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E. Importance of the organization center of the late blastula  
E. Importance of the organization center of the late blastula  
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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.
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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==
==Gastrulation==

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

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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

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


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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


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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


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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.)


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£. 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.




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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


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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).


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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


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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.


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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

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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


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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.)


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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


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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


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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).


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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


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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.)


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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


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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.)


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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



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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.)



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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


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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.)


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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


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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.


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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.


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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.


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GASTRULATION IN VARIOUS CHORDATA


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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


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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


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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.


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PRE-CHORDAL PLATE AND CEPHALIC PROJECTION


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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.


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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

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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.


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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.

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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


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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)

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COMMON CARDINAL VE ---=^'^^-i:-‘^SlNUS VENOSUS

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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).


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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


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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.


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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.)


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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


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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.


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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;


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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


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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


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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


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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

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NASOLATERAL

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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


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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

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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

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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.)


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