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=Part III The Development of Primitive Embryonic Form=
=Part III The Development of Primitive Embryonic Form=
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c. Early Differentiation and Derivatives of the Hypomere  
c. Early Differentiation and Derivatives of the Hypomere  
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  
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)  
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)  
2. Epidermal Area (Ectoderm)  


This area gives origin to:
3. Entodermal Area  
 
(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  
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  
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  
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  
F. Metamerism  
1. Fundamental Metameric Character of the Trunk and
Tail Regions of the Vertebrate Body
Many animals, invertebrate as well as vertebrate, are characterized by the
fact that their bodies are constructed of a longitudinal series of similar parts
or metameres. As each metamere arises during development in a similar
manner and from similar rudiments along the longitudinal or antero-posterior
axis of the embryo, each metamere is homologous with each of the other
metameres. This type of homology in which the homologous parts are arranged serially is known as serial homology. Metamerism is a characteristic
feature of the primitive and later bodies of arthropods, annelids, cephalochordates, and vertebrates.
In the vertebrate group, the mesoderm of the trunk and tail exhibits a type
of segmentation, particularly in the epimeric or somitic area. Each pair of
somites, for example, denotes a primitive body segment. The nervous system


^OPTIC VESICLE
LENS PLACODE .
^ nasal placode
— —maxillary
process
mandibular arch
branchial arch
nasal placode
ORAL OPENING
Laxillary PROCESstl^
.
mandibular ARCH
\ ^nasolateral
PROCESS
^ NaSOMEDIAL -*
process I
''naso-optic furrow
'maxillary process
"mandibular arch
hyomandibular cleft
NASOMEDIAL
process
NASOLATERAL
process
naso-optic
furrow
'hyomandibular
CLEFT
tubercles around
^ hyomandibular CLEFT
§ fusing to form
f external EAR'
...»
NASOLATERAL PROCESS^
NASOMEDIAL PROCESSES
fusing to form PHILTRUM-.
OF LIP
EXTERNAL EAR
ear tubercles around
hyomandibular cleft
-hyoid bone REGlONr
ih j 1
F.O. 256. Developmental features of the human fac. Modified slightly from models by
B. Ziegler, Freiburg, after Karl Peter.
535
536
BASIC FEATURES OF VERTEBRATE MORPHOGENESIS
also manifests various degrees of segmentation (Chap. 19), although the
origin and arrangement of the peripheral nerves in the form of pairs, each
pair innervating a pair of myotomic derivatives of the somites, is the most
constant feature.
In the cephalochordate, Amphioxus, the segmentation of the early mesoderm is more pronounced than that of the vertebrate group. As observed in
Chapter 10, each pair of somites is distinct and entirely separate from other
somitic pairs, and each pair represents all the mesoderm in the segment or
metamere. That is, all the mesoderm is segmented in Amphioxus. However,
in the vertebrate group, only the more dorsally situated mesoderm undergoes
segmentation, the hypomeric portion remaining unsegmented.
2. Metamerism and the Basic Morphology of the
Vertebrate Head
While the primitive, metameric (segmental) nature of the vertebrate trunk
and tail areas cannot be gainsaid, the fundamental metamerism of the vertebrate head has been questioned. Probably the oldest theory supporting a
concept of cephalic segmentation was the vertebral theory of the skull, propounded by Goethe, Oken, and Owen. This theory maintained that the basic
structure of the skull demonstrated that it was composed of a number of
modified vertebrae, the occipital area denoting one vertebra, the basisphenoidtemporo-parietal area signifying another, the presphenoid-orbitosphenoidfrontal area denoting a third vertebra, and the nasal region representing a
fourth cranial vertebra. (Consult Owen, 1848.) This theory, as a serious
consideration of vertebrate head morphology was demolished by the classic
Croonian lecture given in 1858 by Huxley (1858) before the Royal Society
of London. His most pointed argument against the theory rested upon the
fact that embryological development failed to support the hypothesis that the
bones of the cranium were formed from vertebral elements.
A factor which aroused a renewal of interest in a segmental interpretation
of the vertebrate head was the observation by Balfour (1878) that the head
of the elasmobranch fish, Scy Ilium, contained several pairs of pre-otic (prootic) somites (that is, somites in front of the otic or ear region). Since Balfour’s
publication, a large number of studies and dissertations have appeared in an
endeavor to substantiate the theory of head segmentation. The anterior portion of the central nervous system, cranial nerves, somites, branchial (visceral)
arches and pouches, have all served either singly or in combination as proffered
evidence in favor of an interpretation of the primitive segmental nature of
the head region. However, it is upon the head somites that evidence for a
cephalic segmentation mainly depends.
A second factor which stimulated discussion relative to head segmentation
was the work of Locy (1895) who emphasized the importance of so-called
neural segments or neuromcres (Chap. 19) as a means of determining the
ARROWS SHOW water CURRENTS
Fig. 257. Drawings of early frog tadpoles showing development of early systems.
(A) Frog tadpole (R. pipiens) of about 6 7 mm. It is difficult to determine the exact
number of vitelline arteries at this stage of development and the number given in the
figure is a diagrammatic representation. {A') Shows right and left ventral aortal divisions of bulbus cordis. (B) Anatomy of frog tadpole of about 10-18 mm. See also
figures 280 and 335.
537
540
BASIC FEATURES OF VERTEBRATE MORPHOGENESIS
primitive segmental structure of the vertebrate brain. It is to be observed that
the more conservative figure 253, taken from Goodrich, does not emphasize
neuromeres, for, as observed by Kingsbury (’26, p. 85), the evidence is overwhelmingly against such an interpretation. The association of the cranial nerves
with the gill (branchial) region and the head somites, shown in figure 253,
will be discussed further in Chapter 19.
A third factor which awakened curiosity, concerning the segmental theory
of head development, is branchiomerism. The latter term is applied to the
development of a series of homologous structures, segmentally arranged, in
the branchial region; these structures are the visceral arches and branchial
pouches referred to above. As mentioned there, the branchial pouches or outpocketings of the entoderm interrupt a non-segmented mass of lateral plate
(hypomeric) mesoderm, and this mesoderm secondarily becomes segmented
and located within the visceral arches. These arches when formed, other than
possibly the mandibular and the hyoid arches (fig. 253), do not correspond
with the dorsal somitic series. Consequently, “branchiomerism does not, therefore, coincide with somitic metamerism.” (See Kingsbury, ’26, p. 106.)
Undoubtedly, much so-called “evidence” has been accumulated to support
a theory of head segmentation. A considerable portion of this evidence apparently is concerned more with segmentation as an end in itself than with a
frank appraisal of actual developmental conditions present in the head (Kingsbury and Adelmann, ’24 and Kingsbury, ’26). However, the evidence which
does resist critical scrutiny is the presence of the head somites which includes
the pre-otic somites and the first three or four post-otic somites. While the
pre-otic somites are somewhat blurred and slurred over in their development
in many higher vertebrates, the fact of their presence in elasmobranch fishes
is indisputable and consistent with a conception of primitive head segmentation.
Furthermore, aside from a possible relationship with head-segmentation
phenomena, the appearance of the pre-otic and post-otic head somites coincides with basic developmental tendencies. As observed above, for example,
there is a tendency for nature to use generalized developmental procedures in
the early development of large groups of animals (see von Baer’s laws, p. 522,
and also discussion relative to Haeckel’s biogenetic law in Chap. 7). Nature,
in other words, is utilitarian, and one can be quite certain that if general
developmental procedures are used, they will prove most efficient when all
factors are considered. At the same time, while generalized procedures may
be used, nature does not hesitate to mar or elide parts of procedures when
needed to serve a particular end. The obliteration of developmental steps
during development is shown in the early development of the mesoderm in
the vertebrate group compared to that which occurs in Amphioxus. In the
vertebrate embryo, as observed previously, the hypomeric mesoderm is unsegmented except in a secondary way and in a restricted area as occurs in
branchiomerism. However, in Amphioxus, early segmentation of the meso
METAMERISM
541
derm is complete dorso-ventrally, including the hypomeric region of the
mesoderm. It becomes evident, therefore, that the suppression of segmentation
in the hypomeric area in the vertebrate embryo achieves a precocious result
which the embryo of Amphioxus reaches only at a later period of development. Presumably in the vertebrate embryo, segmentation of the epimeric
mesoderm is retained because it serves a definite end, whereas segmentation
of the hypomeric mesoderm is deleted because it also leads to a necessary end
result in a direct manner.
When applied to the developing head region, this procedure principle means
this: A primitive type of segmentation does tend to appear in the pre-otic
area as well as in the post-otic portion of the head, as indicated by the pre-otic
and post-otic somites, and secondarily there is developed a branchial metam
GASSERIAN GANGLION I
ME TENCEPHAUON
geniculate GANGLION OF NERVE Stt
ACOUSTIC GANGLION OF NERVE :
MYf lencephalon
OTIC VESICLE
SUPERIOR GANGLION OF NERVE H
JUGULAR GANGLION OF NERVE X
PETROSAL GANGLION OF NERVE IX ^
NERVE :
NOOOSE ganglion OF nerve::
NERVE
SPINAL CORO-^
pharyngeal pouch in-<:
pharyngeal POUCHBC;
thyroid BODY
BUL0US COROIS
MESENCEPHALON
â– NERVE 33
NERVE m
infundibulum
E’S POCKET
SEESSEL'S POCKET
CHOROID FISSURE
OlENCEPHALON
DORSAL aorta
NOTOCHORD
stomach
LIVER
ventral pancreasdorsal pancreas
gall blaode
MESONEPHROS- —
MIDGUT
AN DUCT
glomeruli
COLLECTING DUCT
HINDGUT
SPINAL GANGLION
Fig. 259. Chick embryo reconstruction of about 100 hrs. of incubation with special
reference to the nervous and urinary systems. See also fig. 336D.
bation. Reference should
5
BASIC HOMOLOGY OF ORGAN SYSTEMS
545
erism (branchiomerism) . However, all these segmental structures serve a
definite end. In other areas, head development proceeds in a manner which
obscures segmentation, for the probable reason that segmentation does not fit
into the developmental pattern which must proceed directly and precociously
to gain a specific end dictated by problems peculiar to head development.
{Note: For a critical analysis of the supposed facts in favor of segmentation,
together with a marshaling of evidence against such an interpretation, consult
Kingsbury and Adelmann (’24) and for a favorable interpretation of the segmental nature of the head region, see Goodrich (’18) and Delsman (’22).
Figure 253 is taken from Goodrich (’18), and the various structures which
favor a segmental interpretation of the head region are shown.)


G. Basic Homology of the Vertebrate Organ Systems  
G. Basic Homology of the Vertebrate Organ Systems  
Line 1,310: Line 642:
1. Definition  
1. Definition  


Homology is the relationship of agreement between the structural parts of
2. Basic Homology of Vertebrate Blastulae, Gastrulae, and Tubulated Embryos  
one organism and the structural parts of another organism. An agreeable
relationship between two structures is established if:
 
( 1 ) the two parts occupy the same relative position in the body,
 
(2) they arise in the same way embryonically and from the same rudiments, and
 
(3) they have the same basic potencies.
 
By basic potency is meant the potency which governs the initial and fundamental development of the part; it should not be construed to mean the
ability to produce the entire structure. To the basic potency, other less basic
potencies and modifying factors may be added to produce the adult form of
the structure.
 
2. Basic Homology of Vertebrate Blastulae, Gastrulae, and  
Tubulated Embryos  
 
In Chapters 6 and 7, the basic conditions of the vertebrate blastula were
surveyed, and it was observed that the formative portion of all vertebrate
blastulae presents a basic pattern, composed of major presumptive organforming areas oriented around the notochordal area and a blastocoelic space.
During gastrulation (Chap. 9), these areas are reoriented to form the basic
pattern of the gastrula, and although round and flattened gastrulae exist, these
form one, generalized, basic pattern, composed of three germ layers arranged
around the central axis or primitive notochordal rod. Similarly, in Chapter
10, the major organ-forming areas are tubulated to form an elongated embryo,
composed of head, pharyngeal, trunk, and tail regions. As tubulation is effected in much the same manner throughout the vertebrate series and as the
pre-chordal plate mesoderm, foregut entoderm, notochord, and somitic meso
 
 
546
 
 
BASIC FEATURES OF VERTEBRATE MORPHOGENESIS
 
 
geniculate ganglion of seventh nerve
 
ACOUSTIC GANGLION OF EIGHTH NERVE
AUDITORY VESICLE
 
 
JU^dLAR GANGLION
 
SUPERIOR GANGLION
NINTH NERV
ACCESSORY ganglion
BASILAR ARTERY
DORSAL ROOT
GANGLION OP FIRST
 
cervical nerve
aortal arch I
 
AORTAL ARCH II
AORTAL ARCH III
AORTAL ARCH IV
AORTAL ARCH VT
PULMONARY ARTERY
TRACHEA
NOTOCHORD
RIGHT ATRIUM
LUNG
 
 
 
SMALL
 
INTESTINE
 
hepatic
 
PORTAL VEIN
DORSAL AORTA
 
 
OMPHALOMESENTERIC
ARTERY
 
(FUTURE SUPERIOR
MESENTERIC ARTERY)
 
 
GLOMERULUS
MESONEPHRIC TUBULE
 
 
DORSAL AORTA
MESONEPHRIC DUCT
 
 
Fig. 261. Drawings of pig embryos of about 9.5 to 12 mm. (A) Reconstruction of about
9.5 to 10 mm. pig embryo with special emphasis on the arterial system.
 
derm appear to be the main organizing influence throughout the series (Chap.
10), the conclusion is inescapable that the tubulated embryos of all vertebrates
are homologous basically, having the same relative parts, arising in the same
manner, and possessing the same basic potencies within the parts. To this
conclusion must be added a caution, namely, that, although the main segments
or specific organ regions along each body tube of one species are homologous
with similar segments along corresponding tubes of other species, variations
may exist and non-homologous areas may be insinuated or homologous areas
 
 
 
BASIC HOMOLOGY OF ORGAN SYSTEMS
 
 
547
 
 
may be deleted along the respective tubes. Regardless of this possibility, a
basic homology, however, appears to exist.
 
During later development through larval and definitive body-form stages,
a considerable amount of molding or plasis by environmental and intrinsic
factors may occur. An example of plasis is given in the development of the
forelimb rudiment of the fish, frog, bird, and pig. In the definitive form, these
structures assume different appearances and are adapted for different func
 
METENCEPHALON
 
 
BASILAR ARTERY
NOTOCHORD
 
ROOT OF TONGUE
THYROID GLAND
developing epiglottis
AORTIC ARCH III
L ARYN X
 
 
ESOPH AGU S
VALVES OF
 
SINUS 'VENOSUS/
LUNG bud'
 
SPINAL CORD
SINUS VENOSU;
 
 
GALL BLADDER
 
NOTOCHORDOORSAL AORTA
 
DEVELOPING
VERTEBRAE
 
MESONEPHRIC
KIDNE
 
 
 
MESENCEPHALON
 
 
TUBERCULUM
 
ju / POSTERIUS
~ — ^INFUNDIBULUM
OIEUCEPHALON
 
rathke's pocket
 
SEESSEL'S POCKET
— -OPTIC CHIASMA
 
-RECESSUS OPTICUS
TELENCEPHALON
AMINA TERMINALIS
TONGUE
BULBUS CORDIS
 
 
EXTRA-EMBRYONIC COELOM
UMBILICAL CORD
 
 
ALLANTOIC DIVERTICULUM
GENITAL EMINENCE
PROCTODAEUM
 
CLOACA
 
 
ALLANTOIC STALK
 
 
B.
 
 
metanephrogenous
 
TISSUE SPINAL GANGLION
 
 
Fig. 261 — (Continued} (B) Median sagittal section of 10 mm. embryo.
 
 
 
VEIN OF maxillary REGION
(BRANCH OF INTERNAL JUGULAR)
 
 
OTIC VESICLE
 
 
VEIN OF
 
MANDIBULAR REGION
BRANCH OF EXTERNAL
JUGULAR)
 
INTERNAL JUGULAR
VEIN
DORSAL
 
jSEGMENTAL VEINS
EXTERNAL
JUGULAR VEIN
 
 
 
LEFT DUCT OF CUVIER
RIGHT VITELLINE VEIN
LIVER
DUCTUS VENOSUS
HEPATIC VEINS
PORTAL VEIN
 
 
UMBILICAL
 
ARTERY
 
 
'TRANSVERSE ANASTOMOSIS
OF SITBCAROINALS
 
 
POSTERIOR CARDINAL
VEIN
 
 
PIG EMBRYO SHOWING RIGHT HALF
OF VENOUS SYSTEM
 
 
Fig. 261 — (Continued) (C) Lateral view of 12 mm. embryo showing venous system.
(C is redrawn and modified from Minot; 1903, A Laboratory Text-book of Embryology,
Blakiston, Philadelphia.)
 
 
548
 
 
 
 
Fig. 262. Sections and stereograms of 10 mm. pig embryo.
 
 
MCSCNCHYME^
 
 
 
Ibl— (Continued) Sections and stereograms of 10 mm. pig embryo
 
 
 
 
BIBLIOGRAPHY
 
 
551
 
 
tional purposes. Basically, however, these structures are homologous, although
plasis produces adult forms which appear to be different.
 
A further statement should be added, concerning that type of molding or
plasis of a developing structure which produces similar structures from conditions which have had a different genetic history. For example, the bat’s fore
limb rudiment is molded to produce a structure resembling superficially that
of the bird, although modern bats and birds have arisen through different lines
of descent. Similarly, the teeth of certain teleost fishes superficially resemble
the teeth of certain mammals, an effect produced from widely diverging lines
of genetic descent. These molding effects or homoplasy, which produce superficially similar structures as a result of adaptations to certain environmental
conditions, are called convergence, parallelism, and analogy. An example of
experimental homoplasy is the induction of eye lenses in the embryo by the
transplantation of optic-cup material to a place in the epidermis which normally does not produce a lens.
 
{Note: For a discussion of homology, homogeny, plasis, convergence, etc.,
see Tait, ’28.)
 
 
Bibliography
 
 
Adelmann, H. B. 1925. The development
of the neural folds and cranial ganglia
of the rat. J. Comp. Neurol. 39:19.
 
. 1927. The development of the eye
 
muscles of the chick. J. Morphol. 44:29.
 
. 1932. The development of the
 
prechordal plate and mesoderm of Ambly stoma piinctatum. J. Morphol. 54:1.
 
Baer, K. E. von. 1828-1837. liber Entwickelungsgeschichte der Thiere. Beobachtung und Reflexion. Erster Theil,
1828; Zweiter Theil, 1837. Konigsberg,
Borntriiger.
 
Balfour, F. M. 1878. Monograph on the
development of elasmobranch fishes. Republished in 1885 in The Works of
Francis Maitland Balfour, edited by M.
Foster and A. Sedgwick, vol. 1. The
Macmillan Co., London.
 
Delsman, H. C. 1922. The Ancestry of
Vertebrates. Valkoff & Co., Amersfoort,
Holland.
 
Goodrich, E. S. 1918. On the development
of the segments of the head of Scy Ilium.
Quart. J. Micr. Sc. 63:1.
 
Hill, J. P. and Tribe, M. 1924. The early
development of the cat {Felis dornestica).
Quart. J. Micr. Sc. 68:513.
 
 
Huxley, T. H. 1858. The Croonian lecture
— on the theory of the vertebrate skull.
Proc. Roy. Soc., London, s.B. 9:381.
 
Kingsbury, B. F. 1915. The development
of the human pharynx. 1. Pharyngeal
derivatives. Am. J. Anat. 18:329.
 
. 1924. The significance of the so
called law of cephalocaudal differential
growth. Anat. Rec, 27:305.
 
. 1926. Branchiomerism and the
 
theory of head segmentation. J. Morphol.
42:83.
 
and Adelmann, H. B. 1924. The
 
morphological plan of the head. Quart.
J. Micr, Sc. 68:239.
 
Kyle, H. M. 1926. The Biology of Fishes.
Sidgwick and Jackson, Ltd., London.
 
Landacre, F. L. 1921. The fate of the
neural crest in the head of urodeles. J.
Comp. Neurol. 33:1.
 
Lewis, W. H. 1910. Chapter 12. The development of the muscular system in
Manual of Human Embryology, edited
by F. Keibel and F. P. Mall. J. B. Lippincott Co., Philadelphia.
 
Locy, W. A. 1895. Contribution to the
structure and development of the vertebrate head. J. Morphol. 11:497.
 
 
 
552
 
 
BASIC FEATURES OF VERTEBRATE MORPHOGENESIS
 
 
Newth, D. R. 1951. Experiments on the
neural crest of the lamprey embryo. J.
Exper. Biol. 28:17.
 
Owen, R. 1848. On the archetype and
homologies of the vertebrate skeleton.
John Van Voorst, London.
 
Raven, C. P. 1933a. Zur Entwicklung der
Ganglienleiste. I. Die Kinematik der
Ganglienleistenentwicklung bei den Urodelen. Arch. f. Entwlngsmech. d. Organ.
125:210.
 
. 1933b. Zur Entwicklung der Ganglienleiste. III. Die Induktionsfahigkeit
des Kopfganglienleistenmaterials von
Rana fusca.
 
 
Stone, L. S. 1922. Experiments on the development of the cranial ganglia and
the lateral line sense organs in Amblystoma pimctatum. J. Exper. Zool. 35:421.
 
. 1926. Further experiments on the
 
extirpation and transplantation of mesectoderm in Amhlystorna punctatum. J.
Exper. Zool. 44:95.
 
. 1929. Experiments showing the
 
role of migrating neural crest (mesectoderm) in the formation of head skeleton and loose connective tissue in Rana
paliistris. Arch. f. Entwicklngsmech. d.
Organ. 118:40.


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

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

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

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

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

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

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

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

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

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

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


Cleavage (Segmentation) and Blastulation

6. Cleavage (Segmentation) and Blastulation

A. General considerations

1. Definitions

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

3. Importance of the cleavage-blastular period of development

a. Morphological relationships of the blastula

b. Physiological relationships of the blastula

1 ) Hybrid crosses

2) Artificial parthenogenesis

3) Oxygen-block studies

4. Geometrical relations of early cleavage

a. Meridional plane

b. Vertical plane

c. Equatorial plane

d. Latitudinal plane

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

a. Mechanisms associated with mitosis or cell division

b. Influence of cytoplasmic substance and egg organization upon cleavage

1) Yolk

2) Organization of the egg

c. Influence of first cleavage amphiaster on polyspermy

d. Viscosity changes during cleavage

e. Cleavage laws

1 ) Sach’s rules

2) Hertwig’s laws

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

B. Types of cleavage in the phylum Chordata

1. Typical holoblastic cleavage

a. Amphioxus

b. Frog (Rana pipiens and R. sylvatica)

c. Cyclostomata

2. Atypical types of holoblastic cleavage

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

1 ) General considerations

2) Early development of the rabbit egg

a) Two-cell stage

b) Four-cell stage

c) Eight-cell stage

d) Sixteen-cell stage

e) Morula stage

f) Early blastocyst

3) Types of mammalian blastocysts (blastulae)

b. Holoblastic cleavage of the transitional or intermediate type

1) Amhystoma maculatum (punctatum)

2) Lepidosiren paradoxa

3) Necturus maculosus

4) Acipenser sturio

5) Amia calva

6) Lepisosteus (Lepidosteus) osseus

7) Gymnophionan amphibia 3. Meroblastic cleavage

a. Egg of the common fowl

1 ) Early cleavages

2) Formation of the periblast tissue

3) Morphological characteristics of the primary blastula

4) Polyspermy and fate of the accessory sperm nuclei

b. Elasmobranch fishes

1 ) Cleavage and formation of the early blastula

2) Problem of the periblast tissue in elasmobranch fishes

c. Teleost fishes

1) Cleavage and early blastula formation

2) Origin of the periblast tissue in teleost fishes

d. Prototherian Mammalia

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

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

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

1. Cytoplasmic inequality of the early blastomeres

2. Nuclear equality of the early blastomeres

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

The Chordate Blastula and Its Significance

7. The Chordate Blastula and Its Significance

A. Introduction

1. Blastulae without auxiliary tissue

2. Blastulae with auxiliary or trophoblast tissue

3. Comparison of the two main blastular types

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

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

D. Introduction of the words ectoderm, mesoderm, endoderm

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

F. Importance of the blastular stage in embryonic development

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

1. Protochordate blastula

2. Amphibian blastula

3. Mature blastula in birds

4. Primary and secondary reptilian blastulae

5. Formation of the late mammalian blastocyst (blastula)

a. Prototherian mammal, Echidna

b. Metatherian mammal, Didelphys

c. Eutherian mammals

6. Blastulae of teleost and elasmobranch fishes

7. Blastulae of gymnophionan amphibia

Late Blastula in Relation to Certain Innate Physiological Conditions: Twinning

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

A. Introduction

B. Problem of differentiation

1. Definition of differentiation; kinds of differentiation

2. Self-differentiation and dependent differentiation

C. Concept of potency in relation to differentiation

1. Definition of potency

2. Some terms used to describe different states of potency

a. Totipotency and harmonious totipotency

b. Determination and potency limitation

c. Prospective potency and prospective fate

d. Autonomous potency c. Competence

D. The blastula in relation to twinning

1. Some definitions

a. Dizygotic or fraternal twins

b. Monozygotic or identical twins

c. Polyembryony •

2. Basis of true or identical twinning

3. Some experimentally produced, twinning conditions

E. Importance of the organization center of the late blastula

Gastrulation

| 9. Gastrulation

A. Some definitions and concepts

1. Gastrulation

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

a. Fundamental body plan of the vertebrate animal

b. The gastrula in relation to the primitive body plan

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

B. General processes involved in gastrulation

C. Morphogenetic movement of cells

1. Importance of cell movements during development and in gastrulation

2. Types of cell movement during gastrulation

a. Epiboly

b. Emboly

3. Description of the processes concerned with epiboly

4. Description of the processes involved in emboly

a. Involution and convergence

b. Invagination

c. Concrescence

d. Cell proliferation

e. Polyinvagination

f. Ingression

g. Delamination

h. Divergence

i. Extension

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

1. The organization center and the primary organizer

2. Divisions of the primary organizer

E. Chemodifferentiation and the gastrulative process

F. Gastrulation in various Chordata 1. Amphioxus

a. Orientation

b. Gastrulative movements

1 ) Emboly

2) Epiboly

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

4) Closure of the blastopore

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

1 ) Emboly

2) Epiboly

2. Gastrulation in Amphibia with particular reference to the frog

a. Introduction

1) Orientation

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

b. Gastrulation

1) Emboly

2) Epiboly

3) Embryo produced by the gastrulative processes

4) Position occupied by the pre -chordal plate material

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

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

1) Emboly

2) Epiboly

3. Gastrulation in reptiles

a. Orientation

b. Gastrulation

4. Gastrulation in the chick

a. Orientation

b. Gastrulative changes

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

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

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

4) Primitive pit notochordal canal

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

5. Gastrulation in mammals

a. Orientation

b. Gastrulation in the pig embryo

c. Gastrulation in other mammals

6. Gastrulation in teleost and elasmobranch fishes

a. Orientation

b. Gastrulation in teleost fishes

1) Emboly

2) Epiboly

3) Summary of the gastrulative processes in teleost fishes

a) Emboly

b) Epiboly

4) Developmental potencies of the germ ring of teleost fishes

c. Gastrulation in elasmobranch fishes

7. Intermediate types of gastrulative behavior

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

H. Autonomous theory of gastrulative movements

I. Exogastrulation

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

K. Blastoporal and primitive-streak comparisons

Development of Primitive Body Form

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

A. Introduction

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

a. Tabulation

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

c. Regional modifications of the tubulated areas

2. Common, vertebrate, embryonic body form

3. Starting point for tabulation

4. Developmental processes which accomplish tabulation

a. Immediate processes

b. Auxiliary processes

5. Blastocoelic space and body-form development

6. Primitive circulatory tubes or blood vessels

7. Extra-embryonic membranes

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

1. Neuralization or the tabulation of the neural plate area

a. Definition

b. Neuralizative processes in the Vertebrata

1) Thickened keel method

2) Neural fold method

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

d. Anterior and posterior neuropores; neurenteric canal

2. Epidermal tabulation

a. Development of the epidermal tube in Amphibia

b. Tabulation of the epidermal area in flat blastoderms

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

a. Regions of primitive gut tube or early metenteron

b. Formation of the primitive metenteron in the frog

c. Formation of the tubular metenteron in flat blastoderms

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

a. Early changes in the mesodermal areas

1) Epimere; formation of the somites

2) Mesomere

3) Hypomere

b. Tabulation of the mesodermal areas

C. Notochordal area

D. Lateral constrictive movements

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

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

a. End-bud growth

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

2. Neuralization and the closure of the blastopore

3. Epidermal tubulation

4. Tubulation of the entodermal area

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

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

5. Tubulation of the mesoderm

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

7. Notochord

F. Early development of the rudiments of vertebrate paired appendages

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

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

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

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

Basic Features of Vertebrate Morphogenesis

11. Basic Features of Vertebrate Morphogenesis

A. Introduction

1. Purpose of This Chapter

2. Definitions

a. Morphogenesis and Related Terms

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

1) Primitive Body Form.

2) Larval Body Form.

3) Definitive Body Form.


3. Basic or Fundamental Tissues

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

1. Processes Involved in Basic System Formation

(a) extension and growth of the body tubes,

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

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

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


2. Fundamental Similarity of Early Organ Systems

C. Laws of von Baer

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

1. Types of Mesodermal Cells

2. Origin of the Mesoderm of the Head Region

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

b. Head Mesoderm Derived from the Pre-chordal Plate

c. Head Mesoderm Contributed by Neural Crest Material

d. Head Mesoderm Originating from Post-otic Somites

3. Origin of the Mesoderm of the Tail

4. Contributions of the Trunk Mesoderm to the Developing Body

a. Early Differentiation of the Somites or Epimere

b. Early Differentiation of the Mesomere (Nephrotome)

c. Early Differentiation and Derivatives of the Hypomere

5. Embryonic Mesenchyme and Its Derivatives


1. Neural Plate Area (Ectoderm)

2. Epidermal Area (Ectoderm)

3. Entodermal Area

4. Notochordal Area

5. Mesodermal Areas

6. Germ-cell Area

F. Metamerism


G. Basic Homology of the Vertebrate Organ Systems

1. Definition

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

Bibliography

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