
Scyllium (Vandebroek, ’36), resembles that of Fundulus (fig. 170A), although some entoderm may arise by a process of delamination from the lower
area of the blastodisc (fig. 179A).
 
b. Gastrulation in Teleost Fishes
 
1) Emboly. As the time of gastrulation approaches, the entire outer edge
of the blastodisc begins to thicken and, thereby, forms a ring-like area around
the edge of the disc, known as the germ ring (figs. 210C; 21 IB). At the
caudal edge of the blastoderm, the germ-ring thickening is not only more pronounced, but it also ‘extends inward for some distance toward the center of
 
 
 
GASTRULATION IN VARIOUS CHORDATA
 
 
437
 
 
the blastoderm (fig. 211 A, B). This posterior prominence of the germ ring
forms the embryonic shield.
 
As gastrulation begins, the entodermal cells of the primary hypoblast at the
caudal edge of the embryonic shield stream forward below the epiblast toward
the anterior end of the blastodisc (figs. 210A, D). Coincident with this forward movement of the primary hypoblast, a small, crescent-shaped opening
 
 
 
Fig. 210. Gastrulation in teleost fishes. (A) Sagittal section of early gastrula. (Modified slightly from Wilson, 1889.) (B) Midsagittal section through late teleost gastrula.
 
The dorsal and ventral lips of the blastopore are shown approaching each other. (Modified slightly from Wilson, 1889.) (C) Beginning gastrula of early blastoderm of brook
 
trout, Salvelinus. Observe inward (forward) migration of primary hypoblast cells and
thickened mass of cells which arises at posterior margin. (After Sumner, ’03.) (D)
 
Later stage in gastrulation of brook trout. (After Sumner, ’03.) (E) Transverse section
 
of late gastrula of brook trout, showing the three germ layers. (After Sumner, ’03.)
(F) Transverse section through late gastrula of sea bass. (After Wilson, 1889.) (G)
Midsagittal section through closing blastopore of sea bass. (After Wilson, 1889.) (H)
 
Longitudinal section through late gastrula of the brook trout. (After Sumner, ’03.)
 
 
 
438
 
 
GASTRULATION
 
 
appears at the caudal edge of the embryonic shield; this opening forms the
dorsal lip of the blastopore (figs. 210A; 211 A, B).
 
In teleost fishes with a primary hypoblast arranged as in Fundulus (fig.
180A, B), as the entodermal cells of the hypoblast move anteriad from the
deeper portions of the blastodisc, the entodermal cells exposed at the caudal
edge of the epiblast move over the blastoporal lip (i.e., involute) and migrate
forward as a part of the entoderm already present in the deeper layer. (See
arrows, fig. 180B.) The primary hypoblast thus becomes converted into the
secondary hypoblast. In teleosts with a primary hypoblast or entodermal arrangement similar to Salmo (fig. 181 A), the secondary hypoblast is formed
by the forward migration and expansion of the entodermal mass located in
the caudal area of the embryonic shield. In both Fundulus and Salmo following the initial forward movement of the entodermal cells, the pre -chordal plate
cells together with the notochordal cells move caudally and involute over the
dorsal blastoporal lip, passing to the inside. (See arrows, figs. 180B; 18 IB.)
The pre -chordal plate and notochordal cells migrate forward along the midline
of the forming embryonic axis. The pre-chordal plate cells lie foremost, while
the notochordal cells are extended and distributed more posteriorly. The
presumptive mesoderm in the meantime converges toward the dorso-lateral
lips of the blastopore (figs. 180B; 18 IB, see arrows), where it involutes,
passing to the inside between the entoderm or secondary hypoblast and epiblast.
Within the forming gastrula, the mesoderm becomes arranged along the upper
aspect of the entoderm and on either side of the median, notochordal material
(fig. 210E, F). The mesoderm in this way becomes inserted between the
flattened entoderm (secondary hypoblast) and the outside ectodermal layer
(Oppenheimer, ’36; Pasteels, ’36; Sumner, ’03; Wilson, 1889).
 
During the early phases of gastrulation, the involuted entodermal, notochordal, and mesodermal tissues may superficially appear as a single, thickened, cellular layer. As gastrulation progresses, however, these three cellular
areas separate or delaminate from each other. When this separation occurs,
the notochordal cells make their appearance as a distinct median rod of cells,
while the mesoderm is present as a sheet of tissue on either side of the notochord. The entoderm may form two sheets or lamellae, one on either side
of the notochord and below the mesodermal cellular areas (fig. 21 OF) or it
may be present as a continuous sheet below the notochord and mesoderm
(fig. 210E, H). The entodermal lamellae, when present, soon grow mediad
below the notochord and fuse to form one complete entodermal layer (Wilson,
1889).
 
2) Epiboly. Emboly involves for the most part the movements of cells in
the caudal and caudo-lateral areas of the blastoderm, i.e., the embryonic
portion of the germ ring. However, while the involution of cells concerned
with the development of the dorsal, axial region of the embryo occurs, the margins of the blastodisc beyond the dorsal-lip area, that is, the extra-embryonic,
 
 
 
GASTRULATION IN VARIOUS CHORDATA
 
 
439
 
 
germ-ring tissue, together with the presumptive epidermal area, proceeds to
expand rapidly. This growth and expansion soon bring about an engulfment
of the yolk mass (figs. 210B; 211C-F). The blastoporal-lip area (i.e., edge of
germ ring) ultimately fuses at the caudal trunk region (figs. 210G; 21 IF).
As the blastoporal region becomes narrower, a small vesicular outpocketing,
known as Kupffer’s vesicle, makes its appearance at the ventro-caudal end of
the forming embryo at the terminal end of the solid, post-anal gut (fig. 210G) .
This vesicle possibly represents a vestige of the enteric portion of the neurenteric canal found in Amphioxus, frog, etc. A certain amount of mesodermal
involution occurs around the edges of the germ ring, in some species more
than in others (fig. 21 OA, B, peripheral mesodermal involution).
 
As the cellular dispositions involved in extra-embryonic expansion of the
epidermal and germ-ring areas are established, the presumptive, neural plate
material (figs. 179, 180, 181) becomes greatly extended antero-posteriorly
in the dorsal midline (figs. 210A, H; 21 IE), where it forms into a thickened,
elongated ridge or keel. The latter gradually sinks downward toward the
underlying notochordal tissue (fig. 210E, F). Also, by the time that the yolk
mass is entirely enveloped, the somites appear within the mesoderm near the
notochordal axis, and the developing body as a whole may be considerably
delimited from the surrounding blastodermic tissue (fig. 21 IG). Therefore,
if the envelopment of the yolk mass is taken as the end point of gastrulation
in teleosts, the stage at which gastrulation is completed does not correspond
to the developmental condition found at the termination of gastrulation in the
chick, frog, and other forms. That is, the embryo of the teleost fish at the
time of blastoporal closure is in an advanced stage of body formation and
corresponds more truly with a chick embryo of about 35 to 40 hours of incubation, whereas the gastrulative processes are relatively complete in the chick
at about 20 to 22 hours of incubation.
 
3) Summary of the Gastrulative Processes in Teleost Fishes:
 
a) Emboly:
 
( 1 ) Formation of the secondary hypoblast. The secondary hypoblast forms
as a result of the forward migration, expansion, and proliferation of
the entodermal cells lying at the caudal margin of the embryonic shield.
This forward migration of the entoderm (primary hypoblast) occurs
below the upper layer or epiblast and thus produces an underlying
entodermal layer or secondary hypoblast.
 
(2) Pre-chordal plate and notochordal involution. As the formation of
the secondary hypoblast is initiated, the presumptive pre-chordal plate
and notochordal cells move posteriad and converge toward the dorsal
lip of the blastopore, where they involute and pass anteriad in the
median line between the hypoblast and epiblast. The hypoblast or
entodermal layer may be separated into two flattened layers or lamellae,
one on either side of the notochord in some species. However, there
 
 
 
440
 
 
GASTRULATION
 
 
 
Fig. 211. Gastrulation in teleost fishes. (A-F after Wilson, 1889; G from Kerr, ’19,
after Kopsch.) (A) Sea bass, 16 hours, embryonic shield becoming evident, marks
beginning of germ ring. (B) Germ ring well developed. Surface view of blastoderm
of 20 hours. (C) Side view of blastoderm shown in (B). (D) Side view, 25 hours.
 
(E) Surface view, 25 hours. (F) Side view, 31 hours. (G) Late gastrula of trout,
Salma fario.
 
is considerable variation among different species as to the degree of
separation of the entodermal layer; in the sea bass it appears to be
definitely separated, whereas in the trout it is reduced to a single layer
of entodermal cells lying below the notochord. The pre-chordal plate,
entoderm, and anterior notochord merge into a uniform mass below
the cranial end of the neural plate.
 
( 3 ) Mesodermal convergence and involution. Along with the migration of
notochordal cells, the presumptive mesoderm converges posteriad to
the dorso-lateral lips of the blastopore, where it involutes and moves
 
 
 
GASTRULATION IN VARIOUS CHORDATA
 
 
441
 
 
to the inside on either side of the median, notochordal mass and above
the forming, secondary hypoblast.
 
b) Epiboly. The germ-ring tissue and the outer areas of the presumptive
epidermal cells gradually grow around the yolk mass and converge toward the
caudal end of the developing embryo. Associated with this migration of cells
is the anterior-posterior extension of the presumptive neural plate material
to form an elongated, thickened, median ridge.
 
4) Developmental Potencies of the Germ Ring of Teleost Fishes. The
germ ring or thickened, marginal area of the teleost late blastula and early
gastrula has interested embryologists for many years. It was observed in
Chapter 8 that various regions of the marginal area of the blastoderm of the
teleost fish have a tendency to form embryos. Luther (’36), working on the
trout (Salmo), found that all sectors of the blastula were able to differentiate
all types of tissue, i.e., they proved to be totipotent. However, in the early
gastrula, only the sector forming the embryonic shield and the areas immediately adjacent to it were able to express totipotency. As gastrulation progresses,
this. limitation becomes more marked. In other words, a generalized potency
around the germ ring, present during blastulation, becomes restricted when
the embryonic shield of the gastrula comes into prominence. The evidence set
forth in the previous chapter indicates that the possibility for twinning in the
trout becomes less and less as the gastrular condition nears. The restriction
of potency thus becomes a function of a developmental sequence.
 
In the case of Fundulus, Oppenheimer (’38) found that various areas of
the germ ring, taken from regions 90 degrees or 180 degrees away from the
dorsal blastoporal lip, were able to differentiate many different embryonic
structures if transplanted into the embryonic shield area. Oppenheimer concludes that: “Since under certain conditions the germ-ring can express potencies for the differentiation of many embryonic organs, it is concluded that its
normal role is limited to the formation of mesoderm by the inhibiting action of
the dorsal lip.” The results obtained by Luther serve to support this conclusion.
 
c. Gastrulation in Elasmobranch Fishes
 
In figure I79B the presumptive major organ-forming areas of the blastoderm of the shark, Scyllium canicula, are delineated. The arrows indicate the
general directions of cell migration during gastrulation. In figure 212A-G are
shown surface views of the dorsal-lip area of different stages of blastodermic
development in this species, while figure 213A-G presents median, sagittal
sections of these blastoderms during inward migration of the presumptive
organ-forming cells. It is to be observed that the dorsal-lip region of the
blastoderm is the focal area over which the cells involute and migrate to the
inside.
 
 
 
 
Fig. 212. Surface views of developing blastoderms of Scyllnim canicula.
 
 
442
 
 
 
Fig. 213. Sagittal sections of blastoderms shown in figure 212A-G, with corresponding
letters, showing migration of presumptive organ-forming areas. (See also fig. 179.) (B)
 
Dorsal lip is shown to left. (H~M) Transverse sections of embryo of Squalus acanthias,
similar to stages shown in 212F and G, for Scyllium. (H) Section through anterior
head fold. (M) Section through caudal end of blastoderm. H-M original drawings
from prepared slides.
 
 
443
 
 
 
NOTOCHORD
 
 
 
 
Fig. 214. Gastrulation in the gymnophionan Amphibia and in the bony ganoid, Amia
calva. (A, B, C, after Brauer, 1897; D, E, after Dean, 1896.) Sections A~C through developing embryo of Hypogeophis alternans. (A) Middle gastrula, sagittal section. Observe that gastrocoel forms by a separation of the entodermal cells. Blastococl forms
similarly through delamination of entoderm from the overlying epiblast and by spaces
which appear between the cells in situ. (B) Transverse section through late gastrula.
(C) Sagittal section through late gastrula. (D) Late gastrula of Amia. Mass of yolk
in center is uncleaved; cellular organization is progressing peripherally around yolk mass.
(E) Later gastrula of Amia. The blastopore is closing, but a large yolk mass still remains uncleaved.
 
 
444
 
 
 
GASTRULATION IN VARIOUS CHORDATA
 
 
445
 
 
In figure 21 3A, B, and C, two general areas of entoderm are shown:
 
(a) that exposed at the surface (cf. fig. 179), and
 
(b) the entoderm lying in the deeper areas of the blastoderm (cf. fig. 179,
cells in black).
 
According to Vandebroek, ’36, the deeper lying entoderm is extra-embryonic
entoderm (in fig. 213, this deeper entoderm is represented as a black area with
fine white stipple), whereas the entoderm exposed at the caudal portion of
the blastoderm in figure 179A and B, and figure 213A is embryonic entoderm.
 
The later distribution of the major presumptive organ-forming areas of
the shark blastoderm is shown in figure 213E-M. In figure 213, observe the
periblast tissue connecting the blastoderm with the yolk substrate.
 
As the notochordal, cntodermal, and mesodermal cells move inward during
emboly, the presumptive epidermal and neural areas become greatly expanded
externally by the forces of epiboly as shown in figures 213B-E, and 213H.
(Compare the positions of these two areas in fig. 179B.)
 
The general result of the gastrulative processes in the shark group is to
produce a blastoderm with three germ layers similar to that shown in figure
21 3L and M. The notochordal and pre-chordal plate cells occupy the median
area below the neural plate as shown in figure 21 3E and F; the mesoderm
and entoderm lie on either side of the median notochord as shown in figure
213M. A little later the entoderm from either side of the notochord grows
mediad to establish a complete floor of entoderm below the notochord as represented in figure 21 3L.
 
7. Intermediate Types of Gastrulative Behavior
 
In certain forms, such as the ganoid fish, Amia, and in the Gymnophiona
among the Amphibia, the gastrulative processes present distinct peculiarities.
In general, gastrulation in the bony ganoid fish, Amia calva, presents a condition of gastrulation which is intermediate between that which occurs in the
teleost fishes and the gastrulative procedures in the frog or the newt. For
example, a blastodisc-like cap of cells is found at the end of cleavage in the
bony ganoid. This cap gradually creeps downward around the yolk masses
which were superficially furrowed during the early cleavages. This process resembles the cellular movement occurring during epiboly in teleost fishes. In
addition, the entodermal, notochordal, and mesodermal materials migrate inward in much the same way as occurs in the teleost fishes, although the formation of the primitive archenteron resembles to a degree the early invaginative
procedure in the frog. However, a distinctive process of entodermal formation
occurs in Amia, for some of the entodermal cells arise as a separation from
the upper portion of the yolk substance where yolk nuclei are found. (See
fig. 214D, E; consult Eycleshymer and Wilson, ’06.)
 
The gastrulative processes in the gymnophionan Amphibia are most pe
 
 
446
 
 
GASTRULATION
 
 
culiar, particularly the behavior of the entoderm. But little study has been
devoted to the group; as a result, our knowledge is most fragmentary. Elusive
and burrowing in their habits and restricted to a tropical climature, they do
not present readily available material for study. Brauer, 1897, described blastulation and gastrulation in Hypogeophis alternans. Our information derives
mainly from this source.
 
In some respects gastrulation in Hypogeophis is similar to that in teleost
and bony ganoid fishes, while other features resemble certain cellular activities
in other Amphibia and possibly also in higher vertebrates. For example, the
blastoderm behaves much like the flat blastoderm of teleost fishes, for a dorsal
blastoporal lip or embryonic portion of the germ ring is formed toward which
the notochordal and mesodermal materials presumably migrate, involute, and
thus pass to the inside below the epiblast layer (tig. 21 4A, B). Also, the rapid
epiboly of the presumptive epidermal area around the yolk material (or yolk
cells) is similar to that of teleost fishes and of the bony ganoid, Amia (fig.
214C-E). However, the behavior of the entodermal cells differs markedly
from that of teleosts. In the first place, there is a double delamination whereby
the solid blastula is converted into a condition having a blastocoel and a gastrocoel (fig. 214A), These processes occur concurrently with the gastrulative phenomena. Blastocoelic formation resembles somewhat the delaminative
behavior of the entoderm in reptiles, birds, and mammals, for the entodermal
layer separates from the deeper areas of the epiblast layer. The formation of
the gastrocoel (archenteron) is a complex affair and is effected by a process
of hollowing or space formation within the entodermal cell mass as indicated
in figure 2 1 4A. The arrangement of the entodermal cells during later gastrulative stages resembles the archenteron in the late gastrula of other Amphibia.
The archenteron possesses a heavily yolked floor, with the roof of the foregut
region complete, but that of the archenteron more posteriorly is incomplete,
exposing the notochord to the archenteric space (fig. 214A-C).
 
G. The Late Gastrula as a Mosaic of Specific, Organ-forming
Territories
 
It was observed above that the presumptive organ-forming areas of the late
blastula become distributed in an organized way along the notochordal axis
during gastrulation. Further, while an interchangeability of different parts of
the epiblast of the late blastula is possible without upsetting normal development, such exchanges are not possible in the late gastrula. For during gastrulation, particular areas of the epiblast become individuated by activities or
influences involved with induction or evocation. (The word “evocation” was
introduced by Waddington and it has come to mean: “That part of the morphogenetic effect of an organizer which can be referred back to the action of
a single chemical substance, the evocator.” See Needham, ’42, p. 42.) As a
 
 
 
AUTONOMOUS THEORY OF GASTRULATIVE MOVEMENTS
 
 
447
 
 
result, the gastrula emerges from the gastrulative process as a general mosaic
of self-differentiating entities or territories. (See Spemann, ’38, p. 107.)
 
It necessarily follows, therefore, that the production of specific areas or
territories of cells, each hax^ing a tendency to differentiate into a specific structure, and the axiation of these areas along the primitive axis of the embryo are
two of the main functions of the gastrulative process. In figure 205A-E, diagrams are presented relative to the chick embryo showing the results of experiments made by Rawles (’36), Rudnick (’44), and others. (See Rudnick,
’44.) These experiments were made to test the developmental potencies of
various limited areas of the chick blastoderm. A considerable overlapping of
territories is shown, which stems, probably, from the fact that transplanted
pieces often show potencies which are not manifested in the intact embryo.
Therefore, these maps should be regarded not with finality but merely as
suggesting certain developmental tendencies.
 
H. Autonomous Theory of Gastrulative Movements
 
Our knowledge concerning the dynamics of gastrulation in the Chordata
is based largely on the classical observations of cell movement made by
Conklin (’05) in Styela, the same author (’32) in Arnphioxus, Vogt (’29)
in various Amphibia, Oppenheimer (’36) in Fundulus, Pasteels (’36, ’37b) in
trout and chick, Vandebroek (’36) in the shark, and Spratt (’46) in the
chick. For detailed discussions, concerning the morphodynamics of the gastrulative period, reference may be made to the works published by Roux
(1895), Spemann (’38), Pasteels (’40), Waddington (’40), and Schechtman
(’42).
 
The theory popularly held, regarding the movements of the major presumptive organ-forming areas of the late blastula, is that a strict autonomy
is present among the various groups of cells concerned with the gastrulative
process. Spemann (’38) p. 107, describes this theory of autonomy as follows:
 
Each part has already previously had impressed upon it in some way or other
direction and limitation of movement. The movements are regulated, not in a
coarse mechanical manner, through pressure and pull of the single parts, but they
are ordered according to a definite plan. . . . After an exact patterned arrangement, they take their course according to independent formative tendencies which
originate in the parts themselves.
 
There are some observations, on the other hand, which point to an interdependence of the various cell groups. For example, we have referred to the
observations of Waddington (’33) and Spratt (’46) which suggest that the
movements of the mesoderm in the bird embryo are dependent upon the
inductive influence of the entoderm. Similarly, Schechtman (’42) points out
that presumptive notochordal material does not have the power to invaginate
(involute) to the inside when transplanted to the presumptive ectodermal
 
 
 
 
animal pole animalpole
 
 
 
 
Fig. 215. Direction of entodermal projection in relation to egg polarity during gastrulation in various Chordata. (A) Amphioxus. (B) Frog. (C) Urodele amphibia.
(D) Chick. For diagrammatic purposes, the positions to the right of the median egg
axis in the diagrams arbitrarily are considered as clockwise positions, whereas those to
the left are regarded as counterclockwise.
 
 
 
Pig. 216. Exogastrulation in the axolotl (Amphibia). (From Huxley and De Beer, *34,
after Holtfreter: Biol. Zentralbl., 53: 1933.) (A, B) Mass outward or exogastrular
 
movements of entoderm and mesoderm, resulting in the separation of these organforming areas from the epidermal, neural areas shown as a sac-like structure in upper
part of figure. (C) Section of (B). Exogastrulation of this character results when the
embolic movements of gastrulation are directed outward instead of inward. Observe that
neural plate does not form in the ectodermal area.
 
 
448
 
 
 
PRE-CHORDAL PLATE AND CEPHALIC PROJECTION
 
 
449
 
 
area, but it does possess the autonomous power to elongate into a slender
column of cells.
 
 
1. Exogastrulation
 
It was demonstrated by Holtfreter (’33) and also by others that embryos
may be made to exogastrulate, i.e., the entoderm, notochord, and mesoderm
evaginate to the outside instead of undergoing the normal processes involved
in emboly (fig. 216). For example, in the axolotl, Ambystoma mexicanurn,
if embryos are placed in a 0.35 per cent Ringer’s solution, exogastrulation
occurs instead of gastrulation, and the entodermal, mesodermal and notochordal areas of the blastula lie outside and are attached to the hollow ectodermal vesicle. The exogastrulated material, therefore, never underlies the
ectodermal cells but comes to lie outside the neural plate and skin ectodermal
areas of the gastrula (fig. 216B).
 
Therefore, the phenomenon of exogastrulation indicates strongly that the
presumptive, neural plate and epidermal areas of the late blastula and early
gastrula are dependent upon the normal gastrulative process for their future
realization in the embryo. Exogastrulation also clearly separates the parts of
the forming gastrula which are concerned with emboly from those which are
moved by the forces of epiboly. That is, exogastrulation results when the jorces
of epiboly are separated from the forces normally concerned with emboly.
Normal gastrulation is concerned with a precise and exact correlation of these
two sets of forces.
 
J. Prc-chordal Plate and Cephalic Projection in Various Chordates
 
It is evident from the descriptions presented in this chapter that the initial
invaginative movements in gastrulation begin in the region of the dorsal lip
of the blastopore in Amphioxus, fishes, and Amphibia. This initial movement
of cells in the region of the dorsal lip consists in the projection forward, toward
the future head region of the embryo, of foregut entoderm, pre-chordal plate
mesoderm, and notochordal cells. The foregut entoderm, pre-chordal mesoderm, and the anterior extremity of the notochord come to lie beneath the
anterior portion of the neural plate. The complex of anterior foregut entoderm
and pre-chordal mesoderm lies in front of the anterior limits of the notochord
— hence, the name pre-chordal plate. As such it represents, as previously
observed, a part of the head organizer (see p. 401 ), the complete organization
of the vertebrate head being dependent upon anterior chordal (notochordal),
as well as pre-chordal, factors.
 
In higher vertebrates a different situation prevails during gastrulation. As
observed in Chapter 7, the late blastula consists of a lower hypoblast and
an upper epiblast in a flattened condition, the hypoblast having separated
from the lower parts of the epiblast. The separation of the hypoblast occurs
shortly before the gastrulative rearrangement of the major, presumptive, organ
 
 
450
 
 
GASTRULATION
 
 
forming areas begins. The organization of the blastoderm (blastula) is such
that presumptive pre -chordal plate mesoderm and notochordal areas lie far
anteriorly toward the midcentral part of the epiblast. In other words, a contiguous relationship between presumptive pre-chordal entoderm (i.e., anterior foregut entoderm) and presumptive pre-chordal mesoderm and the presumptive notochord at the caudal margin of the blastula does not exist.
Consequently, a different procedure is utilized in bringing the foregut entoderm, pre-chordal mesoderm, and anterior notochord together. That is, the
head-organizer materials must be assembled together in one area underneath
the cephalic portion of the neural plate. This is accomplished by two methods:
 
{ 1 ) The use of a large invaginative process, the notochordal canal, which
projects pre-chordal plate mesoderm and notochord cranio-ventrad
toward the foregut entoderm in the hypoblast below, as described in
figure 200 relative to the reptiles or in figure 207B of the human
embryo and
 
(2) the use of another and less dramatic method for getting the headorganizer materials together, the vestigial invaginative process which
produces the primitive pit and Hensen’s nodal area.
 
The latter mechanism succeeds in getting pre-chordal plate mesoderm and
notochord down between the epiblast and hypoblast and forward to unite with
the anterior part of the foregut entoderm. (See Adelmann, ’22, ’26; Pasteels,
’37b.)
 
It is not clear whether the invaginative behavior which produces the primitive pit or notochordal canal is an autonomous affair or whether it may be
dependent upon the inductive activities of the entoderm below. More experimentation is necessary to decide this matter. The work of Waddington (’33),
however, leads one to conjecture that inductive activities may be responsible.
 
Regardless of the factors involved, cephalogenesis or the genesis of the
head is dependent upon the assemblage of anterior foregut, pre-chordal mesoderm, and anterior notochordal cells beneath the cephalic portion of the
neural plate as described on page 401.
 
K. Blastoporal and Primitive-streak Comparisons
 
From the considerations set forth above, it is clear that the area of the notochordal canal or primitive pit (i.e., Hensen’s nodal area) corresponds to the
general region of the dorsal lip of the blastopore of lower vertebrates, whereas
the dorso-lateral and lateral lips of the blastopore of lower forms correspond
to the body of the primitive streak in higher vertebrates (Adelmann, ’32).
 
 
 
Bibliography
 
 
Adelmann, H. B. 1922. The significance
of the prechordal plate: an interpretative study. Am. J. Anat. 31:55.
 
. 1926. The development of the
 
premandibular head cavities and the relations of the anterior end of the notochord in the chick and robin. J. Morphol. 42:371.
 
— . 1932. The development of the
 
prechordal plate and mesoderm of Amhly stoma punctatum. J. Morphol. 54:1.
 
Braiier, A. 1897. I. Beitrage zur Kenntniss
der Entwicklungsgeschichte und der Anatomic der Gymnophionen. Zool. Jahrb.
10:389.
 
Conklin, E. G. 1905. The organization
and cell-lineage of the ascidian egg. J.
Acad. Nat. Sc., Philadelphia. 13:5.
 
. 1932. rhe embryology of Amphi
oxus. J. Morphol. 54:69.
 
. 1933. The development of isolated and partially separated blastomeres
of Amphioxiis. J. Exper. Zool. 64:303.
 
Dalcq, A. and Pasteels, J. 1937. Unc conception nouvclle des bases physiologiques de la morphogenese. Arch, biol.,
Paris. 48:669.
 
Dean, B. 1896. 7'he early development of
Amia. Quart. J. Micr. Sc. New Series.
38:413.
 
Eycleshymer, A. C. and Wilson, J. M.
1906. The gastrulation and embryo formation in Amia calva. Am. J. Anat.
5:133.
 
Haeckel, E. 1874. Anthropogenic oder
Entwickelungsgcschichte des Menschen.
Vols. 1 and 2 in English translation,
1910, The evolution of man, translated
by J. McCabe. G. P. Putnam’s Sons,
New York.
 
Hamburger, V. and Hamilton, H. L. 1951.
A series of normal stages in the development of the chick embryo. J. Morphol.
88:49.
 
Heape, W. 1883. The development of the
mole (Talpa europea). The formation of
the germinal layers and early development of the medullary groove and notochord. Quart. J. Micr. Sc. 23:412.
 
 
Hensen, V. 1876. Beobachtungen iiber die
Befruchtung und Entwicklung des Kaninchens und Meerschweinchens. Zeit. f.
anat. u. Entwicklngesch. 1:213.
 
Holtfrcter, J. 1933. Die totalc Exogastrulation, eine Selbstablosung des Ektoderms vom Entomesoderm. Entwicklung
und funktionelles Verhalten nervenloser
Organe. Roux’ Arch. f. Entwick. d.
Organ. 129:669.
 
. 1948. Concepts on the mechanism
 
of embryonic induction and its relation
to parthenogenesis and malignancy. Symposia of the Soc. Exper. Biol. No. II, p.
17. Growth in Relation to Differentiation
and Morphogenesis. Academic Press,
Inc., New York and Cambridge University Press, England.
 
Holmdahl, D. E. 1926. Die erste Entwicklung des Korpers bei den Vogeln und
Saugetieren, inkl. dem Menschen I-V.
Morph. Jahrb. 54:333; 55:112.
 
Hubrecht, A. A. W. 1906. The gastrulation of the vertebrates. Quart. J. Micr.
Sc. 49:403.
 
Huxley, J. S. and De Beer, G. R. 1934.
The Elements of Experimental Embryology. Cambridge University Press,
London.
 
Keibel, F. 1901. Gastrulation und Keimblattbildung der Wirbelthiere. See chap.
10 of Ergebnisse der Anatomic und Entwickelungsgeschichte. Wiesbaden.
 
Kerr, J. G. 1919. Textbook of Embryology. Vol. 11. Vertebrata with the Exception of the Mammalia. Macmillan & Co.,
Ltd., London.
 
Lankester, R. 1875. On the invaginate
planula, or diploblastic phase of Paludina
vivipara. Quart. J. Micr. Sc. 15:159.
 
Lewis, W. H. 1907. Transplantation of the
lips of the blastopore in Rana palustris.
Am. J. Anat. 7:137.
 
. 1949. Gel layers of cells and eggs
 
and their role in early development. Lecture Series, Rosco B. Jackson Memorial
Laboratories, Bar Harbor, Maine.
 
 
451
 
 
 
452
 
 
GASTRULATION
 
 
Lopaschov, G. 1935. Die Umgestaltung
des prasumtiven Mesoderms in Hirnteile
bei Tritonkeimen. Zool. Jahrb. (Abt. f.
allg. Zool. u. Physiol.) 54:299.
 
Luther, W. 1936. Potenzpriifungen an isolierten Teilstiicken der Forellenkeimscheibe. Arch. f. Entwicklngsmech. d.
Organ. 135:359.
 
Mangold, O. 1923. Transplantationsversuche zur Frage der Spezifitiit und der
Bildung der Keimblatter. Arch. f. Entwicklngsmech. d. Organ. 100:198.
 
. 1928. Neue Experimente zur Analyse der fruhen Embryonal entwicklung
des Amphibienkeims. Naturwissensch.
16:387.
 
. 1928. Probleme der Enlwicklungs
mechanik. Naturwissensch. 16:661.
 
. 1932, Autonome und komplemen
tare Induktionen bei Amphibien. Naturwissensch. 20:371.
 
Morgan, T. H. and Hazen, A. P. 1900.
The gastrulation of Amphioxus. J. Morphol. 16:569.
 
Needham, J. 1942, Biochemistry and Morphogenesis. Cambridge University Press,
London.
 
Nicholas. J. S. 1945. Blastulation, its role in
pregastrular organization in Amhlystoma
punctatum. J. Exper. Zool. 100:265.
 
Oppenheimer, J. M. 1936. Processes of
localization in developing Funduliis. J.
Exper. Zool. 73:405.
 
. 1938. Potencies for differentiation
 
in the teleostean germ ring. J. Exper.
Zool. 79:185.
 
. 1947, Organization of the teleost
 
blastoderm. Quart. Rev. Biol. 22:105.
 
Pasteels, J. 1936. Etudes sur la gastrulation des vertebres meroblastiques. I.
Teleosteens. Arch, biol., Paris. 47:205.
 
. 1937a. Etudes sur la gastrulation
 
des vertebres meroblastiques. II. Reptiles. Arch, biol., Paris. 48:105.
 
. 1937b. Etudes sur la gastrulation
 
des vertebres meroblastiques. III. Oiseaux. IV. Conclusions generales. Arch,
biol., Paris. 48:381.
 
. 1940. Un apergu comparatif de la
 
gastrulation chez les chordes. Biol. Rev.
15:59.
 
 
. 1945. On the formation of the primary entoderm of the duck (Anas domestica) and on the significance of the
bilaminar embryo in birds. Anat. Rec.
93:5.
 
Rawles, M. E. 1936. A study in the localization of organ-forming areas in the
chick blastoderm of the head-process
stage. J. Exper. Zool. 72:271.
 
Roux, W. 1895. Gesammelte Abhandlungen iiber Entwicklungsmechanik der Organismen. II. Engelmann, Leipzig.
 
Rudnick, D. 1944. Early history and mechanics of the chick blastoderm. Quart.
Rev. Biol. 19:187.
 
Rugh, R. 1951. The Frog, Its Reproduction and Development. The Blakiston
Co., Philadelphia.
 
Schechtman, A. M. 1934. Unipolar ingression in Triturus torosus: a hitherto undescribed movement in the pregastrular
stages of a urodele. University of California Publ., Zool. 39:303.
 
, 1935, Mechanism of ingression in
 
the egg of Triturus torosus. Proc. Soc.
Exper. Biol. & Med. 32:1072.
 
. 1942. The mechanism of amphibian gastrulation. I. Gastrulation-promoting interactions between various regions of an anuran egg {Hyla regilla).
University of California Publ., Zool.
51:1.
 
Spemann, H. 1918. fiber die Determination der ersten Organanlagen des Amphibienembryo. I VI. Arch. f. Entwicklngsmech. d. Organ. 43:448.
 
. 1921. Die Erzeugung tierischer
 
Chimaren durch heteroplastische embryonale Transplantation zwischcn Triton cristatus und T. taeniatus. Arch. f.
Entwicklngsmech. d. Organ. 48:533.
 
. 1931. fiber den Anteil von Im
plantat und Wirtskeim an der Orientierung und Beschaffenheit der induzierten
Embryonalanlage. Arch. f. Entwicklngsmech. d. Organ. 123:389.
 
. 1938. Embryonic Development
 
and Induction. Yale University Press,
New Haven.
 
and Mangold, H. 1924. Ober In
duktion von Embryonalanlagen durch
Implantation artfremder Organisatoren.
Arch. f. mikr. Anat, 100:599.
 
 
 
BIBLIOGRAPHY
 
 
453
 
 
Spratt, N. T., Jr. 1942, Location of organspecific regions and their relationship to
the development of the primitive streak
in the early chick blastoderm. J. Exper.
Zool. 89:69 101.
 
. 1946. Formation of the primitive
 
streak in the explanted chick blastoderm
marked with carbon particles. J. Exper.
Zool. 103:259.
 
. 1947. Regression and shortening
 
of the primitive streak in the explanted
chick blastoderm. J. Exper. Zool. 104:69.
 
Streeter, G. L. 1927. Development of the
mesoblast and notochord in pig embryos.
Carnegie Inst., Washington, Publ. No.
380. Contrib. to Embryol. 19:73.
 
Sumner, F. B. 1903. A study of early fish
development. Arch. f. Entwicklngsmech.
d. Organ. 17:92.
 
Vandebroek, G. 1936. Les mouvements
morphogenetiques au cours dc la gastrulation chez Scylliiim ccinicula. Arch, biol.,
Paris. 47:499.
 
 
Vintemberger, P. 1936. Sur le developpement compare des micromeres de I’ocuf
de Rana fusca divise en huit: (a) Apres
isolement. (b) Apres transplantation sur
un socle de cellules vitellines. Compt.
rend. Soc. de biol. 122:127.
 
Vogt, W. 1929. Gestaltungsanalyse am
Amphibienkeim mit ortlicher Vitalfarbung. 11. Teil: Gastrulation und mesodermbildung bei Urodelen und Anuren.
Roux’ Arch. f. Entwick. d. Organ.
120:385.
 
Waddington, C. H. 1933. Induction by the
entoderm in birds. Arch. f. Entwicklngsmech. d. Organ. 128:502.
 
. 1940. Organizers and genes. Cambridge University Press, London.
 
Will, L. 1892. Beitrage zur Entwicklungsgeschichte der Reptilien. Zool. Jahrb.
6 : 1 .
 
Wilson, H. V. 1889. The embryology of
the sea bass (Serranus atrarius). Bull.
U. S. Fish Comm. 9:209.
 
 
 
10
 
 
Tutulation anJ Extension of tke Major Or^an-forming
Areas:
 
==Development of Primitive Body Form==
 
 
A. Introduction
 
1. Some of the developmental problems faced by the embryo after gastrulation
 
a. Tabulation
 
b. Increase in size and antero-posteri(*)r extension of the tubulated, major organforming areas
 
c. Regional modifications of the tubulated areas
 
2. Common, vertebrate, embryonic body form
 
3. Starting point for tabulation
 
4. Developmental processes which accomplish tabulation
 
a. Immediate processes
 
b. Auxiliary processes
 
5. Blastocoelic space and body-form development
 
6. Primitive circulatory tubes or blood vessels
 
7. Extra-embryonic membranes
 
B. Tabulation of the neural, epidermal, entodermal, and mesodermal, organ-forming
areas in the vertebrate group
 
1. Neuralization or the tabulation of the neural plate area
 
a. Definition  
 
b. Neuralizative processes in the Vertebrata
 
1) Thickened keel method
 
2) Neural fold method
 
c. Closure of the blastopore in rounded gastrulae, such as that of the frog
 
d. Anterior and posterior neuropores; neurenteric canal
 
2. Epidermal tabulation
 
a. Development of the epidermal tube in Amphibia
 
b. Tabulation of the epidermal area in flat blastoderms
 
3. Formation of the primitive gut tube (enteric tabulation)
 
a. Regions of primitive gut tube or early metenteron
 
b. Formation of the primitive metenteron in the frog
 
c. Formation of the tubular metenteron in flat blastoderms
 
4. Tabulation (coelom formation) and other features involved in the early differentiation of the mesodermal areas
 
a. Early changes in the mesodermal areas
 
1) Epimere; formation of the somites
 
2) Mesomere
 
3) Hypomere
 
b. Tabulation of the mesodermal areas
 
 
454
 
 
 
 
INTRODUCTION
 
 
455
 
 
C. Notochordal area
 
D. Lateral constrictive movements
 
E. Tubulation of the neural, epidermal, entodermal, and mesodermal, organ-forming
areas in Amphioxus
 
1. Comparison of the problems of tubulation in the embryo of Amphioxus with that
of the embryos in the subphylum Vertebrata
 
a. End-bud growth
 
b. Position occupied by the notochord and mesoderm at the end of gastrulation
 
2. Neuralization and the closure of the blastopore
 
3. Epidermal tubulation
 
4. Tubulation of the entodermal area
 
a. Segregation of the entoderm from the chordamesoderm and the formation of
the primitive metenteric tube
 
b. Formation of the mouth, anus, and other specialized structures of the metenteron
 
5. Tubulation of the mesoderm
 
6. Later differentiation of the myotomic (dorsal) area of the somite
 
7. Notochord
 
F. Early development of the rudiments of vertebrate paired appendages
 
G. The limb bud as an illustration of the field concept of development in relation to the
gastrula and the tubulated embryo
 
H. Cephalic flexion and general body bending and rotation in vertebrate embryos
 
I. Influences which play a part in tubulation and organization of body form
 
J. Basic similarity of body-form development in the vertebrate group of chordate animals
 
A. Introduction
 
1. Some of the Developmental Problems Faced by the
Embryo After Gastrulation
 
a. Tubulation
 
One of the main problems, confronting the embryo immediately following
gastrulation, is the tubulation of the major organ-forming areas, namely,
epidermal, neural, entodermal, and the two, laterally placed, mesodermal
areas. The epidermal, neural, and entodermal areas eventually form elongated, rounded tubes, whereas the mesodermal tubes are flattened. The epidermal and neural tubes extend the entire length of the developing embryo
(fig. 217A-C), while the entodermal tube normally terminates at the beginning of the tail (fig. 21 7B, C), although in some instances it may extend even
to the tail’s end (fig. 217A). Anteriorly, the entodermal tube ends along the
ventral aspect of the developing head (fig. 217A, C). The two mesodermal
tabulations are confined mainly to the trunk region of the embryo, but in the
early embryo of the shark they continue forward into the head almost to the
posterior limits of the developing eyes (fig. 217D). The condition of the
mesodermal tubes in the Amphibia resembles to a degree that in the shark
embryo (fig. 217B, E).
 
An important concept to grasp is that the tubulations of the respective areas
occur synchronously or nearly so. It is true that the initial stages of the epidermal and entodermal tubulations slightly precede the other tubulations in
 
 
 
 
Fig. 217. Primary tubes (tubulations) of the primitive vertebrate body. (A) Schematic representation of epidermal, neural, and entodermal tubes in the early embryo of
the shark. Observe that a well-developed, post-anal or tail gut continues to the end of
the tail. (B) Gut, neural, and epidermal tubes in the amphibian type. (C) Gut,
neural, and epidermal tubes in the chick and mammal type. (D) Mesodermal tube in
the shark embryo. (E) Mesodermal tube in the amphibian embryo. (F) Mesodermal
condition in the early bird and mammal embryo. (G) Transverse section of shark
embryo, showing tubulations of major or^an-forming areas and primary coelomic conditions. (H) Transverse section of frog ^embryo shortly after closure of neural tube,
showing the five fundamental body tubes oriented around the notochord.
 
 
456
 
 
 
Amphioxus, in the frog, and in forms having rounded gastrulae, while in the
chick the neural area is precocious. Viewed in their totality, however, the
tubulations of all of the major organ-forming areas are simultaneous processes
with the exception of the notochord which does not become tubulated but
continues as an elongated rod of cells.
 
b. Increase in Size and Antero-posterior Extension of the Tubulated,
Major Organ-forming Areas
 
Another goal to be achieved by the embryo during the immediate, postgastrular period is an increase in size, together with an antero-posterior extension of the major organ-forming areas. These changes are associated with
tubulation, and they aid in producing the elongated, cylindrical form typical
of the chordate body,
 
c. Regional Modifications of the Tubulated Areas
 
As tubulation of the various major organ-forming areas progresses, specific,
organ-forming areas or fields (see end of chapter), located along the respective primitive body tubes, begin to express themselves and develop in a specialized manner. Thus, regional differentiation of the major organ-forming
areas; comprising each primitive body tube, is another feature of the postgastrular period. As a result, localized areas along each of the body tubes
show changes in shape, and specific, individualized structures begin to make
their appearance. For example, the neural tubulation develops the primitive
parts of the brain at its anterior end, while the posterior portion of the neural
tube, caudal to the brain area, begins to form the spinal cord. Thus, the
primitive brain becomes a specific peculiarity of the head region. Also, the
epidermal tubulation at its cranial end contributes definite structures peculiar
to the head. In the pharyngeal region, special developmental features arise in
the entodermal tube together with the epidermal tube and the mesoderm. In
the trunk region, modifications of the entodermal and mesodermal tubes give
origin to many of the structural conditions peculiar to this area, while in the
tail, the neural and epidermal tubulations together with activities of the mesoderm account for the characterstic structures of the tail appendage. These
special developmental features of the respective, tubulated, organ-forming
areas, which arise in specific areas along the antero-posterior axis of the
embryo, occur in much the same way throughout the vertebrate group with
the result that common or generalized structural conditions of the tubulated
organ-forming areas appear in all vertebrate embryos. That is, the primitive
brains of all vertebrate embryos up to a certain stage of development resemble
each other in a striking manner; the contributions of the epidermal tubulation
to the head also resemble each other, ind the early development of the
pharyngeal and trunk regions is similar. As a result, the early morphogenesis
 
 
 
 
 
INTRODUCTION
 
 
459
 
 
organ systems conform to generalized, basic plans. After the generalized plan
of a particular system is established, it is modified in later development to fit
the requirements of the habitat in which the particular species lives. In the
cephalochordate, Amphioxus, a similar body form also develops, although it
is considerably modified.
 
The common, generalized, primitive embryonic body form of all vertebrate
embryos possesses the following characteristics:
 
( 1 ) It is an elongated structure, cylindrical in shape, and somewhat compressed laterally.
 
(2) It is composed of five, basic, organ-forming tubes, oriented around a
primitive axis, the notochord (fig. 217).
 
(3) It possesses the following regions: (a) head, (b) pharyngeal area,
(c) trunk, and (d) tail (figs. 217, 226, 227, 230, 238, 244, 246).
 
In Chapter 1 1 and the following chapters, various details of these common
regions and other features will be considered. In this chapter, we are concerned mainly with tabulation and antero-posterior extension of the major
organ-forming areas in relation to body-form development.
 
3. Starting Point for Tubulation
 
The starting point for tubulation of the major organ-forming areas and
subsequent, primitive, body formation is the gastrula; which, as observed in
Chapter 9, exists in two forms, namely, rounded and the flattened gastrulae
(figs. 219, 232). Many heavily yolked embryos, such as the embryo of Necturns maculosus, although they form a rounded gastrula, are faced with some
of the problems of the flattened gastrulae (fig. 227). The rounded gastrulae,
found in the frog, Amphioxus, etc., differ from the flattened gastrulae present
in the bird, reptile, mammal, and teleost and elasmobranch fishes, mainly by
the fact that, at the beginning of tubulation and body formation, the epidermal
and gut areas already are partially tubulated in the rounded gastrulae. That is,
in the rounded blastoderm, the initial stages of tubulation occur in these two
major organ-forming areas during gastrulation. This means that the ventral
portion of the trunk area in rounded gastrulae is circumscribed by intact cellular
layers of the embryonic trunk region, with yolk material contained within the
cell layers, while, in flattened gastrulae, the ventro-lateral portions of the
trunk region are spread out flat, the yolk not being surrounded by the future,
ventro-lateral walls of the embryonic trunk region. These conditions are illustrated in figures 219B and C and 234A-F.
 
The developmental problems faced by these two groups of gastrulae, therefore, are somewhat different. Moreover, tubulation of the organ-forming areas
and the development of body form in Amphioxus varies considerably from
that of the rounded gastrulae of the vertebrate group. For this reason, tubulation in Amphioxus is considered separately.
 
 
 
460
 
 
DEVELOPMENT OF PRIMITIVE BODY FORM
 
 
Regardless of differences, however, all vertebrate gastnilae, rounded and
flattened, possess three fundamental or basic regions, to wit, ( 1 ) a cephalic
or head region, containing the rudiments of the future head and pharyngeal
structures, (2) a trunk region, wherein lie the undeveloped fundaments of
the trunk, and (3) an end-bud or tail rudiment, containing the possibilities
of the future tail.
 
4. Developmental Processes Which Accomplish Tubulation
 
a. Immediate Processes
 
The term, immediate processes, signifies the events which actually produce
the hollow tubular condition. In the case of the epidermal, enteric, and neural
tubulations, the immediate process is mainly one of folding the particular,
 
 
 
BRAIN AREA
 
HEAD REGION
 
SENSORY PLATE
 
 
GILL - PLATE
AREA
 
 
TRUNK REGION
 
 
NEURAL FOLD
 
 
TAIL REGION
 
PRE-CHOROAL PLATE
 
NEURAL ECTODERM
EPIDERMAL
 
ECTODERM
 
: S ODE R M \
 
 
 
NOTOCHORD \ D.
 
COELOMIC SPACE ENTODERM
 
 
 
NOTO CHORD
NEURAL
ECTODERM
 
 
VENTRAL
MESODERM
 
 
Fig. 219. Relationships of the major presumptive organ-forming areas at the end of
gastrulation in the anuran amphibia. (A) External view of gastrula, showing the ectodermal layer composed of presumptive epidermis (white) and presumptive neural plate
(black), as viewed from the dorsal aspect. (B) Diagrammatic median sagittal section
of condition shown in (A). (C) Same as (B), showing major organ-forming areas.
 
(D) Section through middorsal area of conditions (B) and (C), a short distance caudal
to foregut and pre-chordal plate region. Observe that the notochord occupies the middorsal area of the gut roof.
 
 
 
INTRODUCTION
 
 
461
 
 
organ-forming area into a hollow tubular affair. With respect to the mesodermal areas, the immediate process is an internal splitting (de lamination),
whereby the mesodermal area separates into an outer and an inner layer
with a space or cavity appearing between the two layers. In the case of the
teleost fishes, a process of internal separation of cells appears to play a part
also in the neural tubulation.
 
b. Auxiliary Processes
 
Aiding the above activities which produce tubulation are those procedures
which extend the tubulated areas into elongated structures. These auxiliary
processes are as follows:
 
( 1 ) The cephalic or head rudiment, with its contained fundaments of the
developing head region, grows forward as a distinct outgrowth. This
anterior protrusion is known as the cephalic or head outgrowth (figs.
223A, B; 2321-L).
 
(2) The trunk rudiments enlarge and the trunk region as a whole undergoes antero-posterior extension (figs. 225 A; 233).
 
(3) The tail-bud area progresses caudally as the tail outgrowth and forms
the various rudimentary structures associated with the tail (figs. 225;
230F; 238).
 
(4) A dorsal upgrowth (arching) movement occurs, most noticeable in
the trunk area. It serves to lift the dorsal or axial portion of the trunk
up above the yolk-laden area below, and the developing body tubes
and primitive body are projected dorsalward (figs. 221, 224, 241).
 
(5) In embryos developing from rounded gastrulae, a ventral contraction
and reshaping of the entire ventro-lateral areas of the primitive trunk
region are effected as the yolk is used up in development. This results
in a gradual retraction of this area which eventually brings the ventrolateral region of the trunk into line with the growing head and tail
regions (cf. figs. 220, 223, 225 on the development of the frog, and
227 on the development of Necturus).
 
(6) In embryos developing from flattened gastrulae, a constriction of the
ventral region of the developing trunk comes to pass. This constriction
is produced by an ingrowth toward the median line of entodermal,
mesodermal, and epidermal cellular layers in the form of folds, the
lateral body folds. Upon reaching the midline, the cellular layers fuse
as follows: The entodermal layer from one side fuses with the entodermal layer of the other; the mesodermal layers fuse similarly; and,
finally, the epidermal layer from one side fuses with the epidermal
layer of the opposite side. The result is a general fusion of the respective body layers from cither side, as shown in figure 24 1C and D,
which establishes the ventral region of the trunk. A complete fusion
throughout jLhe extent of the ventral body wall does not take place
 
 
 
462
 
 
DEVELOPMENT OF PRIMITIVE BODY FORM
 
 
NEURAL PLATE
 
 
.A.B. C. D. E.
 
 
 
NERVOUS LAYER
OF
 
/\ EPIDERMIS
NOTOCHORD
 
 
DORSAL
DIVERTICULUM
OF
 
HINDGUT
 
 
BRAIN AREA
 
 
 
NEUROCOEL
 
NEURENTERIC
CANAL
 
 
DORSAL PROJECTION U
^ OF HINDGUT
 
CLOSING BLASTOPORE
HINDGUT
PROCTOOAEUM
 
 
ORAL'^
 
VENTRAL DIVERTlCULUf
 
DIVERTICULUM
OF HINDGUT
 
 
 
, VENTRAL
 
_ _ DIVERTICULUM
 
MESENCHYME 7" - OF HINDGUT
 
LIVER DIVERTICULUM
 
 
Fig. 220. Beginning neural fold stage of frog embryo from prepared material. (A)
Beginning neural fold stage as seen from dorsal view. (B) Sagittal section near median
plane of embryo similar to that shown in (A). (C) Same as (B), showing organ
forming areas. (D) Midsagittal section of caudal end of frog embryo slightly younger
than that shown in fig. 223 B. Observe that the blastopore practically is closed, while the
dorsal diverticulum of the hindgut connects with the neurocoel to form the neurenteric
canal. Observe, also, ventral diverticulum of hindgut.
 
 
until later in development, and, as a result, a small opening remains,
the umbilicus, where the embryonic and extra-embryonic tissues are
continuous. This discontinuity of the embryonic layers permits the
blood vessels to pass from the embryonic to the extra-embryonic regions. (Note: In the teleost fishes, although a typical, flattened, gastrular form is present, the formation of the ventral body wall of the
trunk through a general retraction of tissues resembles that of the
rounded gastrulae mentioned above.)
 
5. Blastocoelic Space and Body-form Development
 
During the terminal phases of gastrulation in such forms as Amphioxus
and the frog, the blastocoel, as a spacious cavity, disappears for the most
part. Its general area is occupied by cells which migrated into the blastocoel
 
 
INTRODUCTION
 
 
463
 
 
during gastrulation. However, the disappearance of the blastocoelic space
is more apparent than real. For, while most of the original blastocoelic space
is thus occupied and obliterated, a part of the original blastocoel does remain
as an extremely thin, potential area between the outside ectoderm and the
mesoderm-entoderm complex of cells. In flattened blastoderms, as in the
chick, the actual space between the ectoderm, mesoderm, and entoderm is
considerable (fig. 234E, F). To sum up: Though the blastocoelic space appears to disappear during the terminal phases of gastrulation, a residual or
potential space remains between the three germ layers, more pronounced in
some species than in others. This residual space gradually increases during
the tubulation processes of the major organ-forming areas. In doing so, it
permits not only the tubulation of these areas within the outside ectoderm,
but it allows important cell migrations to occur between the various body tubes.
 
 
6. Primitive Circulatory Tubes or Blood Vessels
 
Accompanying the tubulations of the epidermal, neural, entodermal, and
the two mesodermal areas on either side of the notochord, is the formation
 
 
BRAIN AREA OF NEURAL
 
 
 
 
DORSAL
DIVERTICULUM
OF HINOGUT
 
 
VENTRAL
DIVERTICULUM
OF HINDGuT
 
 
 
liver diverticulum
 
CLOSING BLASTOPORE
 
 
 
Fig. 221. Transverse sections through early neural fold embryo of the frog as shown
in fig. 220A and B. (A-J) Sections are indicated in fig. 220B by lines A-J, respectively.
Observe that the dorsal arching (dorsal upgrowth) movement of the dorsally situated
tissues accompanies neural tube formation.
 
 
464
 
 
DEVELOPMENT OF PRIMITIVE BODY FORM
 
 
 
 
Fig. 222. Neural crest cells in Amhystoma punctatum. (A and B from Johnston:
Nervous System of Vertebrates, Philadelphia, Blakiston, ’06; C-F from Stone: J. Exper.
Zool., ’35.) (A) Transverse section of early neural tube of Am by stoma, neural crest
 
cells located dorsally and darkly shaded. (B) Later stage than (A), showing relation
of neural crest cells, epidermis, and neural tube. (C-F) Neural crest cells stippled,
placodes of special lateral line sense organs and cranial nerve ganglia shown in black.
The neural crest cells arise from dorsal portion of neural tube at points of fusion of
neural folds and migrate extensively. A considerable portion of neural crest cells descends
upon the mesoderm of visceral arches as indicated in (D-F) and contributes mesodermal
cells to these arches, where they later form cartilaginous tissue.
 
 
of a delicate system of ve.ssels which function for the transport of the circulatory fluid or blood. The formation of these blood vessels begins below the
forming entodermal tube as two, subenteric (subintestinal) tubes or capillaries.
These capillaries grow forward below the anterior portion of the forming
digestive tube. Near the anterior end of the latter, they separate and pass
upward on either side around the gut tube to the dorsal area, where they come
together again below the notochord and join to form the rudiments of the
dorsal aortae. The latter are two delicate supraenteric capillaries which extend from the forming head area caudally toward the trunk region. In the
 
 
 
TUBULATION OF ORGAN-FORMING AREAS
 
 
465
 
 
latter region, each rudiment of the dorsal aorta sends a small, vitelline blood
vessel laterally into that portion of the gut tube or yolk area containing the
yolk or other nutritional source. In the yolk area, each joins a plexus of
small capillaries extending over the surface of the yolk substance. These
capillaries in turn connect with other capillaries which join ultimately each of
the original subintestinal blood capillaries. Below the anterior or foregut portion of the entodermal tube, the two subintestinal blood vessels fuse and thus
form the beginnings of the future heart (figs. 234-237; 332). The further
development of this system of primitive vessels is described in Chapter 17.
 
7. Extra-embryonic Membranes
 
Associated with the development of body form and tubulation of the major,
organ-forming areas, is the elaboration of the very important extra-embryonic
membranes. As the essential purpose at this time is to gain knowledge of the
changes concerned with tubulation of the major organ-forming areas and the
development of primitive body form, consideration of these membranes is deferred until Chapter 22. The latter chapter is concerned with various activities
relating to the care and nutrition of developing embryos of various vertebrate
species.
 
B. Tubulation of the Neural, Epidermal, Entodermal, and Mesodermal,
Organ-forming Areas in the Vertebrate Group
 
1. Neuralization or the Tubulation of the Neural Plate Area
 
a. Definition
 
The separation of the neural plate material from the skin ectoderm, its migration inward, and its formation into a hollow tube, together with the segregation of the accompanying neural crest cells, is called neuralization.
 
b. Neuralizative Processes in the Vertebrata
 
Neuralization is effected by two general procedures in the vertebrate
subphylum.
 
1) Thickened Keel Method. In tcleost, ganoid, and cyclostomatous fishes,
the neural plate material becomes aggregated in the form of a thickened,
elongated ridge or keel along the middorsal axis of the embryo (figs. 21 OF;
218C). This keel separates from, and sinks below, the overlying skin ectoderm
(fig. 218A). Eventually the keel of neural cells develops a lumen within its
central area and thus gradually becomes transformed into an elongated tube,
coincident with the tubulations of the other major organ-forming areas (fig.
218B). In the cyclostomatous fish, Petromyzon planeri, although neuralization closely resembles the condition in teleost fishes, in certain respects the
behavior of the neuralizative changes represents an intermediate condition
 
 
 
466
 
 
DEVELOPMENT OF PRIMITIVE BODY FORM
 
 
 
Fig. 223. Early neural tube stage of the frog, Rana pipiens, 2Vi to 3 mm. in length.
 
(A) Dorsal view, (B) Midsagittal section of embryo similar to (A). (C) Same as
 
(B) , showing organ-forming areas. Abbreviations: V. HD. = ventral hindgut divertic
ulum; D. HD, = dorsal hindgut diverticulum; PHAR. ~ pharyngeal diverticulum of foregut. (D) Later view of (A). (E) Sec fig. 224.
 
between the keel method of the teleost and neural fold method of other vertebrates described below (Selys-Longchamp.s, ’10).
 
2) Neural Fold Method* In the majority of vertebrates, the neural (medullary) plate area folds inward (i.e., downward) to form a neural groove. This
neural groove formation is associated with an upward and median movement
of the epidermal layers, attached to the lateral margins of the neural plate,
as these margins fold inward to form the neural folds. A change of position in
the mesoderm also occurs at this time, for the upper part which forms the
somites shijts late rad from the notochordal area to a position between the
forming neural tube and the outside epidermis. This mesodermal migration
permits the neural tube to invaginate downward to contact the notochordal
area. Also, this change in position of the somitic mesoderm is a most important
factor in neuralization and neural tube development as mentioned at the end
of this chapter. {Note: In this stage of development, the embryo is often de
 
 
 
 
 
 
468
 
 
DEVELOPMENT OF PRIMITIVE BODY FORM
 
 
 
RHOMBENCEPHALON
MESENCEPHALON
PROSENCEPHALON
 
 
NEURAL TUBE
 
 
EPIPHYSIS
 
 
 
^ HEART RUDIMENT
INFUNDIBULUM^^ ORAL EVAGINATION
ORAL SUCKER
NEURAL TUBE SUBN OTOCHORDAL
HEAD GUT
 
 
(3VrOCT0DAEUM
I'^ANAL OPENING
RECTUM
 
 
VENTRAL MESODERM
NOTOCHORD
 
 
 
FOREGUT’
 
 
DIVERTICULUM
 
 
Fig. 225. Structure of 3’/2- to 4-mm. embryo of Rana pipiens (about eight pairs of
somites are present). (See fig. 226A and B for comparable external views of lateral and
ventral aspects of 5-mm., .yy/v6i//ca embryo.) (A) External dorsal view. (B) Midsagittal view. (C) Same, showing major organ-forming areas.
 
scribed as a neurula, especially in the Amphibia. However, in the bird and
the mammal, the embryo during this period is described in terms of the number
of somitic pairs present, and this stage in these embryos is referred to as the
somite stage.) Each lateral neural fold continues to move dorsad and mesad
until it meets the corresponding fold from the other side. When the two neural
folds meet, they fuse to form the hollow neural tube and also complete the
middorsal area of the epidermal tube (cf. figs. 221, 224, 233, 234, 236, 237,
242, 245A). As a general rule, the two neural folds begin to fuse in the
anterior trunk and caudal hindbrain area. The fusion spreads anteriad and
posteriad from this point (figs. 223, 229, 233, 235, 242, 245A). It is important to observe that there are two aspects to the middorsal fusion process:
 
(a) The lateral edges of the neural plate fuse to form the neural tube; and
 
(b) the epidermal layer from either side fuses to complete the epidermal
layer above the newly formed neural tube.
 
Associated with the fusion phenomena of the epidermis and of the neural
tube, neural crest cells are given off or segregated on either side of the neural
tube at the point where the neural tube ectoderm separates from the skin
 
 
 
TUBULATION OF ORGAN-FORMING AREAS
 
 
469
 
 
ectoderm (figs. 221C-E; 23 4B; 236B). The neural crest material forms a
longitudinal strip of cells lying along either side of the dorsal portion of the
neural tube. As such, it forms the neural or ganglionic crest. In some vertebrate embryos, as in the elasmobranch fish, Torpedo, and in the urodele,
Ambystoma, the cells of the neural crest are derived from the middorsal part
of the neural tube immediately after the tube has separated from the skin
ectoderm (epidermis). (See fig. 222A, B.) In other vertebrates, such as the
frog, chick, and human, the neural crest material arises from the general area
of junction of neural plate and skin ectoderm as fusion of the neural folds is
consummated (fig. 234B).
 
The neural crest gives origin to ganglionic cells of the dorsal root ganglia
of the spinal nerves and the ganglia of cranial or cephalic nerves as described
in Chapter 19. Pigment cells also arise from neural crest material and migrate
extensively within the body, particularly to the forming derma or skin, peritoneal cavity, etc., as set forth in Chapter 12. A considerable part of the
mesoderm of the head and branchial area arises from neural crest material
(fig. 222C-F). (See Chapters 11 and 15.)
 
As the neural plate becomes transformed into the neural tube, it undergoes
extension and growth. Anteriorly, it grows forward into the cephalic outgrowth,
in the trunk region it elongates coincident with the developing trunk, while
posteriorly it increases in length and forms a part of the tail outgrowth.
 
f. Closure of the Blastopore in Rounded Gastrulae, such as that of
 
the Frog
 
Neuralization and the infolding of the neural plate cells begins in the frog
and other amphibia before the last vestiges of the entoderm and mesoderm
have completed their migration to the inside. As mentioned above, the neural
folds begin, and fusion of the neural tube is initiated in the anterior trunk
region. From this point, completion of the neural tube continues anteriad and
posteriad. As the neural tube proceeds in its development caudally, it reaches
ultimately the dorsal lip of the now very small blastopore. As the neural tube
sinks inward at the dorsal blastoporal lip, the epidermal attachments to the
sides of the infolding neural tube fuse in a fashion similar to the fusion of the
edges of the neural tube to complete the dorsal epidermal roof. Associated
with this epidermal fusion at the dorsal lip of the blastopore is the fusion
of the epidermal edges of the very small blastopore. The extreme caudal end
of the archenteron or blastoporal canal in this manner is closed off from the
outside (fig. 220D), and the posterior end of the archenteron (the future
hindgut area), instead of opening to the outside through the blastoporal canal,
now opens into the caudal end of the neural tube. In this way, a canal is
formed connecting the caudal end of the future hindgut with the neural tube.
This neurenteric union is known as the neurenteric canal.
 
It is to be observed in connection with the closure of the blastopore and
 
 
 
 
Fig. 226. External views of embryos of Rana sylvatica and Rana pi pie ns. (A to J
after Pollister and Moore: Anat. Rec., 68; K and L after Shumway: Anat. Rec., 78.)
(A, B) Lateral and ventral views of 5-mm. stage. Muscular movement is evident at this
stage, expressed by simple unilateral flexure; tail is about one-fifth body length. (Pollister
and Moore, stage 18.) (C, D) Lateral and ventral views of 6-rnm. stage. Primitive
 
heart has developed and begins to beat; tail equals one-third length of body. (Pollister
and Moore, stage 19.) (E, F) Similar views of 7-mm. stage. Gill circulation is established;
hatches; swims; tail equals one-half length of body. (Pollister and Moore, stage 20.)
(G, H) Ten-mm. stage, lateral and dorsal views. Gills elongate; tail fin is well developed
and circulation is established within; trunk is asymmetrical coincident with posterior
bend in the gut tube; cornea of eyes is transparent; epidermis is becoming transparent.
(Pollister and Moore, stage 22.) (1, J) Eleven-mm. stage, true tadpole shape. Oper
cular fold is beginning to develop and gradually growing back over gills. (K, L)
Eleven-mm. stage of R. pipiens embryo. Observe that opercular folds have grown back
over external gills and developing limb buds; opercular chamber opens on left side of
body only. Indicated in fig. 257B.
 
470
 
 
 
TUBULATION OF ORGAN-FORMING AREAS
 
 
471
 
 
the formation of the neurenteric canal that two important changes occur in
the future hindgut area of the archenteron at this time, namely, the posterior
dorsal end of the archenteron projects dorso-caudally to unite with the neural
tube (fig. 220D), while the posterior ventral end of the archenteron moves
ventrad toward the epidermis where it meets the epidermal invagination, the
proctodaeum (fig. 220D).
 
d. Anterior and Posterior Neuropores; Neurenteric Canal
 
The fusion of the neural folds in the middorsal area proceeds anteriad and
posteriad from the anterior somitic and hindbrain region as described above.
At the anterior end of the forebrain when fusion is still incomplete, an opening
from the exterior to the inside of the neural canal is present; it forms the
anterior neuropore (figs. 229D; 23 IL; 235B; 242E-G; 245B). When fusion is
complete, this opening is obliterated. The caudal end of the neural tube closes
m a similar manner, and a posterior neuropore is formed (figs. 242E, G;
245). In the chick, as in the mammal, the posterior neuropore at first is a
wide, rhomboidal-shaped trough, known as the rhomboidal sinus. The anterior end of the primitive streak is included within the floor of this sinus
rhomboidalis (fig. 235A, B). The point of posterior neuroporal closure is at
the base of the future tail in most vertebrates (fig. 245B), but, in the elasmobranch fishes, this closure is effected after the tail rudiments have grown
caudally for some distance (fig. 229B-E).
 
The vertebrate tail arises from a mass of tissue, known variously as the
tail bud, caudal bud, or end bud, and the posterior end of the neural tube
comes to lie in the end-bud tissues (figs. 225, 238C). The end bud grows
caudally and progressively gives origin to the tail. It consists of the following:
 
(a) the epidermal tube (i.e., the ectodermal covering of the end bud);
within this epidermal layer are
 
(b) the caudal end of the neural tube;
 
(c) the caudal end of the notochord;
 
(d) mesoderm in the form of a mass of rather compact mesenchyme surrounding the growing caudal ends of the notochord and neural tube;
and
 
(e) a caudal growth from the primitive intestine or gut.
 
This extension of the gut tube into the tail is called, variously, the tail gut,
caudal gut or post-anal gut. It varies in length and extent of development in
embryos of different vertebrate species. In some species it is joined to the
neural tube; in others it is not so united. For example, the tail gut is as long
as the trunk portion of the gut in the young shark embryo of 8 to 10 mm. in
length, and at the caudal extremity it is confluent with the neural tube (figs.
21 7A; 229F). The confluent terminal portions of the neural and gut tubes
form the neurenteric canal. This well-developed neurenteric canal extends
 
 
 
 
TUBULATION OF ORGAN-FORMING AREAS
 
 
473
 
 
around the caudal end or base of the notochord. In the developing frog on the
other hand, the confluence between the neural and gut tubes is present only during the initial stages of tail formation, and it thus represents a transient relationship (fig. 223B, C). Consequently, as the tail bud in the frog embryo grows
caudally, the neurenteric connection is obliterated and the tail gut disappears.
On the other hand, in the European frog, Bombinator, the condition is intermediate between frog and shark embryos (fig. 228). True neurenteric canals
within the developing tail are never formed in the reptile, chick, or mammal,
although a tail or post-anal gut, much abbreviated, develops in these forms.
(See paragraph below.) In teleost fishes, Kupffer’s vesicle possibly represents
a small and transient attempt to form a neurenteric canal (fig. 210G). However, the tail gut here, with the exception of the terminally placed Kupffer’s
 
 
Fig. 227. Stages of normal development of Necturus maculosus. (Slightly modified
from Eycleshymer and Wilson, aided by C. O. Whitman; Chap. 1 1 in Entwicklungsgeschichte cl. Wirheltiere, by F. Keibel, ’10.) (A) Stage 15, 14 days, 19 hours after
 
fertilization. Blastopore is circular and reduced; neural groove is indicated in center
of figure. (B) Stage 18, 17 days, 2 hours old. Blastopore is an elongated, narrow
aperture between caudal ends of neural folds; neural folds prominent and neural groove
is deeper. (C) Stage 21, 18 days, 15 hours old, 3 or 4 pairs of somites. Neural folds
are widely separated in head region, narrower in trunk, and coalesced in tail area. (D)
Stage 22, 20 days, 10 hours, 6 pairs of somites, length about 6 mm. Observe head has
three longitudinal ridges, the middle one represents developing brain, while lateral ones
are common anlagen of optic vesicles and branchial arches. (E) Stage 23, 21 days, 2
hours, 10 to 12 pairs of somites, 7 mm. long. Head projects forward slightly above egg
contour; end of tail is prominent; large optic vesicles protrude laterally from head
area; branchial arch region is caudal to optic vesicle enlargement; anus is below tip of
tail. (F) Stage 24, 22 days, 17 hours, 16 to 18 pairs of somites, 8 mm. long. Anterior
half of head is free from egg contour; optic vesicles and mandibular visceral arch are
well defined. (G) Stage 25, 23 days, 10 hours, 20 to 22 pairs of somites, 9 mm. long.
Head is free from egg surface; tail outgrowth is becoming free; mandibular, hyoid, first
branchial and common rudiment of second and third branchial arches are visible. Otic
vesicle lies above hyoid arch and cleft between hyoid and first branchial arches. (H)
Stage 26, 24 days, 22 hours, 23 to 24 pairs of somites, length-^10 mm. Head and caudal
outgrowths are free from egg surface; heart rudiment is shown as darkened area below
branchial arches; cephalic flexure of brain is prominent. (1) Stage 27, 26 days, 26 to
27 myotomes, length — 1 1 mm. Outline of body is straighter; nasal pits and mouth are
well defined, mandibular arches are long; heart is prominent below branchial arches;
anterior limb buds are indicated; faint outlines of posterior limb buds are evident. (J)
Stage 28, 30 days, 8 hours, 30 to 31 myotomes, length — 13 mm. Trunk of embryo is
straight, head and tail are depressed; surface of yolk is covered by dense network of
capillaries; vitelline veins are prominent; pigment appears below epidermis; anterior limb
bud projects dorsally; nuchal or neck flexure is prominent above heart and limb-bud
area. (K) Stage 29, 36 days, 16 hours, 36 to 38 myotomes, length — 16 mm. Mandibular
arches are forming lower jaw; nuchal and tail flexures are straightening; eye and lens
are well defined; anlagen of gill filament are present on gill bars; pigment cells are
evident on head areas; vitelline veins are prominent; yolk-laden, ventro-lateral portion of
trunk is becoming elongated and contracted toward dorsal region of embryo. (L) Stage
30, 40 days, 20 hours, 44 to 46 myotomes, length — 18 mm. Fore and hind limb buds are
prominent; nasal openings are small. (M) Stage 31, larva 49 days, 21 mm. (N) Stage
32, larva 61 days, 25 mm. (O) Stage 33, larva 70 days, 28 mm. (P) Stage 34, larva
97 days, 34 mm. (Q) Stage 35, young adult form, 126 days, 39 mm.
 
 
 
474
 
 
DEVELOPMENT OF PRIMITIVE BODY FORM
 
 
 
Fig. 228. Sagittal section, showing organ-forming areas of Bomhinator embryo. (After
O. Hertwig: Lehrhuch der Entwicklungsgeschkhte des Menschen und der Wirbeltiere.
1890. Jena, G. Fischer.) Observe elongated tail gut.
 
vesicle, is a solid mass of cells. Thus, the shark and Bomhinator embryos, on
the one hand, and the frog, chick, or mammal embryo, on the other, represent
two extremes in the development of the tail gut in the vertebrate group.
 
In the reptiles, also in some birds, such as the duck, in the human embryo,
and certain other mammals, a transient notochordal-neural canal is present
which connects the enteron or gut tube with the caudal area of the forming
neural tube (figs. 200B, E; 207B; 23 IG-K). This canal is occasionally referred
to as a neurenteric canal. However, it is best to view this condition as a special
type of development within the above group, for it is not strictly comparable
to the neurenteric canal formed in the developing tail of the embryos of the
frog, shark, etc., where the neurenteric canal is formed by a definite union
between neural and taihgut tubes as they project caudalward into the tail
rudiment.
 
 
2. Epidermal Tubulation
 
The formation of the external, epidermal, tubular layer of the vertebrate
body is a complex procedure. Its development differs considerably in the
rounded type of gastrula of the Amphibia from that in the flattened gastrula
of the chick or mammal.
 
a. Development of the Epidermal Tube in Amphibia
 
In the frog and other Amphibia, tubulation of the epidermal area of the
blastula begins during gastrulation. At the end of gastrulation, the changes
involved in epiboly have transformed the ectodermal area of the blastula into
an oval-shaped structure, surrounding the internally placed mesoderm and
entoderm (fig. 219). The neural plate material occupies the middorsal area
of this oval-shaped, ectodermal layer, while the future epidermal area forms
the remainder. Following gastrulation, the anterior end of this oval-shaped
structure, in harmony with the forming neural tube, begins to elongate and
 
 
 
TUBULATION OF ORGAN-FORMING AREAS
 
 
475
 
 
grows forward as the head outgrowth (figs. 220, 223, 225). A cylindrical,
epidermal covering for the entire head, in this manner, is produced as the
cranial or brain portion of the neural plate folds inward (invaginates). A
similar outgrowth in the tail area proceeds posteriorly, although here the
neural tube grows caudally by proliferative activity within the epidermal tube
instead of folding into the epidermal tube as it does in the cephalic outgrowth
(figs. 223, 225). Coincident with these two outgrowths, the trunk area, with
its ventral, yolk-filled, entodermal cells, elongates antero-posteriorly as the
neural plate folds inward. It also grows larger in harmony with the head and
tail outgrowths. VAs these activities continue, yolk substance is used up, and
 
 
 
Fig. 229. Early stages of tubulation of neural and epidermal organ-forming areas
with resultant body-form development in the shark, Squaliis acanthias (drawn from prepared slides). Neural area shown in black; epidermal area is stippled white; neural folds
are outlined in white around edges of black area. (Consult also fig. 230.) (A) Embryonic
 
area is raised upward; neural plate is flattened; bilateral tail outgrowths are indicated.
(B) Embryo is considerably elevated from extra-embryonic blastoderm; brain area is
much expanded; trunk region of neural groove is pronounced. (C) Neuralization is
considerably advanced; tail rudiments are converging. (D) Neural and epidermal areas
are well tubulated; tail rudiments are fusing. (E) Young Squalus embryo, lying on left
side; tail rudiments are fused into single caudal outgrowth. The body now consists of
a flexed cephalic outgrowth, trunk region, and tail outgrowth. (F) Squalus embryo of
about 10 mm. in length.
 
 
 
476
 
 
DEVELOPMENT OF PRIMITIVE BODY FORM
 
 
the ventro-lateral region of the trunk is retracted. A cylindrical shape of the
trunk region thus is established, bringing the trunk area into harmony with
the head and tail outgrowths. (Study particularly fig. 227.) The epidermal
area of the late gastrula thus becomes converted into an elongated, epidermal
tube which forms the external covering or primitive skin (see Chap. 12) for
the developing body. In Amphibia, this primitive epidermal tube is two layered,
consisting of an outer epidermal ectoderm and an inner neural ectoderm (figs.
221, 224). (See Chap. 12.) In the newly hatched larva, the epidermis is
extensively ciliated in all anuran and urodele Amphibia.
 
b. Tubulation of the Epidermal Area in Flat Blastoderms
 
In the flat blastoderms of the elasmobranch fish, chick, reptile, and mammal,
the formation of the external body tube involves processes more complicated
than that of the frog type. The following steps are involved:
 
( 1 ) A head fold produces a cephalic epidermal extension above the general tissues of the blastoderm. This rudimentary fold of the epidermis
contains within it a similar fold of the entodermal layer, together with
the invaginating, neural plate material. The notochordal rod lies between the forming entodermal fold and developing neural tube (figs.
213F; 230A; 232I-L; 242B, C). Shortly, the primitive head fold
becomes converted into a cylindrical head outgrowth of the epidermal
and entodermal layers, associated with the forming neural tube and
notochord (figs. 229C, D; 230C; 233). The general process is similar
to that in the frog, but it is more complicated in that the head rudiment
first must fold or project itself up above the extra-embryonic areas,
before initiating the outgrowth process.
 
(2) A second procedure involved in epidermal tubulation in flattened
blastoderms is the dorsal upgrowth movement of epidermal, mesodermal, and entodermal tissues. This activity lifts the trunk region of
the embryo up above the general blastodermic tissues (figs. 213H-J;
234B; 241 ). In some forms, such as the chick, the dorsal upgrowth
movement is more pronounced in the anterior trunk area at first,
gradually extending caudad to the trunk region later (figs. 233, 235).
However, in the pig, human, and shark embryos, the dorsal elevation
extends along the entire trunk area, coincident with the head outgrowth, and thus quickly lifts the embryonic body as a whole up
above the extra-embryonic tissues (figs. 229, 230, 242, 245).
 
(3) The tail outgrowth, in reptiles, birds, and mammals, begins in a manner
similar to that of the head region, and a tail fold first is developed
which later becomes a cylindrical projection, bounded externally with
epidermal cells, within which are found the notochord, tail mesoderm,
and tail portions of neural and gut tubes (figs. 238C; 239K, L; 245B).
 
 
 
B.
 
 
 
 
X / Gill pouches I ANO II
 
INFUNOiBULUM-OOk'y PRE-ORAL GUT
 
PRE-SOMITE MESODERM FIRST GILL CLEFT NOTOCHORD
 
TBirPMINAL CREST \ FACIAL ACOUSTIC CREST
 
X. V \ / GLOSSOPHARYNGEAL CREST S U B N 0 T 0 C H 0 R D A L ROD
 
vaguscrest j
 
dorsal AORT^^^ ENTERIC
 
 
AORTIC ARCH / / I \ "
 
ORAL PLATE / I HEART VITEL
 
rHYROlD GLAND VENTRAL AORTAE
 
 
P
 
YOLK STALK ' •
 
 
Fig. 230. Sagittal sections of early elasmobranch embryos. (Slightly modified from
Scammon. See Chap. 12 in Entwicklungsgesclnchte d. Wirheltiere, by F. Keibel. ) (A)
 
Graphic reconstruction from sagittal sections of embryo of 2 mm., seen from left side
(condition roughly comparable to stage between fig. 229A and B). Observe that neural
plate is broad and flattened with slight elevation of neural folds. (B) Reconstruction
of embryo of 2.7 mm., viewed from left side, showing mesoderm, forming gut, neural
tubes, etc. (Consult (C) below.) (C) Same as (B) with mesoderm removed. Observe
primitive gut and neural tubes. Note: (B) and (C) are comparable to stage shown in
surface view in fig. 229C. (D, E) Same as (B) and (C), embryo 3.5 mm. in length.
 
(This embryo is comparable to fig. 229D.) (F) Same as (D) with mesoderm removed,
 
showing primitive vascular tubes and neural crest cells.
 
 
 
 
 
 
 
 
 
 
 
 
FOURT M CLE AVAGE
 
 
 
EIGHT-CELL STAGE^
 
'Tight-cell stage following third cleavage,
 
EGG INTACT Blastoderm removed from egg
MARGINAL CELLS
CENTRAL CELLS^^.
 
 
FIFTH CLEAVAGE
 
 
 
N l) T 0 CmOH D AL CANAL EPIMERIC MESODERM
ep'permal Tube head outgrowth
 
( EC TO DERM 1 /
 
, FORM I NG â– 
 
y neural TUBE
 
 
 
CLOSING NOTOCHORDAL CANAL
ANlLRlOR r.AHDlN
 
 
 
NOTOCHORD
 
J
 
epidermal 1 u b e
 
N E U R A I T U B E^
 
 
 
HEART
 
VIT^L^LINE
 
 
Fici. 231. Series of diagrams, showing stages in the development of the turtle. (A~F)
Cleavage stages after Agassiz. (G J ) Stages of gastrulation, drawn from slide preparations. (K T) Stages during development of body fiirm. (P, Q, T from Agassiz; the
others are original.) (See L. Agassiz, 1857, Cont. Nat. Hist, of U. S. A., Vol. 11.)
 
 
479
 
 
 
 
Fig. 232. Early post-gastrular development in the chick. (A-H represent a late
head-process stage-— stage 5 of Hamburger and Hamilton, ’51. Compare with figure
203D I-L show the beginnings of the head fold — intermediate condition between stages
7 and 8 of Hamburger and Hamilton, ’51.) (A) Surface view, showing primitive streak,
 
neural plate, and epidermal areas. (B-F) Cross sections of A at levels indicated on
G. (G) Median sagittal section of (A). (H) Same, showing presumptive, organ
forming areas of entoderm notochord, pre-chordal plate, neural plate, and primitive-streak
mesoderm. (1) Surface view, demonstrating a marked antero-posterior extension of the
neural plate area and beginnings of neural folds. Observe shortening of primitive streak.
(J) Drawing of stained specimen. (K) Median sagittal section of (J). (L) Same,
 
showing major organ-forming areas. In (G) and (H) the entoderm, notochord, and
overlying neural ectoderm are drawn as separate layers. Actually, however, at this stage,
the three layers are intimately associated.
 
 
480
 
 
TUBULATION OF ORGAN-FORMING AREAS
 
 
481
 
 
 
Fig. 233, Early body-form development in chick of 3 to 4 pairs of somites. (Approximately comparable to Hamburger and Hamilton, ’51, stage 8, 26 to 29 hours of incubation.) (A) Surface view, unstained specimen. (B) Stained, transparent preparation.
Observe blood islands in caudal part of blastoderm. (C) Median sagittal section. (D)
Same as (C), showing organ-forming layers.
 
direction of the notochord is much more pronounced in the flattened blastoderms than in the rounded blastoderms of the frog, salamander, etc. (cf.
figs. 224; 237). {Note: Associated with the dorsal invagination of the roof
of the midgut in the frog, is the detachment of a median rod of entodermal
cells from the middorsal area of the gut. This median rod of cells comes to
lie between the notochord and the roof of the midgut. It is known as the
subnotochordal rod (fig. 225C). (See Chapter 15.)
 
The development of the rudimentary hindgut is consummated by caudal
 
 
 
482
 
 
DEVELOPMENT OF PRIMITIVE BODY FORM
 
 
growth and extension of the posterior or tail region of the primitive archenteron of the late gastrula. These changes result in an extension of the
archenteron in the direction of the developing tail and the area ventral to the
tail (compare fig. 220B-D with figs. 223B, C; 225B, C).
 
Three general areas of the primitive gut are thus established:
 
(a) a tubular enlargement and outgrowth into the developing head, the
primitive foregut,
 
(b) a tubular extension and growth in the caudal region toward the tail,
the primitive hindgut, and
 
(c) a midgut area whose ventral wall is filled with yolk substance, while
its roof or dorsal wall assumes a trough-like form extending below
the notochord (figs. 223, 224, 225).
 
The foregut and hindgut areas at this time present the following special
features:
 
( 1 ) Two terminal diverticula or evaginations evolve at the extreme anterior
portion of the foregut; and
 
(2) at the extreme caudal end of the hindgut, similar evaginations occur.
 
In the foregut region, one of these evaginations projects toward the brain
and anterior end of the notochord, while the second diverticulum, more pronounced than the dorsal evagination, moves ventrad toward the epidermis
underlying the developing brain. The dorsal evagination represents the preoral or head gut. In the frog it is much abbreviated (figs. 220B, C; 225B, C).
On the other hand, the antero-ventrally directed, oral, or pharyngeal, evagination is relatively large and projects toward the ectoderm underlying the
brain where it forms the future pharyngeal area of the foregut (figs. 220; 223;
225B, C). Ultimately an invagination from the epidermis, the stomodaeum,
becomes intimately associated with the anterior end of the pharyngeal evagination (see Chap. 13). In the hindgut region, the diverticulum which projects
dorsally into the tail is the tail gut, whereas the ventral evagination toward
the epidermis below the tail represents the future rectal and cloacal areas of
the hindgut (figs. 220; 223; 225B, C). It shortly becomes associated with
an invagination of the epidermis, the proctodaeum (fig. 223B, C). As previously mentioned, the tail gut may be well developed, as in the European frog,
Bombinator (fig. 228), or quite reduced, as in the frog, Rana (fig. 225).
 
c. Formation of the Tubular Metenteron in Flat Blastoderms
 
The development of the cylindrical gut tube in those vertebrate embryos
which possess flattened gastrulae is an involved, complicated affair. The developmental mechanics are not clearly understood. For example, it is not
clear whether the embryonic layers, lying in front of the head fold in figure
23 2G and H, are folded slightly backward in figures 232K and L and still farther
 
 
 
MESODERM
 
 
EXTRA-EMBRYONIC
 
COELOM
 
 
 
 
 
 
 
 
 
....
 
' '^‘**®*55ssaas
 
i
 
 
 
 
 
m
 
 
 
 
 
 
'I
 
F. 1
 
1
 
). (
 
1
 
a. A
 
 
 
Fig. 234. Transverse sections of chick embryo with five pairs of somites. (This embryo is slightly older than that shown in fig. 233; a topographical sketch of this developmental stage is shown at the bottom of the figure with level of sections indicated.) Observe
that a dorsal arching (dorsal upgrowth) movement of the dorsal tissues is associated
with neural tube formation. See A and B.
 
 
483
 
 
 
484
 
 
DEVELOPMENT OF PRIMITIVE BODY FORM
 
 
caudad in figure 23 3C and D by autonomous activities within this tissue, or
whether the actively growing head outgrowth proceeds so rapidly that it
mechanically causes the area in front of the head fold to rotate backward
under the developing foregut and thus contribute to the foregut floor. It is
obvious, however, that the entodermal material, lying in front of the head
fold of the embryo, is folded backward, at least slightly, and thus becomes
a part of the floor of the foregut. The extent, however, varies considerably in
different species. It appears to be greater in the mammal (fig. 242C) than in
the chick. Another example suggesting the integration of different movements
of cellular layers is presented in the formation of the floor of the hindgut of
the developing pig embryo. In figure 242C, the rudiments of the fore gut and
hindgut areas are established. However, in figure 242G, it is difficult to evaluate how much of the floor of the hindgut in this figure is formed by actual
ingrowth forward from point “a” and to what extent the floor is formed by
the rapid extension of tissues and backward growth of the caudal region of
the embryo as a whole, including the allantoic diverticulum.
 
Special processes also aid the formation of foregut and hindgut in many
instances. For example, in the chick, the floor of the foregut is established
in part by a medial or inward growth and fusion of the entodermal folds along
the sides of the anterior intestinal portal, as indicated by the arrows in figure
234C. A similar ingrowth of entoderm occurs in the shark embryo (fig. 213J).
although here the entoderm grows in as a solid layer from either side and is
not present in the form of a lateral fold, as in the chick. However, it should
be observed that the formation of the hindgut in the shark embryo arises by
a most interesting and extraordinary method. In the flattened gastrulae of
reptiles, birds, and mammals, the hindgut is established by the formation of
tail folds, involving entodermal and epidermal layers. In the shark embryo,
on the other hand, an enteric groove with enteric folds is formed, and the
folds eventually move ventrad and fuse to form a hollow tube beneath the
notochord of the developing tail.
 
Though the rudimentary foregut and hindgut areas of the metenteron arise
almost simultaneously in mammalian embryos, such as in the pig and human
embryos, in the chick a different sequence of procedure is present. In the
latter species the foregut begins its development immediately following gastrulation when the first pairs of somites are present (fig. 233). The hindgut,
on the other hand, begins its development at a considerably later period when
the embryo has attained many pairs of somites (fig. 238).
 
Once the rudimentary, pouch-like, foregut and hindgut areas have been
established in embryos developing from flattened gastrulae, their further development assumes morphogenetic features similar to those in the frog embryo.
For example, the foregut possesses an antero-dorsal prolongation toward the
brain, the pre-oral or head gut, while slightly posterior to the pre-oral gut,
the future pharyngeal area makes contact ventrally with the stomodaeal in
 
 
TUBULATION OF ORGAN-FORMING AREAS
 
 
485
 
 
vagination from the epidermal (ectodermal) tube (fig. 242G). Similarly, the
caudal region of the hindgut rudiment contacts the proctodaeal invagination
of the epidermal tube, while a tail gut extension continues into the tail (fig.
217).
 
The formation of definitive walls of the midgut area in embryos developing
from the flattened gastrular condition (including the higher mammals which
do not possess large amounts of yolk substance) occurs as follows:
 
(1) Where the entoderm of the midgut terminates on either side of the
notochord at the end of gastrulation, it grows mesad from either side
 
 
 
Fig. 235. Chick embryo of 9 to 10 pairs of somites. (Approximating Hamburger and
Hamilton, ’51, stage 10; 33 to 38 hours of incubation.) (A) Surface view, unstained.
(B) Stained preparation. (C) Median sagittal section. Observe the following: heart
is bent slightly to the right; three primary brain vesicles are indicated; foregut touches
infundibular outgrowth of prosencephalon; first indication of downward bending of the
head outgrowth, i.e., the cephalic (cranial) flexure is evident. (D) Same, showing
major organ-forming areas.
 
 
 
486
 
 
DEVELOPMENT OF PRIMITIVE BODY FORM
 
 
 
 
Fig. 236. Transverse sections through chick embryo of about 12 to 13 pairs of somites,
about 38 hours of incubation. (Approximately between stages 10 to 11 of Hamburger
and Hamilton, ’51, slightly older than that shown in fig. 235.) Observe that the optic
vesicles are constricting at their bases; heart is bent slightly to the right; anterior neuropore
is evident. (A) Optic vesicles. (B) Stomodaeal area. (C) Anterior end of developing heart. (D) Caudal extremity of forming heart. (E) Anterior intestinal portal
and forming caudal portion of the heart. (F) Well-developed somites. (G) Open
neural groove.
 
below the notochord to complete the roof of the midgut (figs. 201 D;
209C; 21 OF; 213). This process is similar to that which occurs in
the Amphibia (cf. fig. 219D).
 
(2) A dorsal arching or evagination of the entoderm toward the notochordal area, comparable to that found in the frog and other Amphibia, is present also. A study of figures 213H-J; 217G; 23 4B;
237E-G; 241B-D demonstrates the marked dorsal upgrowth of all
the forming body layers in the trunk area. {Note: In the elasmobranch
fishes, a subnotochordah rod of cells of entodermal origin is formed
similar to that in the frog and other Amphibia.)
 
( 3 ) The ventro-lateral walls of the midgut area, in contrast to those found
in the frog, are established largely by actual ingrowth of the entoderm,
mesoderm, and ectoderm with subsequent fusion in the median line
 
 
 
 
Fig. 237. Chick embryo of 17 to 19 pairs of somites. (Approximating Hamburger
and Hamilton, ’51, stage 13, 48 to 52 hours of incubation, sections indicated on outline
drawing.) Head lies partly on left side; auditory pits are deep; cervical flexure is evident
in region of rhombencephalon; cephalic flexure is marked; stomodaeum is a deep indentation touching foregut between the first pair of aortal arches; head fold of amnion
reaches back to anterior part of rhombencephalon (hindbrain). (A) Anterior (telencephalic) portion of prosencephalon, showing closed neuropore; amnion is indicated.
(B) Optic vesicles. (C) Anterior end of foregut, showing anterior extremity of stomodaeal invagination and first (mandibular) pair of aortal arches; notochord ends and
pre -chordal plate area begins at about this section. (D) Anterior end of heart (ventral
aorta); observe thin roof plate of neural tube, characteristic of the later myelencephalic
(medulla) portion of rhombencephalon or hindbrain. (E) Otic (auditory) pits and
anterior region of ventricular portion of heart. (F) Caudal limits of forming heart,
dorsal mesocardium, neural crest cells. (G) Caudal end of heart, showing converging
(vitelline) veins of the heart, sclerotome given off to notochordal area, lateral mesocardium forming. (H) Anterior trunk area, showing differentiation of somite and
typically flattened condition of ectoderm, mesoderm, and entoderm. (I) Caudal trunk
area, showing undifferentiated somite (epimeric mesoderm), intermediate mesoderm
(mesomere), and lateral plate mesoderm (hypomere). (J) Similar to (I). (K) Caudal
 
trunk region, showing closing neural tube. (L) Area of Hensen’s node. (M) Primitive streak.
 
 
487
 
 
 
488
 
 
DEVELOPMENT OF PRIMITIVE BODY FORM
 
 
in elasmobranch fishes, reptiles, birds, and mammals. This process involves the formation of lateral body folds which fold mesially toward
the median plane. (Study fig. 241 A-D.) In teleost fishes the process
is different, for in this group the entoderm and mesoderm grow outward beneath the primitive epidermis (ectoderm) and soon envelop
the yolk. Thus, the end result in teleosts is much the same as in the
frog and Necturus, It is well to observe, at this point, that a complete
retraction of the ventro-lateral walls of the midgut and body-wall
tissues surrounding the yolk or yolk-sac area, as in the frog and
Necturus (fig. 227), does not occur in the higher vertebrates, although
in the elasmobranch and teleost fishes such retraction does occur.
 
 
 
Fig. 238. Chick embryo of about 27 to 28 pairs of somites. (Corresponding approximately to Hamburger and Hamilton, ’51, stage 16, 51 to 56 hours of incubation.) Forebrain (prosencephalon) is divided into telencephalon and diencephalon; epiphysis is appearing on roof of diencephalon; cephalic and cervical flexures are pronounced; tail bud
is short; anterior part of body is rotated to the left back to about the thirteenth pair
of somites; amnion now covers anterior three fifths of body; heart shows strong ventricular loop; three pairs of aortal arches can be seen. (A) External view. (B) Transparent wholemount." (C) Sagittal section, diagrammatic.
 
 
 
TUBULATION OF ORGAN-FORMING AREAS
 
 
489
 
 
MESENCHYME OF
 
 
IDERMAL
 
(ECTODERM AL) TUBE ANTERIOR
SEROSA fifl." S * L
 
 
 
 
INTftA* EMBRYONIC COELOM
NOTOCHORD parietal RECESS OF HIS)
 
FOREGUT yUlV^R EPI M YOC A R DIU
 
endocardium
 
 
 
oermomyotoMe
 
COMMON CARDINAL VE
---=^'^^-i:-‘^SlNUS VENOSUS
 
DORSAL ^OHTA LATERAL MESOCAROlUM
O MPHALOMESENTFRIC
 
 
 
notochord
 
FUSED DORSAL AQRTAI
pharyngeal (BRANCHIAL) POUCH in
 
 
 
 
Fig. 239. Sections through chick embryo of age indicated in fig. 238. Level of sections
 
is shown on diagram.
 
(See Chap. 22.) In the elasmobranch fishes, this retraction of tissues
contributes little to the formation of the wall of the enteron or to that
of the body. However, in teleosts such contribution is considerable.
 
At this point reference should be made to figures 23 8C on the chick, 242C
and G on the pig, and 245 B on the early human embryo to gain a visual image
of the developing foregut, midgut, and hindgut areas of the primitive metenteron. Compare with the frog (fig. 225C).
 
 
 
490
 
 
DEVELOPMENT OF PRIMITIVE BODY FORM
 
 
4. Tubulation (Coelom Formation) and Other Features
Involved in the Early Differentiation of the
Mesodermal Areas
 
The differentiation of the mesodermal areas is an all-important feature of
embryonic development, for the mesoderm contributes much to the substance
of the developing body. (See Chaps. 11 and 15.) While the neural, enteric,
and epidermal tubes are being established, radical changes occur within the
two mesodermal layers on either side of the notochord as follows:
 
a. Early Changes in the Mesodermal Areas
 
1) Epimere; Formation of the Somites. The longitudinal mass of paraxial
mesoderm which lies along the side of the notochord forms the epimere
(figs. 221F, G; 234E, F). The two epimeres, one on either side of the notochord, represent the future somitic mesoderm of the trunk area. In the early
post-gastrula, the epimeric mesoderm, together with the notochord, lies immediately below the neural plate. However, as neuralization is effected^ the
 
 
 
Fig. 240. Chick embryo of about 72 to 75 hours of incubation, about stage 20 of
Hamburger and Hamilton, ’51.
 
 
 
 
 
 
492
 
 
DEVELOPMENT OF PRIMITIVE BODY FORM
 
 
area of the anterior trunk and posterior hindbrain region of the embryo. In
the chick embryo (see Patterson, ’07), the most anterior segment forms first,
and later segmentation progresses in a caudal direction. This probably holds
true for most other vertebrates. However, in elasmobranch fishes, segmentation of the epimeric mesoderm also extends forward from the hindbrain
area into the head region presenting a continuous series of somites from the
eye region caudally into the tail (fig. 217D). (Study figs. 217D, 230D.) Segmentation of epimeric mesoderm appears in the head region of Amphibia.
In many higher vertebrates, three pairs of somitic condensations appear in
the area just caudal to the eye but at a slightly later period of development
than that of the elasmobranch fishes (fig. 217D-F).
 
2 ) Mesomere. The narrow longitudinal band of mesoderm, adjoining the
lateral border of the epimere, is the mesomere (figs. 22 IF, G; 230D; 234E, F) .
This mesoderm ultimately gives origin to much of the excretory (kidney)
tissue and ducts and to certain of the reproductive ducts of many vertebrates.
(See Chap. 18.) Because of the origin of nephric tissue from its substance,
this longitudinal band of mesoderm generally is referred to as the urogenital
or nephrotomic mesoderm. Synonymous terms often used are intermediate
mesoderm or intermediate cell mass. The mesomere undergoes a segmentation
similar to the epimeric area in its more anterior portion where the pronephric
kidney develops in higher vertebrates, while in lower vertebrates, such as the
shark embryo, it may be more extensively segmented.
 
3) Hypomere. The remainder of the mesoderm which extends lateroventrally from the mesomere forms the hypomere or hypomeric mesoderm.
It also is called the lateral plate mesoderm or lateral plate mesoblast. This
portion of the mesoderm does not become segmented in present-day vertebrates. (Compare with the condition in Amphioxus described on p. 505.)
 
b. Tabulation of the Mesodermal Areas
 
Coincident with the formation of the somites, a cavity begins to appear
within the mesoderm. This cavity or primitive coelomic space separates the
mesoderm into two layers, an outer layer near the ectoderm and an inner layer
close to the neural, notochordal, and entodermal cells. This hollowing process
within the mesodermal layer is known as coelom formation or tubulation
of the mesoderm. In many embryos of the lower vertebrates, there is a strong
tendency for the coelomic space to form throughout the entire lateral mass
of mesoderm from the epimeric area ventrad into the lateral plate mesoderm.
For example, in elasmobranch (shark) embryos of about 3 to 4 mm. in length
and also in many early post-gastrular amphibia, the following features of the
primitive coelom are found in the trunk region of each mesodermal mass:
 
(1) The mesoderm possesses a cavity, continuous dorso^ventrally from
the epimere into the lateral plate (figs. 217G, H; 221 E). When the
epimere (and to some extent the nephrotomic region as well) under
 
 
LATERAL CONSTRICTIVE MOVEMENTS
 
 
493
 
 
goes segmentation, the coelomic space within these areas becomes
segregated within the segments and, thus, is present in a discontinuous
condition.
 
(2) The early coelomic cavity in the shark and amphibian embryo, therefore, may be divided into three parts: (a) the myocoelic portion within
the epimeric mesoderm, (b) the nephrocoel within the nephrotomic
mesoderm, and (c) the splanchnocoel contained within the hypomeric
or lateral plate mesoderm. While the myocoelic and nephrocoelic regions of the primitive coelom may become segmented and discontinuous, that within the splanchnocoel is continuous antero-posteriorly
in the trunk region.
 
The coelomic cavities contained within the somites of the shark and amphibian embryo are soon lost. The coelomic cavity or nephrocoel within the
nephrotome is concerned with the development of the lumen within the tubules
and ducts of the excretory (urinary) system, while the splanchnocoels give
origin to the coelomic cavity proper of the adult. The lateral wall of the
splanchnocoel near the primitive epidermis is known as the somatopleural
mesoderm, and the inner or medial wall associated with the gut tube and
developing heart tissues constitutes the splanchnopleural layer. The epidermis
and somatopleural mesoderm together form the somatopleure, while the entoderm and splanchnopleural mesoderm form the splanchnopleure.
 
In the embryos of higher vertebrates, the coelomic space of the somitic
portion of the primitive coelom (i.e., the myocoels) is less pronounced and
appears somewhat later in development than in the shark and amphibian
embryo, but it does tend to appear. This is true also of the nephrocoel or
coelomic cavity within the nephrotome. (See Chap. 18.) The coelomic condition or splanchnocoel within the hypomere forms similarly in all vertebrates.
These matters will be described more in detail in Chapter 20.
 
C. Notochordal Area
 
The notochord is the elongated, median band of cells of the gastrula which
lies between the two mesodermal areas. The notochord thus may be regarded
as a specialized, median portion of the middle germ layer of mesodermal
tissue. During gastrulation and shortly after, there may be a tendency for the
notochordal material in certain forms to canalize or tubulate. Later, the notochordal material becomes converted into a definite rod of notochordal cells
which represents the primitive skeletal axis of the embryo. The notochord
and its relation to the early skeletal system are discussed in Chapter 15.
 
D. Lateral Constrictive Movements
 
While the neural, epidermal, and entodermal tabulations are in progress,
a lateral constriction or invagination of the body wall occurs on either side
in all vertebrate embryos from the fishes to the mammals. These constrictions
 
 
 
494
 
 
DEVELOPMENT OF PRIMITIVE BODY FORM
 
 
are effected at the level of the notochord and lower margin of the somitic
area from the anterior trunk region caudally into the tail. As a result, a
transverse section of the early vertebrate body appears pyriform or pear
shaped, with the neck of the pear directed dorsally (fig. 241 C). The constriction line is shown typically in the developing embryo of Necturus (fig.
227 ) where it extends from the lower aspect of the head outgrowth along the
lower boundary of the somitic area to the base of the tail. A line, drawn
across the body from the general area of the two lateral constrictions and
passing through the notochord, divides the embryonic body into an upper or
epaxial (epiaxial) region above the level of the notochord and a lower or
hypaxial (hypoaxial) region below the level of the notochord.
 
£. Tubulation of the Neural, Epidermal, Entodermal, and Mesodermal,
Organ-forming Areas in Amphioxus
 
1. Comparison of the Problems of Tubulation in the Embryo
OF Amphioxus with that of the Embryos in the
Sub PHYLUM Vertebrata
 
a. End-bud Growth
 
In Amphioxus, the procedures involved in tubulation of the major organforming areas and development of primitive body form differ from those in
the vertebrate group. For example, in the latter group, the basic rudiments of
the head, pharyngeal, trunk, and tail regions appear to be well established
at the end of gastrulation. During tubulation of the major organ-forming areas,
these subregions become extended in an antero-posterior direction and the
rudiments of specific structures begin to express themselves. This is especially
true of the head, pharyngeal, and trunk regions. The vertebrate tail, however,
arises from an end-bud tissue which progressively lays down the various parts
of the tail by means of a proliferative growth in the caudal direction. On the
other hand, in Amphioxus, only a small portion of the anterior end of
the future body is laid down during gastrulation. Further development of the
epidermal, neural, enteric, and mesodermal cellular areas together with the
notochord are dependent upon cell proliferation at the caudal end of the late
gastrula and later embryo. Much of the body of Amphioxus, therefore, is
formed by a caudal proliferative growth of end-bud cells, somewhat comparable to the end-bud growth of the tail in the vertebrate group.
 
b. Position Occupied by the Notochord and Mesoderm at the End
of Gastrulation
 
A second feature of difference in the developing embryo of Amphioxus
from that of the vertebrate embryo lies in the arrangement of the notochordmesoderm complex of cells in the late gastrula. In the late gastrula of
 
 
 
TUBULATION OF ORGAN-FORMING AREAS IN AMPHIOXUS
 
 
495
 
 
Amphioxus, this potential, third germ layer forms a part of the entodermal
roof, although the studies of Conklin (’32) have demonstrated that notochord
and mesoderm are distinct cellular entities even in the blastula. In contrast
to this condition, the notochord and the mesoderm already are segregated as
a middle germ layer between the ectoderm and the entoderm in the late
vertebrate gastrula. The gastrula of Amphioxus, therefore, has the added
problem of segregating the notochordal and mesodermal cells from the entoderm during tubulation of the major organ-forming areas.
 
2. Neuralization and the Closure of the Blastopore
 
In the late gastrula of Amphioxus, a longitudinal middorsal plate of cells,
the neural plate, elaborated by cell division and extension during gastrulation,
represents the future central nervous system (fig. 247E). As the period of
gastrulation comes to its end, the blastopore decreases greatly in size (fig.
247A-D). The archenteric opening also moves dorsally, coincident with a
shifting of the caudal end of the archenteron in such a way that it projects
in a dorso-caudal direction (figs. 189G, H; 247H). This movement of the
archenteron is associated with the migration of the mass of mesodermal cells
from the two lateral areas of the blastoporal lips (fig. 247A, B) to the dorsomedial portion of the blastopore (fig. 247C), where the mesoderm comes to
lie on either side of the notochord below the neural plate (fig. 247C). As
these changes occur, the dorsal area of the gastrula near the blastopore becomes flattened with a subsequent depression of the neural plate (fig. 247C,
D). In sagittal section, the gastrula now appears oval in shape and considerably elongated in the antero-posterior direction (fig. 189G, H); in transverse view, it is triangular, especially at the caudal end (fig. 247D).
 
As the above changes are brought about, the ectoderm of the ventral lip
of the blastopore grows dorsad, while that of the lateral lips grows mediad.
In this way, the opening of the blastopore is closed by the coming together
and fusion of these ectodermal (epidermal) growths (fig. 247D-F). However, the archenteron does not lose its connection, at this time, with the
outside environment of the embryo for two reasons:
 
( 1 ) As observed above, the caudal end of the archenteron previously had
shifted in such a manner that it now projects dorso-caudally; and
 
(2) synchronized with the epidermal growth which closes the blastoporal
opening (fig. 248A), the neural plate sinks downward, becoming detached along its margin from the epidermal area (fig. 248B-D).
 
The downward sinking of the neural plate and its detachment from the
epidermal layer begins at the dorsal lip of the blastopore and spreads anteriad.
(Compare fig. 248D with 248B and C.) Consequently, as the epidermal growth
along the lateral lips of the blastopore reaches the area of the sinking neural
 
 
 
 
Fig. 242. Early development of the pig embryo (B, C, and G from Patten: Embryology
of the Pig, Philadelphia, Blakiston; A is from Streeter: Carnegie Inst. Publ. No. 380;
Contrib. to Embryol. 100; D, E, and F from Heuser and Streeter: Carnegie Inst. Publ.
No. 394, Contrib. to Embryol. 109. All figures have been modified), (A) Early, neural
groove stage. Neural area is shown in black; amnion is cut away as indicated. (B)
Four-somite stage. (C) Median sagittal section, approximating the stage of development shown in (B). Observe foregut, midgut, and hindgut areas. (D) Embryo of about
six pairs of somites. (E) Embryo of about 7 to 8 pairs of somites. (F) Eighteen
pairs of somites. (G) Sagittal .sectional diagram of embryo slightly younger than (F),
showing neural and gut tubes, amnion, allantois, and forming heart.
 
 
496
 
 
 
TUBULATION OF ORGAN-FORMING AREAS IN AMPHIOXUS
 
 
497
 
 
plate in the region of the dorsal blastoporal lip, it continues forward along the
epidermal margins of the insinking neural plate, growing mesad and fusing in
the midline over the neural plate (fig. 247E-G). In this way, the epidermal
growth forms a covering for the neural plate. It follows, therefore, that the posterior end of the archenteron will now open into the space between the neural
plate and its epidermal covering. This new passageway between the epidermalneural plate cavity and the archenteron is the beginning of the neurenteric
canal (figs. 247H; 248A).
 
The flattened neural plate, canopied by the epidermal overgrowth, then
begins to fold itself into the form of a tube. In doing so, its lateral edges
swing gradually toward the middorsal line, as shown in figure 195. The actual
grooving and tubulation of the neural plate starts at a point about midway
along the embryo at the stage of development shown in figure 247F. It proceeds anteriorly and posteriorly from this point. At its extreme anterior end,
the neural tube remains open to the surface as the anterior neuropore (figs.
247H; 249A-D). Eventually the caudal end of the neural plate becomes
tubulated, and a definite canal is formed, connecting neural and enteric tubes.
This canal is the neurenteric canal. The neurenteric canal disappears between
the stage of development shown in figure 249C and that shown in figure 249D.
The continued caudal growth of the neural tube is accomplished by cell
proliferation from the posterior end of the tube and neurenteric canal area.
 
 
 
Fig. 243. Sections of pig embryo of about stage shown in fig. 242 (B) and (C).
(Modified from Patten: Embryology of the Pig, 3d Ed., Philadelphia, Blakiston, ’48.)
(A) Line 1, fig. 242C. (B) Line 2, fig. 242C. (C) Line 3, fig. 242C. (D) Line 4,
 
fig. 242C.
 
 
 
498
 
 
DEVELOPMENT OF PRIMITIVE BODY FORM
 
 
MAXILLARY PROCESS
 
 
ANDI BULAR
ARCH
 
HYOID ARCH
 
 
 
Fig. 244. Development of body form in the pig embryo. (A and B from Keibel:
Normentafel zur Entwicklungsgeschichte des Schweines (Sus scrofa domesticus). 1897.
Jena, G. Fischer. C, D, and E slightly modified from Keibel, previous reference, and from
Minot: A Laboratory Text-book of Embryology. 1903. Philadelphia, P. Blakiston’s Son &
Co.) (A) About 4 to 5 mm. (B) About 6 mm. (C) Ten mm. (crown-rump measurement). (D) Fifteen mm. (E) Twenty mm.
 
3. Epidermal Tubulation
 
After the neural plate sinks downward and becomes separated from the
outside epidermis, the medial growth of the epidermis over the neural plate
completes the middorsal area of the primitive epidermal tube (fig. 247 E-H).
It then comes to enclose the entire complex of growing and elongating neural,
 
 
 
Fig. 245. Human embryo of ten somites. (After G. W. Corner: Contrib. to Embryol
Carnegie Inst., Washington, Publ. No. 394, 112.) (A) Dorsal view. (B) Median
 
sagittal section of model.
 
 
 
 
 
500
 
 
DEVELOPMENT OF PRIMITIVE BODY FORM
 
 
 
Fig. 246. Development of body form in human embryo. (C from Keibel and Mall:
Manual of Human Embryology, Vol. I, 1910. Philadelphia and London, Lippincott.
A, B, D, and E from Keibel and Elze: Normentafel zur Entwicklungsgeschichte des
Menschen. Jena, 1908. G. Fischer.) (A) Early neural fold stage. Somites are beginning
to form; notochordal canal is evident. (B) About nine pairs of somites. (C) His’s
embryo M. (D) About 23 pairs of somites, 4-5 mm. long. (E) About 35 pairs of
trunk somites, 12 mm. long.
 
mesodermal, and entodermal tubes and with them it continues to grow in
length principally by rapid cell proliferation at the caudal end of the embryo.
 
4, Tubulation of the Entodermal Area
 
The primitive metenteron of Amphioxus is derived from the archenteron
of the late gastrula as follows.
 
a. Segregation of the Entoderm from the Chordamesoderm and the
Formation of the Primitive Metenteric Tube
 
The mesoderm and notochord which occupy the roof of the archenteron
of the gastrula evaginate dorsally at the anterior end of the embryo and, thus,
become separated from the entoderm. (Compare fig. 195 with fig. 250A.)
This separation of notochord and mesoderm by dorsal evagination from the
 
 
 
TUBULATION OF ORGAN-FORMING AREAS IN AMPHIOXUS
 
 
501
 
 
entoderm continues slowly in a caudal direction from the anterior end until
an embryonic condition is reached approximating about 13 to 14 pairs of
mesodermal segments. At this level, the notochord and mesoderm become
completely separated from the entoderm. As a result, the enteric or gut tube
from this point in its growth posteriad is a separate entity. (See tabulation
of mesoderm on p. 505. Anterior to the fourteenth somite, after the notochord
and mesoderm separate from the entoderm, the latter grows medially from
either side to complete the entodermal roof below the evaginated notochord
and mesoderm (fig. 250A). A primitive metenteric tube thus is formed, as
shown in figure 249C, whose only opening is that which leads by way of the
neurenteric canal (fig. 249 A, C) into the neurocoel of the neural tube and
from thence to the outside through the anterior neuropore.
 
 
NEURAL PLATE CELLS
 
 
 
Fig. 247. Closure of the blastopore and epidermal overgrowth of neural plate in
Amphioxus (original diagrams, based on data supplied by Conklin, ’32). (A) Vegetal
 
pole view of early stage of gastrulation, showing general areas occupied by notochordal,
entodermal, and mesodermal cells. (B) Same view of gastrula, one hour later, showing
triangular form of blastopore, (C) Posterior view of late gastrula. Blastopore is now
ovoid in shape and dorsally placed. Gastrula is triangular in transverse section with
dorsal surface flattened. (D) Same view, later. Slight epidermal upgrowth, indicated
by arrows (a and a') merges with ingrowing epidermal edges along lateral lips of
blastopore (b and b') which spreads along epidermal edges of neural plate. (E) Dorsal
view a brief period later than (D). Epidermal ingrowth from lateral blastoporal lips is
now closing the blastoporal opening, shown in broken lines, and also is proceeding
craniad along edges of sinking neural plate. (See fig. 248.) (F, G) Later stages of
 
epidermal overgrowth of neural plate. (H) Sagittal section of (G).
 
 
 
502
 
 
DEVELOPMENT OF PRIMITIVE BODY FORM
 
 
 
Fig. 248. Sinking of neural plate and epidermal overgrowth of neural plate in Amphioxus. (Slightly modified from Conklin, ’32.) (A) Sagittal section of embryo comparable
 
to that shown in fig. 247F. (B, C, D) Sections through embryo as shown by lines B,
 
C, D, respectively, on (A). Observe that the neural plate begins to sink downward from
region of closed blastopore and proceeds forward from this point.
 
b. Formation of the Mouth, Anus, and Other Specialized Structures
of the Metenteron
 
At the anterior end of the metenteron, a broad, dorsal outgrowth occurs
which continues up on either side of the notochord and becomes divided into
right and left dorsal diverticula (fig. 249B, H). The left diverticulum remains
small and thick-walled and later fuses with an ectodermal invagination to
form the pre-oral pit, described as a sense organ. The right diverticulum,
however, increases greatly in size, becomes thin-walled, and gives origin to
the so-called head cavity.
 
The mouth develops at a time when the larva acquires about 16 to 18 pairs
of mesodermal segments or somites (fig. 249D). It appears when the overlying epidermis about halfway up on the left side of the body fuses with the
entoderm, a fusion which occurs just posterior to the forming pre-oral pit
(left diverticulum). (See black oval fig. 249D, and fig. 249F.)
 
At the time that the mouth forms, the entoderm opposite the first pair of
somites pushes ventrally and fuses with the ectoderm. This area of fusion
finally perforates and forms the first gill slit. The gill slit, once formed, moves
up on the right side of the body (fig. 249E). The entodermal area from
which the first and later gill slits make their appearance is known as the
branchial rudiment (fig. 249D).
 
At the caudal end of the larva, following the degeneration of the neurenteric canal, a small area of entoderm fuses with the ectoderm and forms the
anal opening. The anus is first ventral in position, but later moves up to the
left side as the caudal fin develops (fig. 249E, G).
 
 
 
TUBULATION OF ORGAN*FORMING AREAS IN AMPHIOXUS
 
 
503
 
 
5. Tubulation of the Mesoderm
 
Tubulation of the mesoderm and the formation of a continuous anteroposterior coelom in Amphioxus differs considerably from that found in the
subphylum Vertebrata. This fact becomes evident in tracing the history of
the mesoderm from the time of its segregation from the entoderm of the late
 
 
 
Fig. 249. Various stages of development of Amphioxus. (A from Kellicott, ’13, and
Conklin, ’32; B from Kellicott, ’13, slightly modified; C-I, slightly modified from Conklin,
’32.) (A) Six -somite stage, comparable to fig. 247G and H. The animal hatches about
 
the time that two pairs of somites are present. (B) Nine-somite stage. The larva at this
stage swims by means of cilia which clothe the entire ectodermal surface. (C) About
fourteen pairs of somites are present at this stage. Neurenteric canal is still patent. (D)
About 16 to 18 pairs of somites. Neurenteric canal is degenerating; mouth is formed.
(E) About 20 to 22 pairs of somites. Anal opening is established between this stage and
that shown in (D). (F) Transverse section, showing oral opening, looking from anterior
 
end of animal. (G) Same through anal area. (H) Frontal section of a 24-hour larva
near dorsal side showing notochord, somites (S-1, S-8, etc.) and undifferentiated tissue
at caudal end. Neural tube shown at anterior end. Nine pairs of somites are present. (I)
Frontal section of a 38-hour larva at the level of the notochord showing section through
the neural tube at the anterior and posterior ends, i.e., in region where larva bends
ventralwards. Thirteen pairs of somites are present with muscle fibrillae along the mesial
borders of the somites.
 
 
504
 
 
DEVELOPMENT OF PRIMITIVE BODY FORM
 
 
gastrula and later embryo to the stage where a continuous antero-posterior
coelomic space is formed, comparable to that found in the vertebrates.
 
The mesoderm of the late gastrula of Amphioxus is present as a dorsomedian band of cells on either side of the notochord, and together with the
notochord, occupies the dorsal area or roof of the archenteron as mentioned
previously. In the region of the blastopore, the two mesodermal bands diverge
ventrally and occupy the inner aspect of the lateral walls of the blastopore
(fig. 190F, G; 247B). At about the time of blastoporal closure, the two
mesodermal masses of cells, located along the lateral lips of the blastopore,
are retracted dorsally, where they come to lie on either side of the notochord
(fig. 247C). In this position the two bands of mesoderm and the notochord
continue to form the dorsal region or roof of the archenteron until approximately the time when the embryo is composed of 13 to 14 pairs of mesodermal segments or somites (fig. 249C). (See Hatschek, 1893, pp. 131, 132;
Willey, 1894, p. 115; Conklin, ’32, p. 106.) When the embryo reaches a
stage of development wherein 15 to 16 pairs of somites are present, the
notochord and mesoderm have separated entirely from the entoderm (fig.
249D). At about this period the neurenteric canal between the metenteron
and the neural tube disappears (fig. 249C, D).
 
 
ENTEROCOEL
 
 
DORSAL FIN-RAY CAVITY
 
 
 
SPLANCMNOCOEL
SPLANCHNOCOELS ^FUSE
 
 
I'’
HORIZONTAL SEPTUM
 
 
Fig. 250. Differentiation of somites in Amphioxus. (A and B from Conklin, ’32; C,
E, and F after Hatschek, 1888 and 1893; D from MacBride, 1898; all figures are modified.) (A) Somites shortly after separation from entoderm. (B) Later stage, the
somites grow ventrally. (C) Semitic wall begins to differentiate into a thickened, dorsal,
myotomic area, located near notochord and neural tube, and thinner somatic and visceral
areas. (D) Horizontal septum formed which separates dorso-myotomic portion of somite
from splanchnocoelic area below. (E, F) Later stages in differentiation of myotome and
myocoelic diverticulum. (See text.)
 
 
 
TUBULATION OF ORGAN-FORMING AREAS IN AMPHIOXUS
 
 
505
 
 
The formation of a continuous, antero-posterior, coelomic cavity in Amphioxus may be described as follows. The mesodermal bands on either side
of the notochord of the post-gastrular embryo become converted into mesodermal grooves as each mesodermal band folds inwards or evaginates into
the residual blastocoelic space between the archenteron and the outside ectoderm (fig. 195). Beginning at the anterior end, these longitudinal grooves
of mesoderm soon become divided into distinct segments or somites by the
appearance of transverse divisions (fig. 249 A, B, H). The first and second
pairs of somites are formed at the anterior ends of the mesodermal grooves
at about the time that the embryo hatches and swims about by means of
ciliary action.
 
Eventually each somite becomes entirely constricted from the notochord
and entoderm. In this segregated condition the somite forms a rounded structure retaining within itself a portion of the original archenteric cavity (fig.
250A). Hence, the cavity within the somite is called an enterocoel and represents the beginnings of the coelomic cavity of later development, at least in
the anterior 13 or 14 pairs of somites. (Note: It is to be observed in this
connection that the primitive somite in Amphioxus is not comparable to the
primitive somite of the vertebrate embryo. In the latter, the somite represents
merely a segment of the epimeric mesoderm, whereas in Amphioxus it is the
entire mesoderm in each half of a particular segment of the embryo.)
 
After hatching, the mesodermal bands continue to form into grooves as the
embryo elongates, and, synchronously, successive pairs of somites are formed.
At about the time 8 to 10 pairs of somites are present (fig. 249B, H), the
enterocoels of the first two pairs of somites have become entirely separated
from the archenteron. The enterocoels of the following six pairs of somites
are small and are not as evident at first as those of the first two pairs. Ultimately a definite enterocoel is found, however, in each somite.
 
Posterior to the eighth or ninth pairs of somites, the forming mesodermal
grooves do not show the enterocoelic pouches as plainly as the more anterior
somites. Slit-like mesodermal grooves tend to be present, however, and, when
the somite is entirely free from the archenteron, this slit-like cavity expands
into the enterocoelic space of the somite. As the region of the fourteenth
pair of somites is approached, the slit-like mesodermal groove becomes more
and more indefinite. Posterior to the fourteenth or fifteenth pair of somites,
the somites originate from a solid mesodermal band on either side of the
notochord. An enterocoelic origin of the cavity within each somite, therefore,
is not possible caudal to this area, and the coelomic space arises by a
hollowing-out process similar to coelomic cavity formation in the vertebrate
group.
 
At about the time when eight pairs of somites are established, a shift of
 
 
 
506
 
 
DEVELOPMENT OF PRIMITIVE BODY FORM
 
 
the mesoderm on either side of the embryo produces a condition wherein the
somites of either side may be slightly intersegmental in relation to the somites
on the other side (fig. 249H).
 
During its later development, each somite grows ventrally (fig. 250B).
That portion of the somite contiguous to the notochord and neural tube
thickens and forms the myotome. The region of the somite near the epidermal
ectoderm is called the somatic or parietal mesoderm, while that associated
with the entoderm forms the visceral or splanchnic mesoderm (fig. 25 OC).
 
As the myotome enlarges, the coelomic space becomes more and more
displaced ventrally, and most of it comes to lie on either side of the enteron
(metenteron). (See fig. 250D.) This ventral coelomic space forms the
splanchnocoel, while the dorsal space, lateral to the myotome, is known as
the myocoel. Eventually, the splanchnocoels of each pair of somites push
ventrally to the lower portion of the enteron, where they ultimately fuse
(fig. 250D-F). Gradually the splanchnocoels of each segment fuse anteroposteriorly and in this way a continuous, antero-posterior, splanchnocoelic
space below and around the gut tube is formed. Tubulation or the formation
of a continuous, antero-posterior, coelomic cavity thus is effected by fusion
of the splanchnocoels of the respective somites on either side (fig. 25 OF). A
horizontal septum, the intercoelomic membrane also appears, separating the
myocoels above from the splanchnocoelic cavity below (fig. 250D).
 
6. Later Differentiation of the Myotomic (Dorsal) Area
OF the Somite
 
While the above events are taking place in the ventral portion of the somite,
the upper, myotomic region undergoes profound modification.
 
As shown in figure 250D, the myotomic portion of the somite has two unequally developed areas;
 
( 1 ) a medial muscular portion, the myotome and
 
(2) the laterally placed, thin-walled, parietal part which surrounds the
coelomic space, or myocoel.
 
The muscular portion enlarges rapidly and, as seen in figure 25 OE and F,
forms the muscle plate or myotome of the adult. These muscle plates very
early assume the typical > shape characteristic of the adult. On the other
hand, the myocoelic portion contributes important connective or skeletal
tissue to the framework of the body. In each segment, the wall of the myocoel
gives origin to three diverticula as follows:
 
(a) a lower sclerotomic diverticulum,
 
(b) a ventral diverticulum, and
 
(c) a dorsal sclerotomic diverticulum.
 
 
 
TUBULATION OF ORGAN-FORMING AREAS IN AMPHIOXUS
 
 
507
 
 
The lower sclerotomic diverticulum (fig. 250D, E) extends up between the
myotome and the medially placed notochord and nerve cord, as diagrammed
in figure 250F. Its walls differentiate into two parts:
 
( 1 ) an inner layer which, together with a similar contribution from the
somite on the opposite side, wraps around the notochord and nerve
cord and, subsequently, gives origin to a skeletogenous sheath of
connective tissue which enswathes these structures; and
 
(2) an outer layer which covers the mesial (inner) aspect of the myotome
with a fascia or connective tissue covering.
 
The outer surface of the myotome does not have a covering of fascia.
 
The ventral diverticulum extends between the lateral wall of the splanchnococl and the epidermal layer of the body wall (fig. 250E, F) and separates
the parietal wall of the splanchnocoel from the epidermal wall (fig. 250F).
This ventral diverticulum or dermatomic fold, together with the external or
parietal wall of the myocoel above, forms the dermatome. The inner and
outer layers of the ventral diverticulum gradually fuse to form the cutis or
dermal layer of the integument or skin in the ventro-lateral portion of the
body, whereas the parietal wall of the myocoel above gives origin to the same
dermal layer in the body region lateral to the myotome. The dorsal sclerotomic
diverticula form the fin-ray cavities in the dorsal fin. These cavities become
entirely isolated from the rest of the myocoelic spaces. Several fin-ray cavities
occupy the breadth of a single myotome. The dorsal myotomic portion of the
somite thus differentiates into three main structural parts:
 
(a) the muscular myotome,
 
(b) the mesial sclerotome or skeletogenous tissue, and
 
(c) the latero- ventral dermatome or dermal tissue of the skin.
 
7. Notochord
 
The notochord arises as a middorsal evagination of the primitive archenteron up to about the stage of about 13 to 14 pairs of somites (fig. 195).
Posterior to this region it takes its origin by proliferative growth from a separate mass of notochordal tissue, lying above the gut and between the two
mesodermal masses of cells. Its origin posterior to the general area of the
thirteenth to fourteenth body segments, therefore, has no relation to the entoderm. It rapidly develops into a conspicuous skeletal rod, lying below the
neural tube and between the mesodermal somites and resting in a slight depression along the dorsal aspect of the metenteron or entodermal tubulation
(fig. 249E, H). It continues forward in the head region, anterior to the brain
portion of the neural tube (fig. 249E).
 
(The student is referred to the following references for further details relative to the early development of Amphioxus: Cerfontaine, ’06; Conklin, ’32;
 
 
 
508
 
 
DEVELOPMENT OF PRIMITIVE BODY FORM
 
 
Hatschek, 1893; Kellicott, ’13; MacBride, 1898, ’00, ’10; Morgan and Hazen,
’00; and Willey, 1894.)
 
F. Early Development of the Rudiments of Vertebrate Paired
Appendages
 
Two pairs of appendages, placed at the anterior (pectoral) and posterior
(pelvic) extremities of the trunk, are common to all vertebrate groups. However, all vertebrates do not possess two pairs of paired appendages. Certain
lizards of the genera Pygopus and Pseudopus have only a posterior pair of
appendages, while in certain other vertebrates the opposite condition is found,
the anterior pair being present without posterior appendages. The latter condition is found in certain teleost and ganoid fishes; the amphibian. Siren
lacertina; the lizard, Chirotes; and among the mammals, the Sirenia and Cetacea. Again, some vertebrates are entirely apodal, e.g., cyclostomatous fishes
and most snakes, although the boa constrictors and pythons possess a pair of
rudimentary posterior appendages embedded in the skin and body wall. Some
have rudimentary appendages only in the embryo, as the legless amphibians
of the order Gymnophiona, and certain lizards. Consequently, the presence
of embryonic rudiments of the paired appendages is a variable feature when
the entire group of vertebrates is considered.
 
The rudiments of the paired appendages also are variable, relative to the
time of their appearance in the vertebrate group as a whole. They are more
constant in the Amniota, i.e., reptiles, birds, and mammals, in time of appearance than in the Anarnniota, i.e., fishes and amphibia. In the reptiles,
birds, and mammals, the limb buds arise when primitive body form is being
evolved. In the anuran amphibia, the anterior rudiments may appear and go
on to a high degree of differentiation before the appearance of the posterior
pair of appendages. For example, in the frog, Rana pipiens, the posterior limb
buds first make their appearance a brief period before the beginning of
metamorphosis of the tadpole into the adult form. However, the anterior
limb buds differentiate earlier but remain concealed beneath the operculum
until they become visible during the later stages of metamorphosis. In urodele
amphibia, the fore limb bud is not covered by an operculum, and it is visible
at the time of its initial appearance which occurs before the hind limb rudiment
arises (fig. 227J-L).
 
In the majority of vertebrates, the limb rudiment first makes its appearance
as an elongated, dorso-ventrally flattened fold of the epidermis, containing a
mass of mesodermal cells within, as shown, for example, in the chick and
mammalian embryos (figs. 240, 244, 246). The contained mesodermal cells
may be in the form of epithelial muscle buds derived directly from the myotomes (e.g., sharks) or as a mass of mesenchyme (chick, pig, human). (See
Chap. 16.) The early limb-bud fold may be greatly exaggerated in certain
elasmobranch fishes, as in the rays, where the anterior and posterior fin folds
 
 
 
LIMB BUD AN ILLUSTRATION OF FIELD CONCEPT OF DEVELOPMENT
 
 
509
 
 
fuse together for a time, forming one continuous lateral body fold. On the
other hand, in the lungfishes (the Dipnoi) and in amphibia (the Anwa and
Urodela), the appendage makes its first appearance, not as an elongated fold
of the lateral body wall, flattened dorso-ventrally, but as a rounded, knob-like
projection of the lateral body surface (fig. 227K-M).
 
G. The Limb Bud as an Illustration of the Field Concept of Development
in Relation to the Gastrula and the Tubulated Embryo
 
In Chapter 9 it was observed that the major presumptive organ-forming
areas are subdivided into many local, organ-forming areas at the end of gastrulation. In the neural and epidermal areas, this subdivision occurs during
gastrulation through influences associated with local inductive action. At the
end of the gastrular period, therefore, each local area within the major organforming area possesses the tendency to give origin to a specific organ or a
part of an organ. The restricted, localized areas within each major organforming area represent the individual, or specific, organ-forming fields. During tubulation, the major organ-forming areas with their individuated,
organ-forming fields are molded into tubes, and, thus, the individual fields
become arranged along each tube. Consequently, each tube possesses a series
of individual, organ-forming areas or fields, distributed antero-posteriorly
along the tube.
 
As a result of the close association of cells and substances during gastrulation and tubulation, many specific organ-forming fields are related to more
than one of the body tubes. Specific organ-forming fields, therefore, may have
intertubular relationships. For example, the lens field is located in the epidermal tube, but, in many species, its origin as a lens field is dependent
upon influences emanating from the optic vesicle of the neural tube (see
Chap. 19). Another example of an association between the parts of two contiguous tubes is the limb-bud field in the urodele, Amby stoma punctatum. As
the limb-bud field in this species illustrates various aspects and properties of
an organ-forming field, it will be described below in some detail.
 
The presumptive anterior limb disc or limb field of Amby stoma is determined as a specific limb-forming area in the middle gastrular stage (Detwiler,
’29, ’33). Later on in the embryo, it occupies a circular-shaped area within
trunk segments three to six. According to Harrison (’18) and Swett (’23),
its properties as a field mainly are resident in the cells of the somatic layer
of the mesoderm in this area. If, for example, the somatic layer of mesoderm
in this area is transplanted to another area, a well-developed limb will result.
Also, the mesoderm of the dorsal half of the field forms a greater part of the
limb than the other parts, with the anterior half of the limb disc next in importance. It appears, therefore, that the limb-forming potencies are greatest
in the dorso-anterior half of the limb field and become less postero-ventrally.
Moreover, not “all of the cells which are potentially limb forming go into
 
 
 
510
 
 
DEVELOPMENT OF PRIMITIVE BODY FORM
 
 
the limb” (Swett, ’23). As demonstrated by Harrison (’18) half discs (half
fields), left intact in the developing embryo or removed and transplanted to
other areas, develop into normal limbs.
 
The above experiments of Harrison, together with those of Detwiler (’29,
’33) suggest that while the limb field is irreversibly determined at an early
stage to form limb tissue, the exact determination of the various parts within
the field is absent at the earlier phases of development. One kind of precise
determination is present, however, for the first digit-radial aspect (i.e., the
pre-axial aspect) of the limb appears to arise only from the anterior end of
the field, whether the field is allowed to develop intact or is split into two
parts. That is, if it is split into two portions, the anterior extremity of the
posterior portion, as well as the original anterior part of the limb field, develops the pre-axial aspect of the limb. This antero-posterior polarization is
present from the first period of field determination. On the other hand, the
dorso-ventral polarity is not so determined; for if the transplanted limb disc
is rotated 180 degrees (i.e., if it is removed and reimplanted in its normal
place dorsal side down) it will develop a limb with the dorsal side up but
with the antero-posterior axis reversed (Harrison, ’21). In these cases the
first digit-radial aspect will appear ventral in position. This result indicates
that the pre-axial aspect of the limb becomes oriented always toward the ventral
aspect of the limb. However, the experiments of Swett (’37, ’39, ’41) tend to
show that the reversal of the dorso-ventral axis occurs only when implanted
below the myotome s; for when the rotated limb field is implanted in the
somitic (myotomic) area, it will remain inverted. Factors other than those
resident within the limb field itself, probably factors in the flank area, appear
thus to induce the normal dorso-ventral axis when the limb disc is implanted
in its normal site.
 
In the descriptions given above, the importance of the somatic layer of
mesoderm as the seat of the limb-forming factors is emphasized. It is obvious,
however, that the epidermal covering of the limbs derived from the epidermal
tubulation also is important in limb formation. For example, epidermal importance is suggested by the experiments of Saunders (’49) on the developing
limb bud of the chick wherein it was found that the apical ridge of ectoderm,
located at the apex of the early limb bud, is essential for normal limb
development.
 
Individual, or specific, organ-forming fields which appear in the gastrula
and early tubulated embryo thus are generalized areas determined to form
individual organs. As development proceeds, two main limitations are imposed upon the field:
 
( 1 ) The cellular contribution of the field actually entering into the organ
becomes restricted; and
 
(2) specific parts of the field become progressively determined to form
specific parts of the organ. •
 
 
 
CEPHALIC FLEXION AND GENERAL BODY BENDING
 
 
511
 
 
It is obvious, therefore, that the fields of influence which govern the development of specific organs may be much more extensive in cellular area
than the actual cellular contributions which take part in the formation of
the specific organ structures. Experiments on the forming limb of Amby stoma
also have demonstrated that a particular area of the field is stronger in its
limb-forming potencies than other regions of the field. This property probably
is true of other fields as well.
 
(For a detailed discussion of the field concept in embryonic development,
reference should be made to Huxley and DeBeer, ’34, Chaps. 8 and 9; Weiss,
’39, p. 289 ff.)
 
H. Cephalic Flexion and General Body Bending and Rotation in
Vertebrate Embryos
 
The anterior end of the neural tubulation is prone to assume a bent or
flexed contour whereby the anterior end of the neural tube is directed downward toward the ventral aspect of the embryo. This general behavior pattern
is strong in vertebrate embryos with the exception of the teleost fishes. In
teleost fishes this bending habit is slight. As the later development of the
head progresses in other vertebrate embryos, the neural tube shows a pronounced cephalic (cranial) flexure in the region of the midbrain, in some
species more than in others. (See Chap. 19.) An additional bending occurs
in the posterior hindbrain area. The latter flexure is the cervical or nuchal
flexure (figs. 231, 238, 240, 244, 246).
 
Aside from the acute bending which takes place in the formation of the
cephalic and the nuchal flexures, there is a definite tendency for many vertebrate embryos to undergo a general body bending, with the result that the
anterior part of the body and the caudal portion of the trunk and tail may
be depressed in a ventral direction (figs. 222C-E; 227; 229F; 238; 240;
244; 246). In the frog embryo, at hatching, the opposite tendency may
prevail for a brief period, and the dorsal trunk region may appear sagging
or hollowed (fig. 226A, C).
 
In addition to these bending movements, in the embryos of higher vertebrates, a rotation or twisting (torsion) of the developing body along the
antero-posterior axis is evident. In the chick embryo, for example, the head
region begins to rotate toward the right at about 38 hours of incubation.
Gradually this torsion continues caudally (figs. 237, 238, 239, 260). At
about 70 to 75 hours, the rotational movement reaches the tail region, and
the embryo then comes to lie on its left side throughout its length (fig. 240).
In exceptional embryos, the rotational movement is toward the left, and the
embryo comes to lie on its right side. Similar movements occur in the pig
and other mammals.
 
This rotational movement is advantageous, particularly in long-bodied
Amniota, such as the snakes, where it permits the developing embryo to coil
 
 
 
512
 
 
DEVELOPMENT OF PRIMITIVE BODY FORM
 
 
in spiral form within the extra-embryonic membranes. The coiling tendency,
however, is not alone confined to the snake group, for the habits of general
body bending, referred to above, essentially is a coiling tendency. Viewed
thus, the rotation or torsion of the developing body along its median axis is
a generalized behavior pattern which permits and aids the coiling habit so
prevalent among the embryos of higher vertebrates. It may be observed further
that the coiling behavior is a common attitude during rest not only among
snakes but also among the adults of many higher vertebrates.
 
I. Influences Which Play a Part in Tubulation and Organization of
 
Body Form
 
In Chapter 9, it was pointed out that the pre-chordal plate material,
that is, organizer material which invaginates first during gastrulation and
which comes to lie in the future head region, induces the organization of
certain head structures and itself may form a part of the pharyngeal wall
and give origin to head mesoderm, etc. On the other hand, the trunk-organizer
material (notochord and somitic mesoderm) which moves to the inside, following the pre-chordal plate material, organizes the trunk region. The following series of experiments based upon work by Spemann, ’31, sets forth the
inductive properties of these two cellular areas:
 
 
Experiment
 
 
1. Head-organizer material, taken from one embryo and placed at head level of a
host embryo, will induce a secondary head, having eyes and ear vesicles
 
2. Head-organizer material, transplanted to trunk and tail levels in host embryos,
induces a complete secondary embryo, including head
 
3. Trunk-organizer material (i.e., notochord and somitic mesoderm), placed at head
level in host embryo, induces a complete secondary embryo, including the head
structures
 
4. Trunk-organizer material, placed at future trunk or tail levels in host embryos,
induces trunk and tail structures only
 
 
The many influences which play a part in the organization of the vertebrate
head and body constitute an involved and an unsolved problem. The extreme
difficulty of this general problem has long been recognized. (See Kingsbury
and Adelmann, ’24.) The above-mentioned work of Spemann represents a
beginning attempt to analyze this aspect of development and to understand
the factors involved. It demonstrates that the organization of the neural tube
and other axial areas is dependent upon specific cellular areas which migrate
inward during gastrulation. However, this is but one aspect of the problem.
As observed in the series of experiments above, trunk-organizer material
is able to organize a complete secondary embryo, including the head, when
 
 
 
 
 
 
TUBULATION AND ORGANIZATION OF BODY FORM
 
 
513
 
 
 
 
Fig. 251. Dependency of neural tube formation upon surrounding tissues. (A) Effect
of notochord without myotomes. (B) Effect of myotomes without notochord. (C)
Absence of notochord and myotomes.
 
 
 
placed at head level in the host but can only organize trunk and tail structures
when placed in trunk and tail areas of the host. In other words, there exists
a mutual relationship between the level of the host tissues and the transplanted
organizer material of the trunk organizer in effecting tlie formation of a head
at the head level.
 
Another forceful example of the interrelationship of developing parts and
formative expression of body structures is shown by the work of Holtfreter
(’33) on the development of the neural tube. This work demonstrates that
the form of the neural tube is dependent upon influences in its environment,
as shown in figure 251. The presence of the later developing notochord determines a thin ventral floor of the neural canal, whereas the contiguous
myotome determines a thick wall of the neural tube. Normally, in development,
the notochord lies below the neural tube, while the somites with their myotomie
parts come to lie lateral to the tube. That is to say, the normal bilateral symmetry of the neural tube is dependent upon the relationship, in their normal
positions, of the notochord and the myotomes.
 
The behavior of the developing neural tube, relative to the notochord and
the myotomes, demonstrates the importance of the’ migration of the somitic
mesoderm from a position contiguous and lateral to the notochord at the
beginning of neuralization to one which is lateral to the forming neural tube
as neuralization and differentiation of the neural tube progresses.
 
A further illustration of the probable influence of the notochordal area in
morphogenesis and organization of body form is the behavior of the developing metenteron or enteric tube. As observed previously, the gut tubulation
tends to invaginate or arch upward toward the notochord not only in embryos
developing from flattened gastrulae but also in amphibia. The movement of
the entoderm toward the notochord strikingly resembles the behavior of the
neural plate ectoderm during the formation of the neural tube. This comparison becomes more striking when one considers the manner of enteron for
 
 
514
 
 
DEVELOPMENT OF PRIMITIVE BODY FORM
 
 
mation in the tail and hindgut regions in the shark embryo, Squalus acanthias,
already mentioned, p. 484. In this species the entoderm of the developing tail
actually invaginates dorsad and closes in a manner similar to the forming
neural tube. That is to say, in the developing tail of the shark, two invaginations toward the notochord are evident, one from the dorsal side, which involves the formation of the neural tube, and the other from the ventral side,
effecting the developing enteric tube.
 
The above facts suggest, therefore, that one of the main organizing influences at work during tubulation and primitive body formation emanates from
the pre-chordal plate area, the notochord, and the epimeric portion of the
mesoderm. From this general area or center, a chain of acting and interacting
influences extends outward, one part acting upon another, to effect the formative expression of the various parts of the developing body.
 
J. Basic Similarity of Body-form Development in the Vertebrate Group
of Chordate Animals
 
In the earlier portion of this chapter, differences in the general procedures
concerned with tubulation and primitive body formation in round and flattened gastrulae were emphasized. However, basically all vertebrate embryos
show the same tendency of the developing body to project itself upward and
forward in the head region, dorsally in the trunk area and dorso-posteriad in
the tail region. Literally, the embryonic body tends to lift itself up out of,
and above, the area which contains the yolk and extra-embryonic tissues.
This proneness to move upward and to protrude its developing head end
forward and its caudal end backward is shown beautifully in the development
of the embryos of the shark (figs. 229, 230), the mud puppy (fig. 227), the
chick (fig. 235C), and the pig (fig. 242). The embryo struggles to be free
from its bed of yolk and extra-embryonic tissue, as it were, and it reminds
one of the superb imagery employed by the poet, John Milton, in his immortal poem, Paradise Lost, where he describes the development of the
lion thus:
 
The grassy clods now calv’d; now half appear’d
 
The tawny lion, pawing to get free
 
His hinder parts, then springs as broke from bonds,
 
And rampant shakes his brinded mane.
 
In summary, therefore, although it appears that rounded and flattened
gastrulae in the vertebrate group may have slightly different substrative conditions from which to start, they ail employ essentially similar processes in
effecting tubulation of the respective, major organ-forming areas and in the
development of primitive body form.
 
 
 
Bibliography
 
 
Cerfontaine, P. 1906. Recherches sur le
developpement de I’ Amphioxus. Arch,
biol., Paris. 22:229.
 
Conklin, E. G. 1932. The embryology of
Amphioxus. J. Morphol. 54:69.
 
Dean, B. 1896. The early development of
Amia. Quart. J. Micr. Sc. (New Series)
38:413.
 
Detwiler, S. R. 1929. Transplantation of
anterior limb mesoderm from Amhlystonia embryos in the slit blastopore
stage. J. Exper. Zool. 52:315.
 
. 1933. On the time of determination of the antero-posterior axis of the
forelimb in Amhly stoma. J. Exper. Zool.
64:405.
 
Hamburger, V. and Hamilton, H. L. 1951.
A series of normal stages in the development of the chick embryo. J. Morphol.
88:49.
 
Harrison, R. G. 1918. Experiments on the
development of the forelimb of Ainblystorna, a self-differentiating equipotential
system. J. Exper. Zool. 25:413.
 
. 1921. On relations of symmetry
 
in transplanted limbs. J. Exper. Zool.
32:1.
 
Hatschek, B. 1888. Uber den Schechtenbau
von Amphioxus. Anat. Anz. 3:662.
 
Hatschek, B. 1893. The Amphioxus and its
development. Translated by J. Tuckey.
The Macmillan Co., New York.
 
Holtfreter, J. 1933. Der Einfluss von Wirtsalter und verschiedenen Organbezirken
auf die Differenzierung von angelagertem Gastrulaektoderm. Arch. f. EntwickIngsmech. d. Organ. 127:619.
 
Huxley, J. S. and De Beer, G. R. 1934.
The Elements of Experimental Embryology. Cambridge University Press,
London.
 
Kcllicott, W. E. 1913. Outlines of Chordate Development. Henry Holt & Co.,
New York.
 
Kingsbury, B. F. and Adelmann, H. B.
1924. The morphological plan of the
head. Quart. J. Micr. Sc. 68:239.
 
MacBride, E. W. 1898. The early development of Amphioxus. Quart. J. Micr. Sc.
40:589.
 
 
. 1900. Further remarks on the development of Amphioxus. Quart. J. Micr.
Sc. 43:351.
 
. 1910. The formation of the layers
 
in Amphioxus and its bearing on the interpretation of the early ontogenetic processes in other vertebrates. Quart. J. Micr.
Sc. 54:279.
 
Morgan, T. H. and Hazen, A. P. 1900.
The gastrulation of Amphioxus. J. Morphol. 16:569.
 
Needham, J. 1942. Biochemistry and Morphogenesis. Cambridge University Press,
London.
 
Patterson, J. T. 1907. The order of appearance of the anterior somites in the chick.
Biol. Bull. 13:121.
 
Saunders, J. W. 1949. An analysis of the
role of the apical ridge of ectoderm in
the development of the limb bud in the
chick. Anat. Rec. 105:567.
 
Selys-Longchamps, M. de. 1910. Gastrulation et L)rmation des feuillets chez Petromyzon planeri. Arch, biol., Paris.
25:1.
 
Spemann, H. 1931. Uber den Anteil von
Implantat und Wirtskeim an der Orienticrung und Beschaffenheit der induzierten Embryonalanlage. Arch. f. EntwickIngsmech. d. Organ. 123:389.
 
Swett, F. H. 1923. The prospective significance of the cells contained in the four
quadrants of the primitive limb disc of
Amhly stoma. J. Exper. Zool. 37:207.
 
. 1937. Experiments upon delayed
 
determination of the dorsoventral limb
axis in Amhly stoma punctatum (Linn.).
J. Exper. Zool. 75:143.
 
. 1939. Further experiments upon
 
the establishment and the reversal of
prospective dorsoventral limb-axis polarity. J. Exper. Zool. 82:305.
 
. 1941. Establishment of definitive
 
polarity in the dorsoventral axis of the
forelimb girdle in Amhly stoma punctatum (Linn.). J. Exper. Zool. 86:69.
 
Weiss, P. 1939. Principles of Development.
Henry Holt & Co., New York.
 
Willey, A. 1894. Amphioxus and the Ancestry of the Vertebrates. The Macmillan Co., New York.
 
 
515
 
 
 
11
 
Basic Features of VerteLrate Morplio^enesis
 
 
A. Introduction
 
1. Purpose of this chapter
 
2. Definitions
 
a. Morphogenesis and related terms
 
b. Primitive, larval, and definitive body forms
 
1 ) Primitive body form
 
2) Larval body form
 
3) Definitive body form
 
3. Basic or fundamental tissues
 
B. Transformation of the primitive body tubes into the fundamental or basic condition
of the various organ systems present in the primitive embryonic body
 
1. Processes involved in basic system formation
 
2. Fundamental similarity of early organ systems
 
C. Laws of von Baer
 
D. Contributions of the mesoderm to primitive body formation and later development
 
1. Types of mesodermal cells
 
2. Origin of the mesoderm of the head region
 
a. Head mesoderm derived from the anterior region of the trunk
 
b. Head mesoderm derived from the pre-chordal plate
 
c. Head mesoderm contributed by neural crest material
 
d. Head mesoderm originating from post-otic somites
 
3. Origin of the mesoderm of the tail
 
4. Contributions of the trunk mesoderm to the developing body
 
a. Early differentiation of the somites or epimere
 
b. Early differentiation of the mesomere (nephrotome)
 
c. Early differentiation and derivatives of the hypomere
 
1) Contributions of the hypomere (lateral plate mesoderm) to the developing
pharyngeal area of the gut tube
 
2) Contributions of the hypomere (lateral plate mesoderm) to the formation
of the gut tube and heart structures
 
3) Contributions of the hypomere (lateral plate mesoderm) to the external
(ectodermal or epidermal) body tube
 
4) Contributions of the hypomere or lateral plate mesoderm to the dorsal body
areas
 
5) Contributions of the lateral plate mesoderm to the walls of the coelomic
cavity
 
5. Embryonic mesenchyme and its derivatives
 
 
516
 
 
 
 
INTRODUCTION
 
 
517
 
 
E. Summary of later derivatives of presumptive, major, organ-forming areas of the late
blastula and gastrula
 
1. Neural plate area (ectoderm)
 
2. Epidermal area (ectoderm)
 
3. Entodermal area
 
4. Notochordal area
 
5. Mesodermal areas
 
6. Germ-cell area
 
F. Metamerism
 
1. Fundamental metameric character of the trunk and tail regions of the vertebrate
body
 
2. Metamerism and the basic morphology of the vertebrate head
 
G. Basic homology of the vertebrate organ systems
 
1. Definition
 
2. Basic homology of vertebrate blastulae, gastrulae, and tubulated embryos
 
==Basic Features of Vertebrate Morphogenesis==
 
A. Introduction
 
1. Purpose of This Chapter
 
In this chapter, the basic morphogenetic features which give origin to the
later organ systems are emphasized. These features arise from the stream of
morphogenetic phenomena which come down from the fertilized egg through
the periods of cleavage, blastulation, gastrulation, and tabulation. This chapter
thus serves to connect the developmental processes, outlined in Chapters 6
to 10, with those which follow in Chapters 12 to 21. As such, it emphasizes
certain definitions and basic structural features involved in the later morphogenetic activities which mold the adult body form.
 
2. Definitions
 
a. Morphogenesis and Related Terms
 
The word morphogenesis means the development of form or shape. It involves the elaboration of structural relationships. The morphogenesis of a
particular shape and structure of a cell is called cytomorphosis or cytogenesis
and is synonymous with the term cellular differentiation, considered from the
structural aspect. In the Metazoa, the body is composed of groups of cells,
each cellular group possessing cells of similar form and function. That is, each
cell group is similarly differentiated and specialized. A cellular group, composed of cells similar in form (structure) and function, is called a tissue.
Histology is the study of tissues, and the word histogenesis relates to that
phase of developmental morphology which deals with the genesis or development of tissues. An organ is an anatomical structure, produced by an association of different tissues which fulfills one or several specialized functions.
For example, the esophagus, stomach, liver, etc., are organs of the body.
During development, each of the major organ-forming areas, delineated in
 
 
 
518
 
 
BASIC FEATURES OF VERTEBRATE MORPHOGENESIS
 
 
Chapters 6, 7, 9 and 10, produce several specific organs. Organogenesis is
concerned with the formation of these specific organs. A group of organs
which are associated together to execute one general function form an organ
system. The digestive system, for example, has for its general function that
of obtaining nourishment for the body. It is composed of a series of o’-gans
integrated toward this end. The nervous system, similarly, is an assemblage
of specific organs devoted to the discharge of nervous functions. So it is with
the other systems of the organism. System development is concerned with
the genesis of such systems. The association of various systems, integrated
together for the maintenance of the body within a particular habitat, constitutes the organism. Finally, the organism acquires a particular body form
because of the form, structure, and activities assumed by its organ systems
as a result of their adaptation to the functional necessities of the particular
habitat in which the organism lives. It should be urged further that this nice
relationship between form and structure, on the one hand, and functional
requirements, on the other, is a fundamental principle of development from
the egg to the adult. It is a principle intimately associated with the morphogenesis of the organ systems described in Chapters 12 to 21.
 
During development from the egg to the adult form, three major types of
body form are evolved in the majority of vertebrate species.
 
b. Primitive, Larval, and Definitive Body Forms (see fig. 255)
 
1) Primitive Body Form. The condition of primitive or generalized, embryonic body form is attained when the embryo reaches a state in which its
developing organ systems resemble the respective developing organ systems
in other vertebrate embryos at the same general period of development. (See p.
520.) Superficially, therefore, the general structure of the primitive embryonic
body of one species resembles that of the primitive embryonic bodies of other
vertebrate species. Such comparable conditions of primitive, body-form development are reached in the 10 to 15-mm. embryo of the shark, Squalus
acanthias, of the frog embryo at about 5 to 7 mm., the chick at about 55 to
96 hrs. of incubation, the pig at 6 to 10 mm., and the human at 6 to 10 mm.
 
2) Larval Body Form. Following primitive body form, the embryo gradually transforms into a larval form. The larval form is present in the period
between primitive body form and definitive body form. The larval period is
that period during which the basic conditions of the various organ systems,
present in primitive body form, undergo a metamorphosis in assuming the
form and structure of the adult or definitive body form. In other words, during
the larval period, the basic or generalized conditions of the various organ
systems are changed into the adult form of the systems, and the larval period
thus represents a period of transition. Embryos which develop in the water
(most fishes, amphibia) tend to accentuate the larval condition, whereas
those which develop within the body of the mother (viviparous teleosts,
 
 
 
INTRODUCTION
 
 
519
 
 
sharks, mammals) or within well-protected egg membranes (turtle, chick)
slur over the larval condition.
 
The larval stage in non-viviparous fishes (see Kyle, ’26, pp. 74-82) and in
the majority of amphibia is a highly differentiated condition in which the
organs of the body are adapted to a free-living, watery existence. The tadpole
of the frog, Rana pipiens, from the 6-mm. stage to the 11 -mm. stage, presents
a period during which the primitive embryonic condition, present at the time
of hatching (i.e., about 5 mm.), is transformed into a well-developed larval
stage capable of coping with the external environment. From this time on to
metamorphosis, the little tadpole possesses free-living larval features. Another
example of a well-developed, free-living, larval stage among vertebrates is
that of the eel, Anguilla rostrata. Spawning occurs in the ocean depths around
the West Indies and Bermuda. Following the early embryonic stage in which
primitive body form is attained, the young transforms into a form very unlike
the adult. This form is called the Leptocephalus. The Leptocephalus was formerly classified as a distinct species of pelagic fishes. After many months in
the larval stage, it transforms into the adult form of the eel. The latter migrates
into fresh-water streams, the American eel into streams east of the Rockies
and the European eel into the European streams (Kyle, ’26, pp. 54-58). The
larval stages in most fishes conform more nearly to the adult form of the fish.
 
The embryo of Squalus acanthias at 20 to 35 mm. in length, the chick
embryo at 5 to 8 days of incubation, the pig embryo of 12- to 18-mm. length,
and the human embryo of 12 to 20-mm. length may be regarded as being
in the stage of larval transition. The young opossum, when it is born, is in a
late larval state. It gradually metamorphoses into the adult body form within
the marsupium of the mother (Chap. 22).
 
3) Definitive Body Form. The general form and appearance of the adult
constitute definitive body form. The young embryo of Squalus acanthias, at
about 40 mm. in length, assumes the general appearance of the adult shark;
the frog young, after metamorphosis, resembles the adult frog (Chap. 21),
the chick of 8 to 13 days of incubation begins to simulate the form of the
adult bird; the pig embryo of 20 to 35 mm. gradually takes on the body features of a pig, and the human fetus, during the third month of pregnancy,
assumes the appearance of a human being. The transformation of the larval
form into the body form of the adult is discussed further in Chapter 21 in
relation to the endocrine system.
 
3. Basic or Fundamental Tissues
 
Through the stages of development to the period when the primitive or
generalized, embryonic body form is attained, most of the cells which take
part in development are closely associated. In the primitive embryonic body,
this condition is found in all the five primitive body tubes and in the notochord. These closely arranged cells form the primitive epithelium. In the de
 
 
520
 
 
BASIC FEATURES OF VERTEBRATE MORPHOGENESIS
 
 
veloping head and tail regions, however, mesoderm is present in the form of
loosely aggregated cells, known as mesenchyme. While the cells of the epithelial variety are rounded or cuboidal in shape with little intercellular substance or space between the cells, mesenchymal cells tend to assume stellate
forms and to have a large amount of intercellular substance between them.
The primitive vascular or blood tubes are composed of epithelium in the sense
that the cells are closely arranged. However, as these cells are flattened and
show specific peculiarities of structure, this tissue is referred to as endothelium.
Also, while the cells of the early neural tube show the typical epithelial features, they soon undergo marked changes characteristic of developing neural
tissue. The primitive or generalized, embryonic body thus is composed of
four fundamental tissues, viz., epithelial, mesenchymal, endothelial, and neural
tissues.
 
B. Transformation of the Primitive Body Tubes into the Fundamental
or Basic Condition of the Various Organ Systems
Present in the Primitive Embryonic Body
 
1. Processes Involved in Basic System Formation
 
As the primitive body tubes (epidermal, neural, enteric, and mesodermal)
are established, they are modified gradually to form the basis for the various
organ systems. While the notochordal axis is not in the form of a tube, it also
undergoes changes during this period. The morphological alterations, which
transform the primitive body tubes into the basic or fundamental structural
conditions of the systems, consist of the following:
 
(a) extension and growth of the body tubes,
 
(b) saccular outgrowths (evaginations) and ingrowths (invaginations)
from restricted areas of the tubes,
 
(c) cellular migrations away from the primitive tubes fo other tubes and
to the spaces between the tubes, and
 
(d) unequal growth of different areas along the tubes.
 
As a result of these changes, the primitive neural, epidermal, enteric, and
mesodermal tubes, together with the capillaries or blood tubes and the notochord, experience a state of gradual differentiation which is directed toward
the production of the particular adult system to be derived from these respective basic structures. The primitive body tubes, the primitive blood capillaries, and the notochord thus come to form the basic morphological conditions of the future or gut ^ systems. The basic structural conditions of the various
systems are described in Chapters 12 to 21.
 
2. Fundamental Similarity of Early Organ Systems
 
The general form and structure of each primitive embryonic system, as it
begins to develop in one vertebrate species, exhibits a striking resemblance
 
 
 
LAWS OF VON BAER
 
 
521
 
 
to the same system in other vertebrate species. This statement is particularly
true of the gnathostomous vertebrates (i.e., vertebrates with jaws). Consequently, we may regard the initial generalized stages of the embryonic or rudimentary systems as fundamental or basic plans of the systems, morphologically
if not physiologically. The problem which confronts the embryo of each
species, once the basic conditions of the various systems have been established,
is to convert the generalized basic condition of each system into an adult
form which will enable that system to function to the advantage of the particular animal in the particular habitat in which it lives. The conversion of
the basic or primitive condition of the various systems into the adult form of
the systems constitutes the subject matter of Chapters 12 to 21.
 
The basic conditions of the various organ systems are shown in the structure
of shark embryos from 10 to 20 mm. in length, frog embryos of 5 to 10 mm.,
chick embryos from 55 to 96 hrs., pig embryos from 6 to 10 mm., crownrump length, and human embryos of lengths corresponding to 6 to 10 mm.
That is to say, the basic or generalized conditions of the organ systems are
present when primitive or generalized embryonic body form is developed.
It is impossible to segregate any particular length of embryo in the abovementioned series as the ideal or exact condition showing the basic condition
of the systems, as certain systems in one species progress faster than those
same systems in other species. However, a study of embryos of these designations serves to provide an understanding of the basic or fundamental conditions of the various systems (figs. 257-262; also fig. 347A).
 
C. Laws of von Baer
 
As indicated above, the species of the vertebrate group as a whole tend to
follow strikingly similar (although not identical) plans of development during
blastulation, gastrulation, tubulation, the development of the basic plan of the
various systems and primitive body form. As observed in the chapters which
follow, the fundamental or basic plan of any particular, organ-forming system,
in the early embryo of one species, is comparable to the basic plan of that
system in other species throughout the vertebrate group. However, after these
basic parallelisms in early development are completed, divergences from the
basic plan begin to appear during the formation of the various organ systems
of a particular species.
 
The classical statements or laws of Karl Ernst von Baer (1792-1876) describe a tendency which appears to be inherent in the developmental procedure
of any large group of animals. This developmental tendency is for generalized
structural features to arise first, to be remodeled later and supplanted by features specific for each individual species. To interpret these laws in terms of
the procedure principle mentioned in Chapter 7, it may be assumed that
general, or common, developmental procedures first are utilized, followed by
 
 
 
522
 
 
BASIC FEATURES OF VERTEBRATE MORPHOGENESIS
 
 
specific developmental procedures which change the generalized conditions
into specific conditions.
 
The laws of von Baer ( 1 828-1837, Part I, p. 224) may be stated as follows:
 
(a) The general features of a large group of animals appear earlier in development than do the special features;
 
(b) after the more general structures are established, less general structures
arise, and so on until the most special feature appears;
 
(c) each embryo of a given adult form of animal, instead of passing
through or resembling the adult forms of lower members of the group,
diverges from the adult forms, because
 
(d) the embryo of a higher animal species resembles only the embryo of
the lower animal species, not the adult form of the lower species.
 
D. Contributions of the Mesoderm to Primitive Body Formation and
Later Development
 
The mesoderm is most important to the developing architecture of the
body. Because the mesoderm enters so extensively into the structure of the
many organs of the developing embryo, it is well to point out further the
sources of mesoderm and to delineate the structures and parts arising from
this tissue.
 
 
1. Types of Mesodermal Cells
 
Most of the mesoderm of the early embryo exists in the form of epithelium
(see p. 519). As development proceeds, much of the mesoderm loses the close
arrangement characteristic of epithelium. In doing so, the cells separate and
assume a loose connection. They also may change their shapes, appearing
stellate, oval, or irregular, and may wander to distant parts of the body. This
loosely aggregated condition of mesoderm forms the primitive mesenchyme.
Though most of the mesoderm becomes transformed into mesenchyme, the
inner layer of cells of the original hypomeric portion of the mesodermal tubes
retains a flattened, cohesive pattern, described as mesothelium. Mesothelium
comes to line the various body cavities, for these cavities are derived directly
from the hypomeric areas of the mesodermal tubes (Chap. 20).
 
2. Origin of the Mesoderm of the Head Region
 
The primary cephalic outgrowth (Chap. 10), which later forms the head
structures, contains two basic regions, namely, the head proper and the
pharyngeal or branchial region. During its early development, the heart lies
at the ventro-caudal extremity of the general head region; it recedes gradually
backward as the head and branchial structures develop. The exact origin of
the mesoderm which comes to occupy the head proper and pharyngeal areas
varies in different gnathostomous vertebrates. The general sources of the head
mesoderm may be described in the following manner.
 
 
 
CONTRIBUTIONS OF THE MESODERM TO PRIMITIVE BODY FORMATION
 
 
523
 
 
a. Head Mesoderm Derived from the Anterior Region of the Trunk
 
The mesoderm of the branchial area in lower vertebrates, such as the snarks
and, to some degree, the amphibia, represents a direct anterior extension of
the mesoderm of the trunk (figs. 217D, E; 230D; 252E) . It is divisible into two
parts: (1) a ventro-lateral region, the hypomeric or lateral plate mesoderm,
and (2) a dorsal or somitic portion. The latter represents a continuation into
the head region of the epimeric (somitic) mesoderm of the trunk. That portion of the mesoderm of the branchial area which may be regarded specifically
as part of the mesoderm of the head proper is the mesoderm associated with
the mandibular and hyoid visceral arches, together with the hyoid and mandibular somites located at the upper or dorsal ends of the hyoid and mandibular visceral arches (fig. 217D, E).
 
In the higher vertebrates (reptiles, birds, and mammals), the mesoderm
of the branchial region appears early, not as a continuous epithelium, as in
the shark and amphibian embryo, but as a mass of mesenchyme which wanders into the branchial area from the anterior portion of the developing trunk
region (figs. 217F; 23 3B; 234B). This mesenchyme assumes branchial region
characteristics, for it later condenses to form the mandibular, hyoid, and more
posteriorly located, visceral arches. Also, mesenchymal condensations appear
which correspond to the pre-otic head somites formed in the early shark
embryo. For example, in the chick, there is an abducent condensation, which
corresponds to the hyoid somite of the shark embryo, and a superior oblique
condensation corresponding probably to the mandibular somite of the shark
embryo (cf. fig. 217D, F). (See also Adelmann, ’27, p. 42.) Both of these
condensations give origin to eye muscles (Chap. 16). Somewhat similar condensations of mesenchyme which form the rudiments of eye muscles occur in
other members of the higher vertebrate group.
 
b. Head Mesoderm Derived from the Pre-chordal Plate
 
The term pre-chordal plate mesoderm signifies that portion of the head
mesoderm which derives from the pre-chordal plate area located at the anterior end of the foregut. The pre-chordal plate mesoderm is associated closely
with the foregut entoderm and anterior extremity of the notochord in the late
blastula and gastrula in the fishes and amphibia. However, in reptiles, birds,
and mammals, this association is established secondarily with the foregut entoderm by means of the notochordal canal and primitive-pit invaginations during
gastrulation. (See Chap. 9 and also Hill and Tribe, ’24.)
 
(Note: It is advisable to state that Adelmann, ’32, relative to the 19-somite
embryo of the urodele Ambystoma pimctatum, distinguishes between a prechordal mesoderm, which forms the core of the mandibular visceral arch, and
the pre-chordal plate mesoderm, which earlier in development is associated
with the dorsal anterior portion of the foregut entoderm. See figure 252E.)
 
During the period when the major organ-forming areas are being tubulated.
 
 
 
NEURAL ECTODERM
 
 
 
.RANCHIAL POUCHBRANCHIAL GROOVE
OR gill- SL 1 T AREA
 
 
HYPOMERICl
MESOOERMAI^
CONTRIBUTION ^
TO LATERAL
BODY WALL
 
 
MESENCHYMAL
CONTRIBUTION
FROM SPLANCHNIC
LAYER OF HYPOMERE
 
 
Fig. 252. Mesodermal contributions to developing body. (A-D) Sections through
developing chick of 48-52 hrs. of incubation. (A) Section through somites of caudal
trunk area showing primitive area of mesoderm and coelomic spaces. (B) Section
through anterior trunk area depicting early differentiation of somite. (C) Section
through trunk area posterior to heart revealing later stage of somite differentiation than
that shown in B. (D) Section through developing heart area. Observe dermomyotome,
sclerotomic mesenchyme, and mesenchymal contributions of hypomere to forming body
substance. (E) Mesodermal contributions to anterior end of developing embryo of
Ambystoma of about 19 somites. (Redrawn and modified from Adelmann: 1932, J.
Morphol. 54.) (F) Frontal section of early post-hatching larva of Rana pipiens show
ing mass of mesoderm lying between gut, epidermal and neural tubes, together with the
contributions of the mesoderm to the visceral arches.
 
 
524
 
 
 
 
 
CONTRIBUTIONS OF THE MESODERM TO PRIMITIVE BODY FORMATION
 
 
525
 
 
the pre-chordal plate mesoderm separates as a mass of mesenchyme from the
antero-dorsal aspect of the foregut, anterior to the cephalic terminus of the
notochord (fig. 232G, H). It migrates forward as two groups of mesenchyme
connected at first by an interconnecting bridge of mesenchyme. Eventually
these two mesenchymal masses become separated and each forms a dense
aggregation of mesodermal cells over the mandibular visceral arch and just
caudal to the eye (fig. 252E). In the shark embryo and in the chick it gives
origin to the pre-mandibular somites (condensations) which probably give
origin to the eye muscles innervated by the oculomotor or third cranial nerves.
In Ambystoma, Adelmann (’32, p. 52) describes the pre-chordal plate mesoderm as giving origin to “the eye muscles” and “probably much of the head
mesenchyme ahead of the level of the first (gill) pouch, but its caudal limit
cannot be exactly determined.” Thus it appears that a portion of the head
mesoderm in the region anterior to the notochordal termination is derived
from the pre-chordal plate mesoderm in all vertebrates.
 
c. Head Mesoderm Contributed by Neural Crest Material
 
A conspicuous phase of the development of the head region in vertebrate
embryos is the extensive migration of neural crest cells which arise in the middorsal area as the neural tube is formed (Chap. 10; fig. 222C-F). Aside
from contributing to the nervous system (Chap. 19), a portion of the neural
crest material migrates extensively lateroventrally and comes to lie within the
forming visceral (branchial) arches, contributing to the mesoderm in these
areas (figs. 222C-F; 230D, F). Also, consult Landacre (’21); Stone (’22,
’26, and ’29); and Raven (’33a and b). On the other hand, Adelmann (’25)
in the rat and Newth (’51 ) in the lamprey, Lampetra planeri, were not able
to find evidence substantiating this view. However, pigment cells (melanophores) of the skin probably arise from neural crest cells in the head region
of all vertebrate groups.
 
d. Head Mesoderm Originating from Post-otic Somites
 
There is good evidence that the musculature of the tongue takes its origin
in the shark embryo and lower vertebrates from cells which arise from the
somites of the trunk area, immediately posterior to the otic (ear) vesicle, from
whence they migrate ventrad to the hypobranchial region and forward to
the area of the developing tongue (fig. 253). In the human embryo, Kingsbury
(’15) suggests this origin for the tongue and other hypobranchial musculature.
However, Lewis (’10) maintains that, in the human, the tongue musculature
arises from mesenchyme in situ.
 
3. Origin of the Mesoderm of the Tail
 
In the Amphibia, the tail mesoderm has been traced by means of the Vogt
staining method to tail mesoderm in the late blastular and early gastrular
 
 
 
526
 
 
BASIC FEATURES OF VERTEBRATE MORPHOGENESIS
 
 
stages. At the time of tail-rudiment formation, this mesoderm forms two bilateral masses of cells located within the “tail bud” or “end bud.” These cellular
masses proliferate extensively as the tail bud grows caudally and give origin
to the mesoderm of the tail. Similarly, in other vertebrates, the mesoderm of
the future tail is present as mesenchyme in the terminal portion of the tail
bud. These mesenchymal cells proliferate, as the tail grows caudalward, and
leave behind the mesoderm, which gradually condenses into the epithelial
masses or segments (myotomes) along either side of the notochord and
neural tube.
 
4. Contributions of the Trunk Mesoderm to the
Developing Body
 
The mesoderm of the trunk area contributes greatly to the development of
the many body organs and systems in the trunk region. Details of this contribution will be described in the chapters which follow, but, at this point, it
is well to survey the initial activities of the mesodermal tubes of the trunk
area in producing the vertebrate body.
 
a. Early Differentiation of the Somites or Epimere
 
The somites (figs. 217, 237, 252) contribute much to the developing structure of the vertebrate body. This fact is indicated by their early growth and
differentiation. For example, the ventro-mesial wall of the fully developed
somite gradually separates from the rest of the somite and forms a mass of
mesenchymal cells which migrates mesad around the notochord and also
dorsad around the neural tube (fig. 252A-C). The mesenchyme which thus
arises from the somite is known as the sclerotome. In the somite of the higher
vertebrates just previous to the origin of the sclerotome, a small epithelial
core of cells becomes evident in the myocoel; this core contributes to the
sclerotomic material (fig. 252B). As a result of the segregation of the sclerotomic tissue and its migration mesad to occupy the areas around the notochord
and nerve cord, the latter structures become enmeshed by a primitive skeletogenous mesenchyme. The notochord and sclerotomic mesenchyme are the
foundation for the future axial skeleton of the adult, including the vertebral
elements and the caudal part of the cranium as described in Chapter 15.
 
After the departure of sclerotomic material, myotomic and dermatomic
portions of the somite soon rearrange themselves into a hollow structure (fig.
252C, D), in which the myotome forms the inner wall and the dermatome
the outer aspect. This composite structure is the dermomyotome, and the
cavity within, the secondary myocoei. In many vertebrates (fishes, amphibia,
reptiles, and birds), the dermatome gives origin to cells which migrate into
the region of the developing dermis (Chap. 12) and contributes to the formation of this layer of the skin.
 
 
 
CONTRIBUTIONS OF THE MESODERM TO PRIMITIVE BODY FORMATION
 
 
527
 
 
b. Early Differentiation of the Mesomere (Nephrotome)
 
The differentiation of the nephrotome or intermediate mesoderm will be
considered later (Chap. 18) in connection with the urogenital system.
 
c. Early Differentiation and Derivatives of the Hypomere
 
The lateral-plate mesoderm (hypomere), figure 252A, performs an extremely important function in embryological development. The cavity of the
hypomere (splanchnocoel) and the cellular offspring from the hypomeric mesoderm, which forms the wall of this cavity, give origin to much of the structural material and arrangement of the adult body.
 
1) Contributions of the Hypomere (I^ateral Plate Mesoderm) to the Developing Pharyngeal Area of the Gut Tube. The developing foregut (Chap.
13) may be divided into four main areas, namely, (1) head gut, (2) pharyngeal, (3) esophageal, and (4) stomach areas. The head gut is small and
represents a pre-oral extension of the gut; the pharyngeal area is large and
expansive and forms about half of the forming foregut in the early embryo;
the esophageal segment is small and constricted; and the forming stomach
region is enlarged. At this point, however, concern is given specifically to the
developing foregut in relation to the early development of the pharyngeal
region.
 
In the pharyngeal area the foregut expands laterally. Beginning at its anterior end, it sends outward a series of paired, pouch-like diverticula, known
as the branchial (pharyngeal or visceral) pouches. These pouches push outward toward the ectodermal (epidermal) layer. In doing so, they separate
the lateral plate mesoderm which synchronously has divided into columnar
masses or cells (fig. 252E, F). Normally, about four to six pairs of branchial
(pharyngeal) pouches are formed in gnathostomous vertebrates, although in
the cyclostomatous fish, Petromyzon, eight pairs appear. In the embryo of the
shark, Squalus acanthias, six pairs are formed, while in the amphibia, four
to six pairs of pouches may appear (fig. 252F). In the chick, pig, and human,
four pairs of pouches normally occur (figs. 259, 261). Also, invaginations or
inpushings of the epidermal layer occur, the branchial grooves (visceral furrows); the latter meet the entodermal outpocketings (figs. 252F; 262B).
 
The end result of all these developmental movements in the branchial area
is to produce elongated, dorso-ventral, paired columns of mesodermal cells
(figs. 252E; 253), the visceral or branchial arches, which alternate with the
branchial-groove-pouch or gill-slit areas (figs. 252F; 253). The most anterior
pair of visceral arches forms the mandibular visceral arches; the second pair
forms the hyoid visceral arches; and the succeeding pairs form the branchial
(gill) arches (figs. 239C, D; 240; 244; 246; 252E; 253). The branchial arches
with their mesodermal columns of cells will, together with the contributions
from the neural crest cells referred to above, give origin to the connective,
muscle, and blood-vessel-forming tissues in this area.
 
 
 
528
 
 
BASIC FEATURES OF VERTEBRATE MORPHOGENESIS
 
 
2) Contributions of the Hypomere (Lateral Plate Mesoderm) to the Formation of the Gut Tube and Heart Structures. Throughout the length of the
forming gut tube, from the oral area to the anal region, the lateral plate mesoderm (mesoblast) contributes much to the forming gut tube. This is occasioned to a great extent posterior to the pharyngeal area by the fact that the
inner or mesial walls of the two hypomeres enswathe the forming gut tube
as they fuse in the median plane (fig. 241), forming the dorsal and ventral
mesenteries of the gut. However, in the heart area, due to the dorsal displacement of the foregut, the dorsal mesentery is vestigial or absent while
the ventral mesentery is increased in extent. Each mesial wall of the hypomeric
mesoderm, forming the ventral mesentery in the region of the developing
heart, becomes cupped around the primitive blood capillaries, coursing anteriad in this area to form the rudiments of the developing heart. The ventral
mesentery in the heart area thus gives origin to the dorsal mesocardium, the
ventral mesocardium, and the rudimentary, cup-shaped, cpimyocardial structures around the fusing blood capillaries (figs. 236C-D; 254A). The primitive
blood capillaries soon unite to form the rudiment of the future endocardium
of the heart, while the enveloping epimyocardium establishes the rudiment of
the future muscle and connective tissues of the heart (Chap. 17).
 
On the other hand, in the region of the stomach and continuing posteriorly
to the anal area of the gut, the movement mediad of the mesial walls of the
two lateral plate (hypomeric) mesodermal areas occurs in such a way as to
 
 
 
Fig. 253. Diagram illustrating the basic plan of the vertebrate head based upon the
shark, Scy Ilium canicula. (Modified from Goodrich: 1918, Quart. Jour. Micros. Science, 63.)
 
 
 
CONTRIBUTIONS OF THE MESODERM TO PRIMITIVE BODY FORMATION
 
 
529
 
 
 
the hypomeres to the developing heart and gut structures in reptiles, birds, and mammals.
Sections are drawn through the following regions: (A) Through primitive tubular heart
anterior to sinus venosus. (B) Through caudal end of sinus venosus and lateral meso*
cardia. (C) Through liver region. (D) Through region posterior to liver. (E)
Through posterior trunk in region of urinary bladder.
 
envelop or enclose the gut tube. This enclosure readily occurs because in this
region of the trunk, the gut tube lies closer to the ventral aspect of the embryo
than in the heart area. Consequently, a dorsal mesentery above and a ventral
mesentery below the primitive gut tube are formed (fig. 25 4C). The dorsal
and ventral mesenteries may not persist everywhere along the gut (fig. 254D).
The degree of persistence varies in different vertebrates; these variations will
be mentioned later (Chap. 20) when the coelomic cavities are discussed.
However, there is a persistence of the ventral mesentery below the stomach
and anterior intestinal area of all vertebrates, for here the ventral mesentery
(i.e., the two medial walls of the lateral plate mesoderm below the gut) contributes to the development of the liver and the pancreas. These matters are
discussed in Chapter 13.
 
Aside from the formation of the dorsal and ventral mesenteries by the inward movement and fusion of the medial walls of the lateral plate mesoderm
above and below the primitive enteron or gut tube, that part of the medial
walls of the lateral plate mesoderm which envelops the primitive gut itself is
of great importance. This importance arises from the fact that the entoderm
of the gut only forms the lining tissue of the future digestive tract and its
various glands, such as the liver, pancreas, etc., whereas mesenchymal contributions from the medial wall of the lateral plate mesoderm around the
 
 
 
530
 
 
BASIC FEATURES OF VERTEBRATE MORPHOGENESIS
 
 
entodermal lining give origin to smooth muscle tissue, connective tissue, etc.
(figs. 254C, D; 258; 260; 262; 278C). It is apparent, therefore, that the gut
throughout its length is formed from two embryonic contributions, namely,
one from the entoderm and the other from the mesenchyme given off by the
medial walls of the lateral plate or hypomeric mesoderm.
 
{Note: The word splanchnic is an adjective and is derived from a Greek
word meaning entrails or bowels. That is, it pertains to the soft structures
within the body wall. The plural noun viscera (singular, viscus) is derived
from the Latin and signifies the same structures, namely, the heart, liver,
stomach, intestine, etc., which lie within the cavities of the body. It is fitting,
therefore, to apply the adjective splanchnic to the medial portion of the hypomere because it has an intimate relationship with, and is contributory to, the
development of the viscera. The somatic mesoderm, on the other hand, is the
mesoderm of the lateral or body-wall portion of the hypomere. The word
splanchnopleure is a noun and it designates the composite tissue of primitive
entoderm and splanchnic mesoderm, while the word somatopleure is applied
to the compound tissue formed by the primitive lateral wall of the hypomere
(somatic mesoderm) plus the primitive ectoderm overlying it. The coelom
proper or spianchnocoel is the space or cavity which lies between the splanchnic
and somatic layers of the lateral plate or hypomeric mesoderm. During later
development, it is the cavity in which the entrails lie.
 
3) Contributions of the Hypomere (Lateral Plate Mesoderm) to the External (Ectodermal or Epidermal) Body Tube. The somatopleural mesoderm
gives origin to a mass of cellular material which migrates outward to lie along
the inner aspect of the epidermal tube in the lateral and ventral portions of
the developing body (fig. 252A, D). In the dorsal and dorso-lateral regions of
the body, contributions from the sclerotome and dermatome apparently aid
in forming this tissue layer. The layer immediately below the epidermis constitutes the embryonic rudiment of the dermis. (See Chap. 12.)
 
4) Contributions of the Hypomere or Lateral Plate Mesoderm to the Dorsal
Body Areas. Many cells are given off both from splanchnic and somatic layers
of the hypomeric mesoderm to the dorsal body areas above and along either
side of the dorsal aorta (fig. 254), contributing to the mesenchymal “packing tissue” in the area between the notochord and differentiating somite, extending outward to the dermis.
 
5) Contributions of the Lateral Plate Mesoderm to the Walls of the Coelomic Cavities. The pericardial, pleural, and peritoneal cavities are lined, as
stated above, by an epithelial type of tissue called mesothelium (fig. 254A-E).
These coelomic spaces (see Chap. 20) are derived from the fusion of the
two primitive splanchnocoels or cavities of the two hypomeres. External to
the mesothelial lining of the coelomic spaces, there ultimately is developed a
fibrous, connective tissue layer. Thus, mesothelium and connective tissue form.
 
 
 
 
Fig. 255. This figure illustrates different types of body form in various vertebrates
during embryonic development. A, D, H, M, and Q show primitive embryonic body
form in the developing shark, rock fish, frog, chick, and human. B, larval form of
shark; E and F, larval forms of rock fish; I and J, larval forms of frog; N and O, larval
forms of chick; R, larval form of human. C, G, K, L, P, and S represent definitive
body form in the above species. (Figures on rockfish development (Roccus saxatilis) redrawn from Pearson: 1938, Bull. Bureau of Fisheries, L). S. Dept, of Commerce, vol.
49; figures on chick redrawn from Hamburger and Hamilton: 1951, J. Morphol., vol.
88; figure Q, of developing human embryo, redrawn and modified from model based
upon Normentafeln of Keibel and Elze: 1908, vol. 8, G. Fischer, Jena; Dimensions of
human embryos in R and S, from Mall: Chap. 8, vol. 1, Human Embryology, by
F. Keibel and F. P. Mall, 1910, Lippincott, Philadelphia.)
 
 
531
 
 
 
532
 
 
BASIC FEATURES OF VERTEBRATE MORPHOGENESIS
 
 
in general, the walls of the coelomic spaces. These two tissues arise directly
from the hypomeric mesoderm.
 
5. Embryonic Mesenchyme and Its Derivatives
 
The mesenchymal cells given off from the mesodermal tubes of the trunk
area, namely, (1) sclerotomic mesenchyme, (2) dermatomic mesenchyme,
(3) mesenchymal contributions from the lateral plate mesoblast (hypomere)
to the gut, skin, heart, and (4) the mesenchyme contributed to the general
regions of the body lying between the epidermal tube, coelom, notochord,
and neural tube, form, together with the head and tail mesoderm, the general
packing tissue which lies between and surrounding the internal tubular structures of the embryo (fig. 254). Its cells may at times assume polymorphous
or stellate shapes. This loose packing tissue of the embryo constitutes the
embryonic mesenchyme. (See Chap. 15.)
 
This mesenchyme ultimately will contribute to the following structures of
the body:
 
(a) Myocardium (cardiac musculature, etc.) and the epicardium or covering coelomic layer of the heart (Chap. 17),
 
(b) endothelium of blood vessels, blood cells (Chap. 17),
 
(c) smooth musculature and connective tissues of blood vessels (Chaps.
16 and 17),
 
(d) spleen, lymph glands, and lymph vessels (Chap. 17),
 
(e) connective tissues of voluntary and involuntary muscles (Chap. 16),
 
(f) connective tissues of soft organs, exclusive of the nerve system (Chap.
15),
 
(g) connective tissues in general, including bones and cartilage (Chap. 15),
 
(h) smooth musculature of the gut tissues and gut derivatives (Chap. 16),
 
(i) voluntary or striated muscles of the tail from tail-bud mesenchyme
(Chap. 16),
 
(j) striated (voluntary) musculature of face, jaws, and throat, derived
from the lateral plate mesoderm in the anterior pharyngeal region
(Chap. 16),
 
(k) striated (voluntary) extrinsic musculature of the eye (Chap. 16),
 
(l) intrinsic, smooth musculature of the eye (Chap. 16),
 
(m) tongue and musculature of bilateral appendages, derived from somitic
muscle buds (sharks) or from mesenchyme possibly of somitic origin
(higher vertebrates) (Chap. 16), and
 
(n) chromatophores or pigment cells of the body from neural crest mesenchyme (Chap. 12).
 
 
 
SUMMARY OF DERIVATIVES OF ORGAN-FORMING AREAS
 
 
533
 
 
£. Summary of Later Derivatives of the Major Presumptive Organforming Areas of the Late Blastula and Gastrula
 
1. Neural Plate Area (Ectoderm)
 
This area gives origin to the following:
 
(a) Neural tube,
 
(b) optic nerves and retinae of eyes,
 
(c) peripheral nerves and ganglia,
 
(d) chromatophores and chromaffin tissue (i.e., various pigment cells of
the skin, peritoneal cavity, etc., chromaffin cells of supra-renal gland),
 
(e) mesenchyme of the head, neuroglia, and
 
(f) smooth muscles of iris.
 
2. Epidermal Area (Ectoderm)
 
This area gives origin to:
 
(a) Epidermal tube and derived structures, such as scales, hair, nails,
feathers, claws, etc.,
 
(b) lens of the eye, inner ear vesicles, olfactory sense area, general, cutaneous, sense organs of the peripheral area of the body,
 
(c) stomodaeum and its derivatives, oral cavity, anterior lobe of pituitary,
enamel organs, and oral glands, and
 
(d) proctodaeum from which arises the lining tissue of the anal canal.
 
3. Entoderm AL Area
 
From this area the following arise:
 
(a) Epithelial lining of the primitive gut tube or metenteron, including:
(1) epithelium of pharynx; epithelium pharyngeal pouches and their
derivatives, such as auditory tube, middle-ear cavity, parathyroids, and
thymus; (2) epithelium of thyroid gland; (3) epithelial lining tissue
of larynx, trachea, and lungs, and (4) epithelium of gut tube and gut
glands, including liver and pancreas,
 
(b) most of the lining tissue of the urinary bladder, vagina, urethra, and
associated glands,
 
(c) Seessel’s pocket or head gut, and
 
(d) tail gut.
 
4. Notochordal Area
 
This area:
 
(a) Forms primitive antero-posterior skeletal axis of all chordate forms,
 
(b) aids in induction of central nerve tube.
 
 
 
534
 
 
BASIC FEATURES OF VERTEBRATE MORPHOGENESIS
 
 
(c) gives origin to adult notochord of Amphioxus and cyclostomatous fishes
and to notochordal portions of adult vertebral column of gnathostomous
fishes and water-living amphibia, and
 
(d) also, comprises the remains of the notochord in land vertebrates, such
as “nucleus pulposus” in man.
 
5. Mesodermal Areas
 
These areas give origin to:
 
(a) Epimeric, mesomeric, and hypomeric areas of primitive mesodermal
tube,
 
(b) epimeric portion also aids in induction of central nerve tube,
 
(c) muscle tissue, involuntary and voluntary,
 
(d) mesenchyme, connective tissues, including bone, cartilage,
 
(e) blood and lymphoid tissue,
 
(f) gonads with exception of germ cells, genital ducts, and glandular tissues of male and female reproductive ducts, and
 
(g) kidney, ureter, musculature and connective tissues of the bladder,
uterus, vagina, and urethra.
 
6. Germ-cell Area
 
This area gives origin to:
 
(a) Primordial germ cells and probably to definitive germ cells of all vertebrates below mammals and
 
(b) primordial germ cells of mammals and possibly to definitive germ cells.
 
F. Metamerism
 
1. Fundamental Metameric Character of the Trunk and
Tail Regions of the Vertebrate Body
 
Many animals, invertebrate as well as vertebrate, are characterized by the
fact that their bodies are constructed of a longitudinal series of similar parts
or metameres. As each metamere arises during development in a similar
manner and from similar rudiments along the longitudinal or antero-posterior
axis of the embryo, each metamere is homologous with each of the other
metameres. This type of homology in which the homologous parts are arranged serially is known as serial homology. Metamerism is a characteristic
feature of the primitive and later bodies of arthropods, annelids, cephalochordates, and vertebrates.
 
In the vertebrate group, the mesoderm of the trunk and tail exhibits a type
of segmentation, particularly in the epimeric or somitic area. Each pair of
somites, for example, denotes a primitive body segment. The nervous system
 
 
 
 
 
^OPTIC VESICLE
LENS PLACODE .
 
^ nasal placode
— —maxillary
process
 
mandibular arch
 
 
branchial arch
 
 
 
 
 
nasal placode
 
 
ORAL OPENING
 
Laxillary PROCESstl^
 
.
 
 
 
mandibular ARCH
 
 
\ ^nasolateral
PROCESS
 
 
^ NaSOMEDIAL -*
 
process I
''naso-optic furrow
'maxillary process
"mandibular arch
 
 
hyomandibular cleft
 
 
NASOMEDIAL
 
process
 
 
NASOLATERAL
 
process
 
 
naso-optic
 
furrow
 
 
'hyomandibular
 
CLEFT
 
 
 
 
 
tubercles around
^ hyomandibular CLEFT
§ fusing to form
 
f external EAR'
 
 
 
...»
 
 
NASOLATERAL PROCESS^
 
NASOMEDIAL PROCESSES
 
fusing to form PHILTRUM-.
OF LIP
 
EXTERNAL EAR
 
ear tubercles around
hyomandibular cleft
 
-hyoid bone REGlONr
 
 
ih j 1
 
 
 
F.O. 256. Developmental features of the human fac. Modified slightly from models by
B. Ziegler, Freiburg, after Karl Peter.
 
 
535
 
 
 
536
 
 
BASIC FEATURES OF VERTEBRATE MORPHOGENESIS
 
 
also manifests various degrees of segmentation (Chap. 19), although the
origin and arrangement of the peripheral nerves in the form of pairs, each
pair innervating a pair of myotomic derivatives of the somites, is the most
constant feature.
 
In the cephalochordate, Amphioxus, the segmentation of the early mesoderm is more pronounced than that of the vertebrate group. As observed in
Chapter 10, each pair of somites is distinct and entirely separate from other
somitic pairs, and each pair represents all the mesoderm in the segment or
metamere. That is, all the mesoderm is segmented in Amphioxus. However,
in the vertebrate group, only the more dorsally situated mesoderm undergoes
segmentation, the hypomeric portion remaining unsegmented.
 
2. Metamerism and the Basic Morphology of the
Vertebrate Head
 
While the primitive, metameric (segmental) nature of the vertebrate trunk
and tail areas cannot be gainsaid, the fundamental metamerism of the vertebrate head has been questioned. Probably the oldest theory supporting a
concept of cephalic segmentation was the vertebral theory of the skull, propounded by Goethe, Oken, and Owen. This theory maintained that the basic
structure of the skull demonstrated that it was composed of a number of
modified vertebrae, the occipital area denoting one vertebra, the basisphenoidtemporo-parietal area signifying another, the presphenoid-orbitosphenoidfrontal area denoting a third vertebra, and the nasal region representing a
fourth cranial vertebra. (Consult Owen, 1848.) This theory, as a serious
consideration of vertebrate head morphology was demolished by the classic
Croonian lecture given in 1858 by Huxley (1858) before the Royal Society
of London. His most pointed argument against the theory rested upon the
fact that embryological development failed to support the hypothesis that the
bones of the cranium were formed from vertebral elements.
 
A factor which aroused a renewal of interest in a segmental interpretation
of the vertebrate head was the observation by Balfour (1878) that the head
of the elasmobranch fish, Scy Ilium, contained several pairs of pre-otic (prootic) somites (that is, somites in front of the otic or ear region). Since Balfour’s
publication, a large number of studies and dissertations have appeared in an
endeavor to substantiate the theory of head segmentation. The anterior portion of the central nervous system, cranial nerves, somites, branchial (visceral)
arches and pouches, have all served either singly or in combination as proffered
evidence in favor of an interpretation of the primitive segmental nature of
the head region. However, it is upon the head somites that evidence for a
cephalic segmentation mainly depends.
 
A second factor which stimulated discussion relative to head segmentation
was the work of Locy (1895) who emphasized the importance of so-called
neural segments or neuromcres (Chap. 19) as a means of determining the
 
 
 
 
 
ARROWS SHOW water CURRENTS
 
 
Fig. 257. Drawings of early frog tadpoles showing development of early systems.
(A) Frog tadpole (R. pipiens) of about 6 7 mm. It is difficult to determine the exact
number of vitelline arteries at this stage of development and the number given in the
figure is a diagrammatic representation. {A') Shows right and left ventral aortal divisions of bulbus cordis. (B) Anatomy of frog tadpole of about 10-18 mm. See also
figures 280 and 335.
 
 
537
 
 
 
 
 
 
540
 
 
BASIC FEATURES OF VERTEBRATE MORPHOGENESIS
 
 
primitive segmental structure of the vertebrate brain. It is to be observed that
the more conservative figure 253, taken from Goodrich, does not emphasize
neuromeres, for, as observed by Kingsbury (’26, p. 85), the evidence is overwhelmingly against such an interpretation. The association of the cranial nerves
with the gill (branchial) region and the head somites, shown in figure 253,
will be discussed further in Chapter 19.
 
A third factor which awakened curiosity, concerning the segmental theory
of head development, is branchiomerism. The latter term is applied to the
development of a series of homologous structures, segmentally arranged, in
the branchial region; these structures are the visceral arches and branchial
pouches referred to above. As mentioned there, the branchial pouches or outpocketings of the entoderm interrupt a non-segmented mass of lateral plate
(hypomeric) mesoderm, and this mesoderm secondarily becomes segmented
and located within the visceral arches. These arches when formed, other than
possibly the mandibular and the hyoid arches (fig. 253), do not correspond
with the dorsal somitic series. Consequently, “branchiomerism does not, therefore, coincide with somitic metamerism.” (See Kingsbury, ’26, p. 106.)
 
Undoubtedly, much so-called “evidence” has been accumulated to support
a theory of head segmentation. A considerable portion of this evidence apparently is concerned more with segmentation as an end in itself than with a
frank appraisal of actual developmental conditions present in the head (Kingsbury and Adelmann, ’24 and Kingsbury, ’26). However, the evidence which
does resist critical scrutiny is the presence of the head somites which includes
the pre-otic somites and the first three or four post-otic somites. While the
pre-otic somites are somewhat blurred and slurred over in their development
in many higher vertebrates, the fact of their presence in elasmobranch fishes
is indisputable and consistent with a conception of primitive head segmentation.
 
Furthermore, aside from a possible relationship with head-segmentation
phenomena, the appearance of the pre-otic and post-otic head somites coincides with basic developmental tendencies. As observed above, for example,
there is a tendency for nature to use generalized developmental procedures in
the early development of large groups of animals (see von Baer’s laws, p. 522,
and also discussion relative to Haeckel’s biogenetic law in Chap. 7). Nature,
in other words, is utilitarian, and one can be quite certain that if general
developmental procedures are used, they will prove most efficient when all
factors are considered. At the same time, while generalized procedures may
be used, nature does not hesitate to mar or elide parts of procedures when
needed to serve a particular end. The obliteration of developmental steps
during development is shown in the early development of the mesoderm in
the vertebrate group compared to that which occurs in Amphioxus. In the
vertebrate embryo, as observed previously, the hypomeric mesoderm is unsegmented except in a secondary way and in a restricted area as occurs in
branchiomerism. However, in Amphioxus, early segmentation of the meso
 
 
METAMERISM
 
 
541
 
 
derm is complete dorso-ventrally, including the hypomeric region of the
mesoderm. It becomes evident, therefore, that the suppression of segmentation
in the hypomeric area in the vertebrate embryo achieves a precocious result
which the embryo of Amphioxus reaches only at a later period of development. Presumably in the vertebrate embryo, segmentation of the epimeric
mesoderm is retained because it serves a definite end, whereas segmentation
of the hypomeric mesoderm is deleted because it also leads to a necessary end
result in a direct manner.
 
When applied to the developing head region, this procedure principle means
this: A primitive type of segmentation does tend to appear in the pre-otic
area as well as in the post-otic portion of the head, as indicated by the pre-otic
and post-otic somites, and secondarily there is developed a branchial metam
 
GASSERIAN GANGLION I
ME TENCEPHAUON
 
geniculate GANGLION OF NERVE Stt
ACOUSTIC GANGLION OF NERVE :
 
MYf lencephalon
OTIC VESICLE
 
SUPERIOR GANGLION OF NERVE H
JUGULAR GANGLION OF NERVE X
PETROSAL GANGLION OF NERVE IX ^
 
NERVE :
 
NOOOSE ganglion OF nerve::
 
NERVE
 
SPINAL CORO-^
 
pharyngeal pouch in-<:
pharyngeal POUCHBC;
thyroid BODY
BUL0US COROIS
 
 
MESENCEPHALON
 
 
â– NERVE 33
 
NERVE m
 
infundibulum
E’S POCKET
SEESSEL'S POCKET
CHOROID FISSURE
OlENCEPHALON
 
 
 
DORSAL aorta
NOTOCHORD
stomach
LIVER
 
 
ventral pancreasdorsal pancreas
gall blaode
MESONEPHROS- —
 
 
MIDGUT
AN DUCT
 
glomeruli
 
COLLECTING DUCT
HINDGUT
 
SPINAL GANGLION
 
 
Fig. 259. Chick embryo reconstruction of about 100 hrs. of incubation with special
reference to the nervous and urinary systems. See also fig. 336D.
 
 
 
 
bation. Reference should
5
 
 
BASIC HOMOLOGY OF ORGAN SYSTEMS
 
 
545
 
 
erism (branchiomerism) . However, all these segmental structures serve a
definite end. In other areas, head development proceeds in a manner which
obscures segmentation, for the probable reason that segmentation does not fit
into the developmental pattern which must proceed directly and precociously
to gain a specific end dictated by problems peculiar to head development.
 
{Note: For a critical analysis of the supposed facts in favor of segmentation,
together with a marshaling of evidence against such an interpretation, consult
Kingsbury and Adelmann (’24) and for a favorable interpretation of the segmental nature of the head region, see Goodrich (’18) and Delsman (’22).
Figure 253 is taken from Goodrich (’18), and the various structures which
favor a segmental interpretation of the head region are shown.)
 
G. Basic Homology of the Vertebrate Organ Systems
 
1. Definition
 
Homology is the relationship of agreement between the structural parts of
one organism and the structural parts of another organism. An agreeable
relationship between two structures is established if:
 
( 1 ) the two parts occupy the same relative position in the body,
 
(2) they arise in the same way embryonically and from the same rudiments, and
 
(3) they have the same basic potencies.
 
By basic potency is meant the potency which governs the initial and fundamental development of the part; it should not be construed to mean the
ability to produce the entire structure. To the basic potency, other less basic
potencies and modifying factors may be added to produce the adult form of
the structure.
 
2. Basic Homology of Vertebrate Blastulae, Gastrulae, and
Tubulated Embryos
 
In Chapters 6 and 7, the basic conditions of the vertebrate blastula were
surveyed, and it was observed that the formative portion of all vertebrate
blastulae presents a basic pattern, composed of major presumptive organforming areas oriented around the notochordal area and a blastocoelic space.
During gastrulation (Chap. 9), these areas are reoriented to form the basic
pattern of the gastrula, and although round and flattened gastrulae exist, these
form one, generalized, basic pattern, composed of three germ layers arranged
around the central axis or primitive notochordal rod. Similarly, in Chapter
10, the major organ-forming areas are tubulated to form an elongated embryo,
composed of head, pharyngeal, trunk, and tail regions. As tubulation is effected in much the same manner throughout the vertebrate series and as the
pre-chordal plate mesoderm, foregut entoderm, notochord, and somitic meso
 
 
546
 
 
BASIC FEATURES OF VERTEBRATE MORPHOGENESIS
 
 
geniculate ganglion of seventh nerve
 
ACOUSTIC GANGLION OF EIGHTH NERVE
AUDITORY VESICLE
 
 
JU^dLAR GANGLION
 
SUPERIOR GANGLION
NINTH NERV
ACCESSORY ganglion
BASILAR ARTERY
DORSAL ROOT
GANGLION OP FIRST
 
cervical nerve
aortal arch I
 
AORTAL ARCH II
AORTAL ARCH III
AORTAL ARCH IV
AORTAL ARCH VT
PULMONARY ARTERY
TRACHEA
NOTOCHORD
RIGHT ATRIUM
LUNG
 
 
 
SMALL
 
INTESTINE
 
hepatic
 
PORTAL VEIN
DORSAL AORTA
 
 
OMPHALOMESENTERIC
ARTERY
 
(FUTURE SUPERIOR
MESENTERIC ARTERY)
 
 
GLOMERULUS
MESONEPHRIC TUBULE
 
 
DORSAL AORTA
MESONEPHRIC DUCT
 
 
Fig. 261. Drawings of pig embryos of about 9.5 to 12 mm. (A) Reconstruction of about
9.5 to 10 mm. pig embryo with special emphasis on the arterial system.
 
derm appear to be the main organizing influence throughout the series (Chap.
10), the conclusion is inescapable that the tubulated embryos of all vertebrates
are homologous basically, having the same relative parts, arising in the same
manner, and possessing the same basic potencies within the parts. To this
conclusion must be added a caution, namely, that, although the main segments
or specific organ regions along each body tube of one species are homologous
with similar segments along corresponding tubes of other species, variations
may exist and non-homologous areas may be insinuated or homologous areas
 
 
 
BASIC HOMOLOGY OF ORGAN SYSTEMS
 
 
547
 
 
may be deleted along the respective tubes. Regardless of this possibility, a
basic homology, however, appears to exist.
 
During later development through larval and definitive body-form stages,
a considerable amount of molding or plasis by environmental and intrinsic
factors may occur. An example of plasis is given in the development of the
forelimb rudiment of the fish, frog, bird, and pig. In the definitive form, these
structures assume different appearances and are adapted for different func
 
METENCEPHALON
 
 
BASILAR ARTERY
NOTOCHORD
 
ROOT OF TONGUE
THYROID GLAND
developing epiglottis
AORTIC ARCH III
L ARYN X
 
 
ESOPH AGU S
VALVES OF
 
SINUS 'VENOSUS/
LUNG bud'
 
SPINAL CORD
SINUS VENOSU;
 
 
GALL BLADDER
 
NOTOCHORDOORSAL AORTA
 
DEVELOPING
VERTEBRAE
 
MESONEPHRIC
KIDNE
 
 
 
MESENCEPHALON
 
 
TUBERCULUM
 
ju / POSTERIUS
~ — ^INFUNDIBULUM
OIEUCEPHALON
 
rathke's pocket
 
SEESSEL'S POCKET
— -OPTIC CHIASMA
 
-RECESSUS OPTICUS
TELENCEPHALON
AMINA TERMINALIS
TONGUE
BULBUS CORDIS
 
 
EXTRA-EMBRYONIC COELOM
UMBILICAL CORD
 
 
ALLANTOIC DIVERTICULUM
GENITAL EMINENCE
PROCTODAEUM
 
CLOACA
 
 
ALLANTOIC STALK
 
 
B.
 
 
metanephrogenous
 
TISSUE SPINAL GANGLION
 
 
Fig. 261 — (Continued} (B) Median sagittal section of 10 mm. embryo.
 
 
 
VEIN OF maxillary REGION
(BRANCH OF INTERNAL JUGULAR)
 
 
OTIC VESICLE
 
 
VEIN OF
 
MANDIBULAR REGION
BRANCH OF EXTERNAL
JUGULAR)
 
INTERNAL JUGULAR
VEIN
DORSAL
 
jSEGMENTAL VEINS
EXTERNAL
JUGULAR VEIN
 
 
 
LEFT DUCT OF CUVIER
RIGHT VITELLINE VEIN
LIVER
DUCTUS VENOSUS
HEPATIC VEINS
PORTAL VEIN
 
 
UMBILICAL
 
ARTERY
 
 
'TRANSVERSE ANASTOMOSIS
OF SITBCAROINALS
 
 
POSTERIOR CARDINAL
VEIN
 
 
PIG EMBRYO SHOWING RIGHT HALF
OF VENOUS SYSTEM
 
 
Fig. 261 — (Continued) (C) Lateral view of 12 mm. embryo showing venous system.
(C is redrawn and modified from Minot; 1903, A Laboratory Text-book of Embryology,
Blakiston, Philadelphia.)
 
 
548
 
 
 
 
Fig. 262. Sections and stereograms of 10 mm. pig embryo.
 
 
MCSCNCHYME^
 
 
 
Ibl— (Continued) Sections and stereograms of 10 mm. pig embryo
 
 
 
 
BIBLIOGRAPHY
 
 
551
 
 
tional purposes. Basically, however, these structures are homologous, although
plasis produces adult forms which appear to be different.
 
A further statement should be added, concerning that type of molding or
plasis of a developing structure which produces similar structures from conditions which have had a different genetic history. For example, the bat’s fore
limb rudiment is molded to produce a structure resembling superficially that
of the bird, although modern bats and birds have arisen through different lines
of descent. Similarly, the teeth of certain teleost fishes superficially resemble
the teeth of certain mammals, an effect produced from widely diverging lines
of genetic descent. These molding effects or homoplasy, which produce superficially similar structures as a result of adaptations to certain environmental
conditions, are called convergence, parallelism, and analogy. An example of
experimental homoplasy is the induction of eye lenses in the embryo by the
transplantation of optic-cup material to a place in the epidermis which normally does not produce a lens.
 
{Note: For a discussion of homology, homogeny, plasis, convergence, etc.,
see Tait, ’28.)
 
 
Bibliography
 
 
Adelmann, H. B. 1925. The development
of the neural folds and cranial ganglia
of the rat. J. Comp. Neurol. 39:19.
 
. 1927. The development of the eye
 
muscles of the chick. J. Morphol. 44:29.
 
. 1932. The development of the
 
prechordal plate and mesoderm of Ambly stoma piinctatum. J. Morphol. 54:1.
 
Baer, K. E. von. 1828-1837. liber Entwickelungsgeschichte der Thiere. Beobachtung und Reflexion. Erster Theil,
1828; Zweiter Theil, 1837. Konigsberg,
Borntriiger.
 
Balfour, F. M. 1878. Monograph on the
development of elasmobranch fishes. Republished in 1885 in The Works of
Francis Maitland Balfour, edited by M.
Foster and A. Sedgwick, vol. 1. The
Macmillan Co., London.
 
Delsman, H. C. 1922. The Ancestry of
Vertebrates. Valkoff & Co., Amersfoort,
Holland.
 
Goodrich, E. S. 1918. On the development
of the segments of the head of Scy Ilium.
Quart. J. Micr. Sc. 63:1.
 
Hill, J. P. and Tribe, M. 1924. The early
development of the cat {Felis dornestica).
Quart. J. Micr. Sc. 68:513.
 
 
Huxley, T. H. 1858. The Croonian lecture
— on the theory of the vertebrate skull.
Proc. Roy. Soc., London, s.B. 9:381.
 
Kingsbury, B. F. 1915. The development
of the human pharynx. 1. Pharyngeal
derivatives. Am. J. Anat. 18:329.
 
. 1924. The significance of the so
called law of cephalocaudal differential
growth. Anat. Rec, 27:305.
 
. 1926. Branchiomerism and the
 
theory of head segmentation. J. Morphol.
42:83.
 
and Adelmann, H. B. 1924. The
 
morphological plan of the head. Quart.
J. Micr, Sc. 68:239.
 
Kyle, H. M. 1926. The Biology of Fishes.
Sidgwick and Jackson, Ltd., London.
 
Landacre, F. L. 1921. The fate of the
neural crest in the head of urodeles. J.
Comp. Neurol. 33:1.
 
Lewis, W. H. 1910. Chapter 12. The development of the muscular system in
Manual of Human Embryology, edited
by F. Keibel and F. P. Mall. J. B. Lippincott Co., Philadelphia.
 
Locy, W. A. 1895. Contribution to the
structure and development of the vertebrate head. J. Morphol. 11:497.
 
 
 
552
 
 
BASIC FEATURES OF VERTEBRATE MORPHOGENESIS
 
 
Newth, D. R. 1951. Experiments on the
neural crest of the lamprey embryo. J.
Exper. Biol. 28:17.
 
Owen, R. 1848. On the archetype and
homologies of the vertebrate skeleton.
John Van Voorst, London.
 
Raven, C. P. 1933a. Zur Entwicklung der
Ganglienleiste. I. Die Kinematik der
Ganglienleistenentwicklung bei den Urodelen. Arch. f. Entwlngsmech. d. Organ.
125:210.
 
. 1933b. Zur Entwicklung der Ganglienleiste. III. Die Induktionsfahigkeit
des Kopfganglienleistenmaterials von
Rana fusca.
 
 
Stone, L. S. 1922. Experiments on the development of the cranial ganglia and
the lateral line sense organs in Amblystoma pimctatum. J. Exper. Zool. 35:421.
 
. 1926. Further experiments on the
 
extirpation and transplantation of mesectoderm in Amhlystorna punctatum. J.
Exper. Zool. 44:95.
 
. 1929. Experiments showing the


role of migrating neural crest (mesectoderm) in the formation of head skeleton and loose connective tissue in Rana
2. Basic Homology of Vertebrate Blastulae, Gastrulae, and Tubulated Embryos
paliistris. Arch. f. Entwicklngsmech. d.
Organ. 118:40.


Tait, J. 1928. Homology, analogy and
==Bibliography==
plasis. Quart. Rev. Biol. Ill: 151.

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Nelsen OE. Comparative embryology of the vertebrates (1953) Mcgraw-Hill Book Company, New York.

   Comparative Vertebrate Embryology 1953: 1. The Period of Preparation | 2. The Period of Fertilization | 3. The Development of Primitive Embryonic Form | 4. Histogenesis and Morphogenesis of the Organ Systems | 5. The Care of the Developing Embryo | Figures
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Part III The Development of Primitive Embryonic Form

Part III - The Development of Primitive Embryonic Form: 6. Cleavage (Segmentation) and Blastulation | 7. The Chordate Blastula and Its Significance | 8. The Late Blastula in Relation to Certain Innate Physiological Conditions: Twinning | 9. Gastrulation | 10. Tubulation and Extension of the Major Organ-forming Areas: Development of Primitive Body Form | 11. Basic Features of Vertebrate Morphogenesis

The general procedures leading to the development of primitive embryonic body form in the chordate group of animals are:

( 1 ) Cleavage. Cleavage is the division of the egg into progressively smaller cellular units, the blaslomeres (Chap. 6).

(2) Blastulation. Blastulalion results in the formation of the blastula. The blastula is composed of a cellular blastoderm in relation to a fluid-filled cavity, the blastocoel. The blastoderm of the late blastula is composed of neural, epidermal, notochordal, mesodermal, and entodermal major presumptive organ-forming areas. In the phylum Chordata, the notochordal area is the central region around which the other areas are oriented (Chap. 7). The major presumptive organ-forming areas of the late blastula exist in various degrees of differentiation (Chap. 8).

(3) Gastrulation. This is the process which effects a reorientation of the presumptive organ-forming areas and brings about their axiation antero-posteriorly in relation to the notochordal axis and the future embryonic body (Chap. 9). During gastrulation the major organ-forming areas are subdivided into minor areas or fields, each field being restricted to the development of a particular organ or part. (Pp. 378, 446, 447.

(4) Following gastrulation, the next step in the development of embryonic body form is tubulation and extension of the major organ-forming areas (Chap. 10).

(5) As tubulation and extension of the organ-forming areas is effected, the basic or fundamental conditions of the future organ systems are established, resulting in the development of primitive body form. As the development of various vertebrate embryos is strikingly similar up to this point, the primitive embryonic body forms of all vertebrates resemble each other (Chap. II).

In the drawings presented in Part III, the following scheme for designating the major organ-forming areas existing within the three germ layers is adhered to:


Cleavage (Segmentation) and Blastulation

6. Cleavage (Segmentation) and Blastulation

A. General considerations

1. Definitions

2. Early history of the cleavage (cell-division) concept

3. Importance of the cleavage-blastular period of development

a. Morphological relationships of the blastula

b. Physiological relationships of the blastula

1 ) Hybrid crosses

2) Artificial parthenogenesis

3) Oxygen-block studies

4. Geometrical relations of early cleavage

a. Meridional plane

b. Vertical plane

c. Equatorial plane

d. Latitudinal plane

5. Some fundamental factors involved in the early cleavage of the egg

a. Mechanisms associated with mitosis or cell division

b. Influence of cytoplasmic substance and egg organization upon cleavage

1) Yolk

2) Organization of the egg

c. Influence of first cleavage amphiaster on polyspermy

d. Viscosity changes during cleavage

e. Cleavage laws

1 ) Sach’s rules

2) Hertwig’s laws

6. Relation of early cleavage planes to the antero-posterior axis of the embryo

B. Types of cleavage in the phylum Chordata

1. Typical holoblastic cleavage

a. Amphioxus

b. Frog (Rana pipiens and R. sylvatica)

c. Cyclostomata

2. Atypical types of holoblastic cleavage

a. Holoblastic cleavage in the egg of the metatherian and eutherian mammals

1 ) General considerations

2) Early development of the rabbit egg

a) Two-cell stage

b) Four-cell stage

c) Eight-cell stage

d) Sixteen-cell stage

e) Morula stage

f) Early blastocyst

3) Types of mammalian blastocysts (blastulae)

b. Holoblastic cleavage of the transitional or intermediate type

1) Amhystoma maculatum (punctatum)

2) Lepidosiren paradoxa

3) Necturus maculosus

4) Acipenser sturio

5) Amia calva

6) Lepisosteus (Lepidosteus) osseus

7) Gymnophionan amphibia 3. Meroblastic cleavage

a. Egg of the common fowl

1 ) Early cleavages

2) Formation of the periblast tissue

3) Morphological characteristics of the primary blastula

4) Polyspermy and fate of the accessory sperm nuclei

b. Elasmobranch fishes

1 ) Cleavage and formation of the early blastula

2) Problem of the periblast tissue in elasmobranch fishes

c. Teleost fishes

1) Cleavage and early blastula formation

2) Origin of the periblast tissue in teleost fishes

d. Prototherian Mammalia

e. Cleavage in the California hagfish, Polistotrema (Bdellostorna) stouti

C. What is the force which causes the blastomeres to adhere together during early cleavage?

D. Progressive cytoplasmic inequality and nuclear equality of the cleavage blastomeres

1. Cytoplasmic inequality of the early blastomeres

2. Nuclear equality of the early blastomeres

E. Quantitative and qualitative cleavages and their influence upon later development

The Chordate Blastula and Its Significance

7. The Chordate Blastula and Its Significance

A. Introduction

1. Blastulae without auxiliary tissue

2. Blastulae with auxiliary or trophoblast tissue

3. Comparison of the two main blastular types

B. History of the concept of specific, organ-forming areas

C. Theory of epigenesis and the germ-layer concept of development

D. Introduction of the words ectoderm, mesoderm, endoderm

E. Importance of the blastular stage in Haeckel's theory of The Biogenetic Law of Embryonic Recapitulation

F. Importance of the blastular stage in embryonic development

G. Description of the various types of chordate blastulae with an outline of their organforming areas

1. Protochordate blastula

2. Amphibian blastula

3. Mature blastula in birds

4. Primary and secondary reptilian blastulae

5. Formation of the late mammalian blastocyst (blastula)

a. Prototherian mammal, Echidna

b. Metatherian mammal, Didelphys

c. Eutherian mammals

6. Blastulae of teleost and elasmobranch fishes

7. Blastulae of gymnophionan amphibia

Late Blastula in Relation to Certain Innate Physiological Conditions: Twinning

8. The Late Blastula in Relation to Certain Innate Physiological Conditions: Twinning

A. Introduction

B. Problem of differentiation

1. Definition of differentiation; kinds of differentiation

2. Self-differentiation and dependent differentiation

C. Concept of potency in relation to differentiation

1. Definition of potency

2. Some terms used to describe different states of potency

a. Totipotency and harmonious totipotency

b. Determination and potency limitation

c. Prospective potency and prospective fate

d. Autonomous potency c. Competence

D. The blastula in relation to twinning

1. Some definitions

a. Dizygotic or fraternal twins

b. Monozygotic or identical twins

c. Polyembryony •

2. Basis of true or identical twinning

3. Some experimentally produced, twinning conditions

E. Importance of the organization center of the late blastula

Gastrulation

| 9. Gastrulation

A. Some definitions and concepts

1. Gastrulation

2. Primitive vertebrate body plan in relation to the process of gastrulation

a. Fundamental body plan of the vertebrate animal

b. The gastrula in relation to the primitive body plan

c. Chart of blastula, gastrula, and primitive, body-form relationships (fig. 188)

B. General processes involved in gastrulation

C. Morphogenetic movement of cells

1. Importance of cell movements during development and in gastrulation

2. Types of cell movement during gastrulation

a. Epiboly

b. Emboly

3. Description of the processes concerned with epiboly

4. Description of the processes involved in emboly

a. Involution and convergence

b. Invagination

c. Concrescence

d. Cell proliferation

e. Polyinvagination

f. Ingression

g. Delamination

h. Divergence

i. Extension

D. The organization center and its relation to the gastrulative process

1. The organization center and the primary organizer

2. Divisions of the primary organizer

E. Chemodifferentiation and the gastrulative process

F. Gastrulation in various Chordata 1. Amphioxus

a. Orientation

b. Gastrulative movements

1 ) Emboly

2) Epiboly

3) Antero-posterior extension of the gastrula and dorsal convergence of the mesodermal cells

4) Closure of the blastopore

c. Resume of cell movements and processes involved in gastrulation of Amphioxus

1 ) Emboly

2) Epiboly

2. Gastrulation in Amphibia with particular reference to the frog

a. Introduction

1) Orientation

2) Physiological changes which occur in the presumptive, organ-forming areas of the late blastula and early gastrula as gastrulation progresses

b. Gastrulation

1) Emboly

2) Epiboly

3) Embryo produced by the gastrulative processes

4) Position occupied by the pre -chordal plate material

c. Closure of the blastopore and formation of the neurenteric canal

d. Summary of morphogenetic movements of cells during gastrulation in the frog and other Amphibia

1) Emboly

2) Epiboly

3. Gastrulation in reptiles

a. Orientation

b. Gastrulation

4. Gastrulation in the chick

a. Orientation

b. Gastrulative changes

1) Development of primitive streak as viewed from the surface of stained blastoderms

2) Cell movements in the epiblast involved in primitive-streak formation as indicated by carbon-particle marking and vital-staining experiments

3) Cell movements in the hypoblast and the importance of these movements in primitive-streak formation

4) Primitive pit notochordal canal

5) Resume of morphogenetic movements of cells during gastrulation in the chick

5. Gastrulation in mammals

a. Orientation

b. Gastrulation in the pig embryo

c. Gastrulation in other mammals

6. Gastrulation in teleost and elasmobranch fishes

a. Orientation

b. Gastrulation in teleost fishes

1) Emboly

2) Epiboly

3) Summary of the gastrulative processes in teleost fishes

a) Emboly

b) Epiboly

4) Developmental potencies of the germ ring of teleost fishes

c. Gastrulation in elasmobranch fishes

7. Intermediate types of gastrulative behavior

G. The late gastrula as a mosaic of specific, organ-forming territories

H. Autonomous theory of gastrulative movements

I. Exogastrulation

J. Pre-chordal plate and cephalic projection in various chordates

K. Blastoporal and primitive-streak comparisons

Development of Primitive Body Form

10. Tubulation and Extension of the Major Organ-forming Areas: Development of Primitive Body Form

A. Introduction

1. Some of the developmental problems faced by the embryo after gastrulation

a. Tabulation

b. Increase in size and antero-posteri(*)r extension of the tubulated, major organforming areas

c. Regional modifications of the tubulated areas

2. Common, vertebrate, embryonic body form

3. Starting point for tabulation

4. Developmental processes which accomplish tabulation

a. Immediate processes

b. Auxiliary processes

5. Blastocoelic space and body-form development

6. Primitive circulatory tubes or blood vessels

7. Extra-embryonic membranes

B. Tabulation of the neural, epidermal, entodermal, and mesodermal, organ-forming areas in the vertebrate group

1. Neuralization or the tabulation of the neural plate area

a. Definition

b. Neuralizative processes in the Vertebrata

1) Thickened keel method

2) Neural fold method

c. Closure of the blastopore in rounded gastrulae, such as that of the frog

d. Anterior and posterior neuropores; neurenteric canal

2. Epidermal tabulation

a. Development of the epidermal tube in Amphibia

b. Tabulation of the epidermal area in flat blastoderms

3. Formation of the primitive gut tube (enteric tabulation)

a. Regions of primitive gut tube or early metenteron

b. Formation of the primitive metenteron in the frog

c. Formation of the tubular metenteron in flat blastoderms

4. Tabulation (coelom formation) and other features involved in the early differentiation of the mesodermal areas

a. Early changes in the mesodermal areas

1) Epimere; formation of the somites

2) Mesomere

3) Hypomere

b. Tabulation of the mesodermal areas

C. Notochordal area

D. Lateral constrictive movements

E. Tubulation of the neural, epidermal, entodermal, and mesodermal, organ-forming areas in Amphioxus

1. Comparison of the problems of tubulation in the embryo of Amphioxus with that of the embryos in the subphylum Vertebrata

a. End-bud growth

b. Position occupied by the notochord and mesoderm at the end of gastrulation

2. Neuralization and the closure of the blastopore

3. Epidermal tubulation

4. Tubulation of the entodermal area

a. Segregation of the entoderm from the chordamesoderm and the formation of the primitive metenteric tube

b. Formation of the mouth, anus, and other specialized structures of the metenteron

5. Tubulation of the mesoderm

6. Later differentiation of the myotomic (dorsal) area of the somite

7. Notochord

F. Early development of the rudiments of vertebrate paired appendages

G. The limb bud as an illustration of the field concept of development in relation to the gastrula and the tubulated embryo

H. Cephalic flexion and general body bending and rotation in vertebrate embryos

I. Influences which play a part in tubulation and organization of body form

J. Basic similarity of body-form development in the vertebrate group of chordate animals

Basic Features of Vertebrate Morphogenesis

11. Basic Features of Vertebrate Morphogenesis

A. Introduction

1. Purpose of This Chapter

2. Definitions

a. Morphogenesis and Related Terms

b. Primitive, Larval, and Definitive Body Forms (see fig. 255)

1) Primitive Body Form.

2) Larval Body Form.

3) Definitive Body Form.


3. Basic or Fundamental Tissues

B. Transformation of the Primitive Body Tubes into the Fundamental or Basic Condition of the Various Organ Systems Present in the Primitive Embryonic Body

1. Processes Involved in Basic System Formation

(a) extension and growth of the body tubes,

(b) saccular outgrowths (evaginations) and ingrowths (invaginations) from restricted areas of the tubes,

(c) cellular migrations away from the primitive tubes fo other tubes and to the spaces between the tubes, and

(d) unequal growth of different areas along the tubes.


2. Fundamental Similarity of Early Organ Systems

C. Laws of von Baer

D. Contributions of the Mesoderm to Primitive Body Formation and Later Development

1. Types of Mesodermal Cells

2. Origin of the Mesoderm of the Head Region

a. Head Mesoderm Derived from the Anterior Region of the Trunk

b. Head Mesoderm Derived from the Pre-chordal Plate

c. Head Mesoderm Contributed by Neural Crest Material

d. Head Mesoderm Originating from Post-otic Somites

3. Origin of the Mesoderm of the Tail

4. Contributions of the Trunk Mesoderm to the Developing Body

a. Early Differentiation of the Somites or Epimere

b. Early Differentiation of the Mesomere (Nephrotome)

c. Early Differentiation and Derivatives of the Hypomere

5. Embryonic Mesenchyme and Its Derivatives


1. Neural Plate Area (Ectoderm)

2. Epidermal Area (Ectoderm)

3. Entodermal Area

4. Notochordal Area

5. Mesodermal Areas

6. Germ-cell Area

F. Metamerism


G. Basic Homology of the Vertebrate Organ Systems

1. Definition

2. Basic Homology of Vertebrate Blastulae, Gastrulae, and Tubulated Embryos

Bibliography