Difference between revisions of "Histology and Embryology 1941 - Embryology"

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==Part One - General Development==
==Part One - General Development==
For the formation and structure of the germ cells see pp. 77, 83.  
* [[Histology and Embryology 1941 - Embryology 1#Fertilization, Cleavage and the Origin of the Germ Layers|Fertilization, Cleavage and the Origin of the Germ Layers]]
** [[Histology and Embryology 1941 - Embryology 1#|Fertilization|Fertilization]]
** [[Histology and Embryology 1941 - Embryology 1#II. Cleavage (Segmentation) of the Mammalian Ovum|II. Cleavage (Segmentation) of the Mammalian Ovum]]
** [[Histology and Embryology 1941 - Embryology 1#III. Gastrlation|III. Gastrlation]]
** [[Histology and Embryology 1941 - Embryology 1# IV. Formation of the Mesoderm|IV. Formation of the Mesoderm]]
* [[Histology and Embryology 1941 - Embryology 1#Early Differentiations of the Germ Layers; Development of the External Form|Early Differentiations of the Germ Layers; Development of the External Form]]
** [[Histology and Embryology 1941 - Embryology 1#Early Differentiations of the Germ Layers|Early Differentiations of the Germ Layers]]
** [[Histology and Embryology 1941 - Embryology 1#Development of the External Form|Development of the External Form]]
* [[Histology and Embryology 1941 - Embryology 1#Human Fetal Membranes, Placentation and Deciduae|Human Fetal Membranes, Placentation and Deciduae]]
** [[Histology and Embryology 1941 - Embryology 1#I. Fetal Appendages and Membranes|I. Fetal Appendages and Membranes]]
** [[Histology and Embryology 1941 - Embryology 1#II. Placentation|II. Placentation]]
** [[Histology and Embryology 1941 - Embryology 1#III. The Deciduae|III. The Deciduae]]
** [[Histology and Embryology 1941 - Embryology 1#IV. The Placenta|IV. The Placenta]]
The processes of fertilization and cleavage of the human ovum
have never been observed; in fact the earliest human embryos
known are nearly two weeks old and have the three germ layers
already formed. What follows is based on observations in mammals.
I. Fertilization
It consists essentially in the fusion of the gametes (spermatozoon
and ovum), each of which carries one half the number of chromosomes characteristic of the species; the original number is thus
A. Site of fertilization. In man it occurs in the upper third of the
oviduct. As proof of this can be cited the fact that the embryo may
be implanted in the oviduct (tubal pregnancy).
B. The process of fertilization.
1. Penetration of the spermatozoon. It usually takes place after
the second maturation mitosis has been completed. The spermatozoon crosses the zona pellucida of the ovum (p. 82) and enters
the cytoplasm, where it loses its tail. Its middle piece furnishes the
centrosomes and spindle for the dividing ovum.
2. Formation of the pronuclei. After the second maturation mitosis the nucleus of the ovum is termed the female pronucleus.
1 1°5 ]
The compact nucleus (head) of the spermatozoon becomes vesicular and shows a distinct chromatin network (male pronucleus).
3. Fusion of the pronuclei. The male pronucleus moves toward
the female pronucleus; when the two finally come into contact
they merge into a single (segmentation) nucleus.
4. First cleavage mitosis. Immediately after fusion of the pronuclei the segmentation nucleus enters the prophase of the first
cleavage mitosis. The centrosomes contributed by the spermatozoon form a spindle.
II. Cleavage (Segmentation) of the Mammalian Ovum
Cleavage is greatly influenced by the amount of yolk present in
the ovum. If the yolk is very abundant it does not affect the whole
ovum (meroblastic cleavage of fishes, reptiles and birds). If it is
scarce or present in moderate amounts the whole ovum will divide
(holoblastic cleavage of the amphibia and mammals). Although the
microscopic mammalian ovum contains little yolk its cleavage does
not correspond to the segmentation of similar ova in other forms
(i.e. Amphioxus) because it does not give rise to a typical blastula.
A. Early cleavage. The fertilized egg divides into two blastomeres;
each of the latter divides in turn and a total of four is produced.
The first two divisions are meridional. The third is equatorial and
gives rise to eight blastomeres which upon further division become
sixteen, etc.
B. Morula. When this stage is reached the cell aggregation shows
two types of blastomeres: one light, the other dark (i.e. more granular). The light blastomeres arrange themselves as a capsule around
the dark cells; the latter are termed the inner cell mass while the
layer of light blastomeres is known as the trophoblast (or trophectoderm).
C. Formation of the blastocyst. Fluid collects between the inner cell
mass and the trophoblast; as a result of this the morula is changed
into a hollow vesicle in which the inner cell mass remains in contact
with the trophoblast at one of the poles.
D. Significance of the inner cell mass. It corresponds to the blastoderm of the chick since it will give rise to the embryo, while the
trophoblast becomes associated intimately with the uterine mucosa
and is concerned with the nutrition, respiration and excretion of the
[ 106]
III. Gastrulation
Gastrulation in mammals is much more similar to the corresponding process of reptiles and birds than to the typical gastrulation
observed in Amphioxus and the amphibians.
A. Formation of the endoderm. It does not arise by invagination
but it is constituted by the arrangement of cells in a layer on the
under surface of the inner cell mass, which is now the ectoderm.
B. Extension of the endoderm. In most mammals the endoderm
spreads rapidly on the inner surface of the blastocyst, which it lines
IV. Formation of the Mesoderm
The formation of the mammalian mesoderm closely resembles
the production of the same layer in birds.
A. Primitive streak. It appears in the pear-shaped blastoderm and
marks the axis of the embryo. It ends anteriorly in a primitive knot
(of Hensen). In vertical section is seen to be a thickened band continuous with the ectoderm; its under-surface produces mesodermic
cells which spread laterally and caudally between the ectoderm and
B. Formation of the head process. This arises as a forward extension of Hensen’s knot and in man and many mammals it has a
cavity (notochordal canal) which opens at the primitive pit. Its floor
fuses with the endoderm, after which both disappear in the area of
fusion. The roof (notochordal plate) becomes the notochord.
C. Formation of the coelom. The mammalian mesoderm grows
rapidly between the ecto- and endoderm. At first it is a single sheet
but it soon splits into two layers, one associated with the ectoderm
(somatic mesoderm) and the other with the endoderm (splanchnic
mesoderm). The cavity between the two mesodermic layers is the
coelom or body cavity.
D. Extra-embryonic coelom. Since the mesoderm spreads between
the ecto- and endoderm throughout the blastodermic vesicle its splitting will extend beyond the embryonic area. An extra-embryonic
coelom is thus formed.
[ 107]
After gastrulation and the formation of the mesoderm the three
germ layers of the embryo begin to differentiate and produce the
primordia of the most important organic systems.
I. Early Differentiations of the Germ Layers
A. Formation of the neural tube. The neural tube is the primordium
of the central nervous system; its anterior (or cephalic) portion
gives rise to the brain, the rest to the spinal cord. The first indication of the formation of the neural tube is the differentiation of the:
1. Neural (medullary) plate. This is a thickening of the ectoderm
along the longer axis of the blastoderm. It begins in Hensen’s
node and extends anteriorly (i.e. cephalad).
2. Neural (medullary) folds. The edges of the medullary plate
are converted into folds, which diverge posteriorly enclosing Hensen’s node. The neural plate is thus changed into a groove.
3. Closure of the neural groove. The neural folds increase in
height and curve toward each other, finally meeting in the midline at a point which corresponds to the neck region of the adult.
This changes the neural groove into a tube, the lumen of which
persists throughout adult life (p. 158) .
4 . The neural crest. A longitudinal band of cells between the ectoderm and the edge of the plate sinks into the neural fold and
becomes this structure.
5. Anterior and posterior neuropores. The closure of the neural
groove does not take place simultaneously along its entire length
but proceeds slowly cephalad and caudad. For a considerable
period there are two openings in the ends of the tube, the anterior
and posterior neuropores, respectively.
6. Primary brain vesicles. The anterior, expanded region of the
neural tube is divided through constrictions into the three primary
brain vesicles: the fore-, mid-, and hindbrain.
B. Formation of the notochord. The roof of the head process (p.
107) is known as the notochordal plate. It loses all connection with
both endoderm and mesoderm and becomes a rod, the notochord,
which extends beneath the neural tube and ends anteriorly under
[ 108]
the midbrain. The notochord is the axis around which the vertebral
column will develop. Remnants are occasionally found as the pulpy
nuclei of the intervertebral disks.
C. Differentiation of the mesoderm. The splitting of the mesoderm
into somatic and splanchnic layers has been mentioned (p. 107).
The two layers of each side are continuous near the midline, i.e.
lateral to the notochordal plate.
1. Formation of the somites. At the junction of somatic and
splanchnic layers the mesoderm is much thicker. This dorsal or
median mesoderm becomes divided transversely into a number
of more or less cuboidal, usually solid masses, the somites.
a. Order of appearance. The first somite appears behind the
future occipital region of the adult. The segmentation of the
dorsal mesoderm proceeds caudad until 38 pairs have developed
in the neck and trunk regions of the body, in addition to those
that are developed in the occipital region of the head (probably
b. Differentiation. Each somite becomes differentiated into
three distinct portions:
(1) Sclerotome. The cells of that portion of the somite next
to the notochord grow inward toward the midline to surround the notochord and lateral walls of the neural tube.
They will form the body of the vertebrae and the vertebral
(2) Myotome. The middle portion of the somite will give
rise to a part of the skeletal (voluntary) musculature of the
(3) Dermatome. The outer or lateral portion of the somite
is believed to be transformed into the derma of the skin (p.
46). Its existence in mammals and man has been denied,
2. The ventral (lateral) mesoderm. It never becomes segmented
in the neck and trunk regions. The coelom occurs between its
somatic and splanchnic layers.
a. Somatic layer. It is continuous laterally with the mesodermic
layer which lines the outer surface of the amnion (p. 115).
Mesially it passes into the:
b. Splanchnic layer, which is applied closely to the endoderm
of the:
(1) Digestive tract, derived from the dorsal portion of the
yolk sac (p. 114). The splanchnic mesoderm becomes converted into mesenchyme out of which the muscular coats
will develop.
(2) Yolk sac. The splanchnic layer surrounding the yolk sac
is the source of the first blood vessels and blood of the embryo
and corresponds to the vascular area of the chick blastoderm.
3. The intermediate cell mass (nephrotome). This is a narrow
area underlying the original longitudinal groove which separates
the somite area from the ventral mesoderm. It produces the
pronephroi, Wolffian bodies (mesonephroi) and the mesonephric
ducts (p. 141).
II. Development of the External Form
The early appearance of the embryo differs greatly from that of
the child and has features in common with the other vertebrates:
this is the embryonic period, which lasts two months. After this
the resemblance with the child is more striking (fetal period).
A. Embryonic period.
1. Separation of the embryo from the yolk sac. At the end of the
formation of the mesoderm the embryonic disk (blastoderm)
forms the roof of the yolk sac, the whole being connected with
the chorion by the body stalk, occupied by the rudimentary
a. Folding of the blastoderm. A groove which appears at the
periphery of the blastoderm, between the latter and the sac,
marks the beginning of the separation of these two portions.
As it deepens the flat blastoderm is gradually transformed into
a cylinder.
b. Formation of the digestive tract. The endoderm of the roof
of the yolk sac is incorporated into the embryo and constitutes
the primordium of the digestive tract.
c. Formation of the yolk stalk. With the deepening of the
groove the connection between the yolk sac and the embryo is
very much reduced and becomes the yolk stalk.
2. Formation of the cephalic and caudal folds. These are due to
rapid elongation of the body of the embryo.
a. The cephalic fold. With the closure of the neural groove the
[ HO ]
anterior end of the embryo rises and projects beyond the yolk
sac. The primitive brain vesicles become distinct.
b. The fore-gut. As the cephalic fold rises it carries along the
endoderm, which forms a blind diverticulum, the fore-gut.
Beneath the fore-gut the heart is developing.
c. The caudal fold and hind-gut. A less developed fold is
formed at the posterior end of the embryo; it also contains an
endodermic diverticulum, the hind-gut.
3. Establishment of the primary flexures. Continued elongation
of the embryo during the 5th week causes the bending of the body
in three different regions:
a. Cephalic flexure. This is a sharp bend at the level of the midbrain.
b. Cervical flexure. A second more prominent flexure occuring
more posteriorly, in the region of the future neck.
c. Caudal (sacral) flexure. Is present toward the posterior end
of the body, which ends in a short, pointed tail.
4. Appearance of the limb buds. These are already present at the
end of the fifth week. The bud for the arm is seen on each side
of the body a little posterior to the cervical flexure. The lower
limb buds, located in the region of the caudal flexure, are slightly
5. The branchial arches. In common with other vertebrates the
human embryo shows on each side of the region of the future
pharynx four grooves which separate five branchial arches. Their
formation will be considered later (p. 123).
a. Mandibular arch. The first arch consists of a main portion
which gives rise to the jaw, and a maxillary process appearing
as a wedge between the eye and the mandibular process. Retardation of the development of the mandibular process causes
an abnormally small jaw (micrognathus) ; its complete absence
is also possible (agnathus).
b. Hyoid arch. It is separated from the mandibular by the first
branchial groove which sometimes is actually a cleft.
c. Other arches. They are less developed, especially the fifth,
placed behind the fourth groove and poorly defined posteriorly.
d. Cervical sinus. After the sixth week the first two arches overlap the other three, which sink into a triangular depression
called the cervical sinus. Later the posterior edge of the hyoid
arch fuses with the thoracic wall and the sinus is cut off from
the outside.
6. Development of the neck. The neck develops in the area occupied by the branchial arches whose mesoderm gives rise to muscles, bones and blood vessels. From the lining of the endodermic
pouches 'several important organs arise (p. 124). When this differentiation has been completed the embryo has acquired a neck,
not present in earlier stages in which the mandibular arch rests
on the thorax. The neck results from an elongation of the region
between the mandibular arch and the pericardium. Incomplete
closure of the branchial clefts causes cervical fistulae.
7 . The face. The formation of the face is a complex process. It
takes place chiefly between the 5th and 8th weeks.
a. The olfactory pits. They are first represented by ectodermic
thickenings (olfactory placodes) on the ventrolateral aspect of
the head. The placodes sink and become converted into shallow
b. Fronto-nasal process. This is the region of the head between
the olfactory pits in early embryos. In later stages the olfactory
pits subdivide the fronto-nasal process into:
(1) Lateral nasal processes, which with the
(2) Median nasal processes, bound the nostrils externally.
c. Fusion of the median nasal processes with the maxillary
processes. This fusion forms the upper jaw. When incomplete
it causes hare-lip.
d. Fusion of the lateral nasal processes with the maxillary processes. It obliterates the naso-lacrimal groove, which extends
between the maxillary process and the lateral nasal process and
is thus changed into the naso-lacrimal canal of the adult. This
fusion also forms the wings (alae) of the nose and the cheek
region. Its failure causes oblique facial cleft.
e. The nose. When first formed the nose is broad and flat with
the nostrils set far apart. In later fetal months the bridge of the
nose rises and the nostrils are approximated.
f. The mouth. The mouth is also very wide in early stages but
during later development it is much reduced in width.
8. The external ear. This is formed around the first branchial
groove (between the mandibular and hyoid arches) by the fusion
[ 112]
of several small tubercles in the two arches. The groove becomes
the external auditory meatus.
9. The eye. During the embryonic period the eye lacks eyelids.
The iris and pupil are clearly seen. The eyelids develop during
the second month.
10. The limbs. At the earliest stage of their development (5th
week) the limbs are lateral swellings (limb buds), the location
of which has already been given (p. 111). The upper limb buds
arise first.
a. Division into proximal and distal regions. The distal end of
the limb bud flattens and a constriction separates this portion
from a more proximal, cylindrical segment. The flattened portion becomes the hand and the foot, respectively. Radial ridges
indicate the formation of the digits.
b. Subdivision of the proximal region. A second constriction
separates the proximal region into two segments: the arm and
forearm, and the thigh and leg, respectively.
c. Rotation of the limbs. During their development the limbs
undergo changes in position. At the beginning they point
caudad, but soon project outwards at right angles to the body
wall. Later the palmar and plantar surfaces face the body.
Further rotation brings them into the position of the adult.
d. Anomalies. They are frequent and range from complete or
almost complete absence of the limbs (amelus) to a partial duplication of the hand or foot (dichirus).
(1) The distal portion may resemble a stump (hemimelus)
or the proximal segments may be missing so that the hand
or foot spring directly from the body (phocomelus).
(2) The digits may be fused (syndactyly), or be excessively
short (brachydactyly) or they may have more phalanges than
usual (hyperphalangism).
(3) More than the normal number of digits may also occur
(4) Clubhand and clubfoot also result from primary defects
in the development of the limb buds.
B. Fetal period. The fetus definitely resembles the child, but at the
beginning of the period (3rd lunar month) the head is still disproportionately large; the embryonic flexures have disappeared. The
sex can be distinguished readily.
[ 113]
1. Appearance of lanugo. This fine, silky hair covers the face and
body during the 5th month, and begins to be shed before birth
(10th lunar month) except on the face.
2. Growth of eyebrows and lashes. The eyelids fuse during the
3rd month and remain so until the 7th month. During the 6th
month the eyebrows and lashes grow.
3. Growth of the nails. They begin to form during the 3rd month
and project at the finger tips during the 9th.
4 . Descent of the testes into the scrotum. This takes place during
the 8th month.
I. Fetal Appendages and Membranes
The human fetal appendages and membranes are: the yolk sac,
chorion, amnion, allantois and umbilical cord. They all appear early
since they are concerned with important protective and nutritional
A. The yolk sac. Although it does not contain yolk it is an important structure whose roof will provide the endodermic epithelium
of the whole digestive system (p. 121).
1. Vitelline (omphalomesenteric) vessels. The splanchnic mesoderm which invests the outer surface of the sac is a source of blood
for the embryo since the vitelline vessels arise in it.
2. Yolk stalk. This is a constriction between the sac proper and
the body of the embryo. It becomes thinner as development proceeds and is located within the umbilical cord.
3. Fate of the sac. It shrinks somewhat and becomes a small, solid
structure containing detritus, usually seen near the umbilical cord
in the after-birth.
4 . Meckel’s diverticulum. This is the persisting proximal end of
the yolk stalk. When it opens at the umbilicus (navel) is called
an umbilical fistula.
B. The chorion. It is the continuation of the trophoblast of the blastocyst to which is added an inner lining of extra-embryonic (somatic) mesoderm. The human chorion is studded with villi, each of
which contains a mesodermic core. Blood vessels enter the latter,
[ 114]
especially in those villi next to the uterine wall, which are incorporated into the placenta (p. 117).
C. The amnion. While in most mammals this membrane arises by
a process of folding, in man and certain mammals (bat, guinea pig,
apes) the amnion is formed by a different method at a very early
stage of development.
1. Origin. A cavity appears in the solid ectodermal cell mass
which remains after formation of the endoderm (p. 106). The
roof and sides of the cavity become the amnion through a process
of thinning, while the floor remains as the dorsal ectoderm of the
2. Structure. It is a thin, transparent, non-vascular membrane
covered externally by mesenchyme; its interior is lined by a low
cuboidal, ectodermic epithelium. It contains the amniotic fluid in
which is suspended the embryo; the amount of fluid at birth is
about one liter.
3. Fate of the amnion. After the 2nd month of pregnancy the
amnion is loosely fused with the chorionic wall, with the resulting obliteration of the extra-embryonic coelom. It breaks during
the early stages of childbirth and the fluid escapes as the “waters.”
It is expelled attached to the after-birth (p. 120).
D. The allantois. It is rudimentary in man, whereas in other mammals (i.e. the pig) it attains great development. In man, however,
it is important in that it conveys the umbilical vessels to the chorion.
1. Origin. It appears very early, even before the gut begins to assume a tubular form. Due to this it cannot be properly regarded
as an evagination of the hind-gut, as in other mammals. It occupies the body stalk, a mesodermic bridge connecting the embryo
to the chorion, which it reaches.
2. Fate. Growth of the allantois soon ceases, and it becomes obliterated. Remnants are still discernible in the proximal part of the
umbilical cord in early pregnancy.
E. The umbilical cord. The human cord is fully formed during the
6th week of pregnancy through the wrapping of the amnion around
the body stalk, yolk stalk and sac.
1. Contents. The young cord contains the body stalk (with the
enclosed allantois) and a prolongation of the coelom occupied by
an intestinal loop to which is attached the yolk sac. The yolk stalk
carries the vitelline vessels. In addition there are the umbilical
vessels (two arteries and a vein).
2. Structure at term. The walls of the cord develop a peculiar
form of connective tissue (‘mucous,’ or jelly of Wharton) which
causes the obliteration of the coelomic prolongation within the
cord after the intestinal loop has been withdrawn into the body.
Externally it is covered with simple cuboidal epithelium. Remnants of the allantois and yolk stalk may still be seen.
3. Umbilical hernia. It is caused by failure of the intestinal loop
to be withdrawn into the body cavity.
II. Placentation
This includes the implantation of the early embryo in the uterine
wall and the formation of the placenta.
A. Preparation of the uterus for the embryo. It is accomplished during the premenstrual (progravid) stage of the uterine cycle (p. 86),
during which the endometrium undergoes changes resulting in
increased vascularity, increased glandular secretions rich in glycogen, and a general loosening of the tissues of the endometrium.
B. Passage of the segmenting ovum into the uterus. When the
ovum enters the uterus it is probably in the morula stage. Several
days elapse before the blastocyst becomes embedded in the uterine
wall. The period between fertilization and implantation is estimated
at nine or ten days.
C. Implantation. The blastocyst becomes embedded in the endometrium. How this is accomplished in the human is not exactly
known, but the whole process is supposed to take no more than a
day. Implantation has been studied in detail in the guinea pig.
1. Destruction of the epithelium. Contact of the trophoblast with
the uterine epithelium causes destruction of the latter, probably
under the influence of trophoblastic enzymes.
2. Invasion of the tunica propria. After destruction of the epithelium trophoblastic processes grow into the tunica propria. The
blastocyst gradually sinks into the endometrium. The orifice of
entrance is closed later.
3. Site of implantation. It varies somewhat but it is usually at or
near the fundus of the uterus on either the anterior or the posterior wall.
[ n6]
D. Establishment of the embryo in the endometrium.
1. Formation of embryotroph. As a result of the enzymatic activity of the trophoblast spaces are formed which contain cellular
debris and some blood. This material (embryotroph) is absorbed
by the trophoblast and serves as food for the embryo.
2. Hemotrophic nutrition. The rapid growth of the blastocyst
after implantation results in additional destruction of the surrounding tissues. Dissolution of the walls of the capillaries and
small veins leads to the formation of blood sinuses; a hemotrophic
type of nutrition is thus established.
3. Primary villi. The enzymatic activities of the trophoblast are
exerted only by the more superficial cells of this layer, which form
syncytial projections (primary villi).
4. Secondary villi. The chorionic villi (secondary villi) are lined
by the more deeply placed trophoblastic cells which are concerned
with absorption of maternal nutritive substances from the blood
sinuses. Hemotrophic nutrition is definitely established when
branches of the umbilical vessels enter the chorion; this happens
about three weeks after fertilization.
a. Differentiation of the chorionic villi. Since the umbilical vessels reach the chorion by way of the body stalk, the villi which
are remote from the attachment of the latter to the chorion
do not receive an abundant blood supply and gradually atrophy.
The region in which they formerly occurred is the chorion
laeve. The other villi continue their growth and become profusely branched (chorion frondosum).
b. Structure. Each young villus consists of a core of mesoderm
containing blood vessels, enclosed within a double-layered epithelium.
(1) Syncytium. This is the outer epithelial covering consisting of a continuous layer of cytoplasm with evenly distributed
nuclei. In the older villi and in the placenta at term it is
represented by scattered cytoplasmic clumps containing many
nuclei (syncytial knots).
(2) Langhans layer. The inner layer consists of epithelial
cells (of Langhans) with definite outlines. They gradually
atrophy as the villus grows, persisting only in small scattered
(3) Blood vessels. The mesodermic core contains usually two
[ 117]
arterioles and two somewhat larger venules connected by
capillaries situated mainly at the tips of the villus.
( 4 ) Canalized fibrin. After degeneration of portions of the
syncytium and of the Langhans layer the vacant spaces are
occupied by depositions of fibrin. These areas of canalized
fibrin increase in extent in the older placentae.
III. The Deciduae
Before describing the placenta it will be necessary to consider the
changes which take place in the endometrium following implantation of the embryo. Since the endometrium is cast off after birth of
the fetus it is known as the decidua. Three different regions are distinguished: The decidua vera, the decidua capsularis and the decidua basalis.
A. Decidua vera (parietalis). This is the general lining of the pregnant uterus exclusive of the region of the embryo and the cervix.
Two layers are distinguished.
1. Compact layer. Contains the straight, dilated segments of the
uterine glands. Its surface epithelium disappears by the end of the
third (lunar) month due to contact with the decidua capsularis.
2. Spongy layer. In early pregnancy it is characterized by the
greatly enlarged and tortuous portions of the uterine glands, with
their long axes perpendicular to the surface of the endometrium.
After the second month stretching of the decidua reduces the
glands to elongated clefts parallel to the uterine surface.
3. Decidual cells. They are large, polygonal elements with one or
more nuclei. Decidual cells arise from reticulo-endothelial elements which abound in the tunica propria (p. 86 ). They contain
glycogen and are highly characteristic of pregnancy. Their function is not clearly understood.
4. Regression. The period of growth of the decidua vera is limited
to the first three or four months of pregnancy; later it becomes
thinner, less vascular and shows regressive changes. The decidual
cells become smaller and many degenerate.
B. Decidua capsularis (reflexa). This is the endometrial portion
which covers the area of implantation of the embryo. In the early
stages of pregnancy it shows some of the endometrial characteristics
and is covered by columnar epithelium. As the chorionic sac expands it becomes thin and atrophic. At the end of the third month
it fuses with the decidua vera and then degenerates, allowing the
chorion laeve to adhere to the decidua vera.
C. Decidua basalis. Since the blastocyst is implanted superficially in
the endometrium the deeper part of the compact and the spongy
layer, respectively, remain intact. The two together become the
basalis, which is an important part of the placenta. Decidual cells
also occur in this layer.
IV. The Placenta
The placenta is a structure, part of which (chorion frondosum)
is derived from the embryo (fetal placenta), the other (decidua
basalis) from the mother (maternal placenta). The two portions
are intimately associated into a solid, disk-shaped organ to which
is attached the umbilical cord.
A. Fetal placenta. The structure of the villi of the chorion frondosum has already been described (p. 117).
1. Cotyledons. The villi of the frondosum are evenly distributed
at first but in the older placentae become separated into 15 to 20
groups or cotyledons by the growth of trabeculae (placental septa)
from the walls of the uterus.
2. Fixation (anchoring) villi. These are attached to the decidual
wall and to the placental septa.
3. Free villi. In the older villous trees there are many villi which
float in the cavity of the blood sinuses.
4. Chorionic plate. This is the portion of the chorion between the
bases of the villi. It contains the larger chorionic vessels which
converge in the center, where they enter the umbilical cord.
a. Epithelium. The plate is lined externally by a layer of trophoblastic epithelium which rests on a layer of mesoderm.
b. Fusion with the amnion. By the end of the second month
the amnion is brought into contact with the chorion. In the
placental area it fuses with the chorionic plate.
c. Production of fibrin. During the last half of pregnancy the
epithelium is replaced by canalized fibrin.
B. Maternal placenta. This is represented by the decidua basalis.
1. Glands of the spongy layer. They become stretched into clefts
by the third month.
2. Basal plate. This is what remains of the compact layer of the
endometrium; it is incorporated into the placenta. It consists of a
[ 119]
connective tissue stroma containing decidual cells, fibrinoid material and portions of the trophoblast.
3. Intervillous spaces. They arise through fusion of the blood
sinuses (p. 117) during the early stages of development, following implantation. Arteries and veins open into them. The ends
of the free villi float in these large spaces which are supposed to
contain circulating maternal blood. This point, however, has been
recently questioned.
4. Placental septa (or trabeculae). They are the septa which separate the villi of the chorion frondosum into cotyledons.
C. The placenta at term. It is convex on the uterine surface (covered by the decidua basalis) and concave on the fetal side (covered
by the amnion), but after it is expelled these relations are reversed.
Its margin is continuous with a membrane produced through fusion
1. The decidua vera.
2. The decidua capsularis.
3. The chorion laeve.
4. The amnion.
==Part Two - Organogenesis==
The organs of the vertebrate body arise from the three germ
layers. The endoderm gives rise to the digestive and respiratory systems. The mesoderm contributes the supporting tissues, the vascular, urogenital and muscular systems, as well as the body cavities.
The ectoderm is the source of the integument, central and peripheral nervous systems, and the sense organs.
I. Introductory Remarks
A. Ectodermic contributions.
1. Stomodaeum. In the early embryo the fore-gut ends blindly;
its endoderm is fused with the ectoderm to form the oral (pharyngeal) membrane which is the floor of an external depression, the
[ 120 ]
stomodaeum. The latter develops into the front part of the mouth
and gives rise to the enamel of the teeth, salivary glands and
mucosa of the nose and palate, which are, therefore, ectodermic.
The oral membrane ruptures at the beginning of the 5th week.
2 . Proctodaeum. The hind-gut becomes the cloaca which later is
divided into the rectum and the urogenital sinus. The cloacal
membrane (endoderm pips ectoderm) ruptures at the end of the
7th week. After this a short ectodermic proctodaeum is added to
the rectum as its anal canal.
B. Relation to the primitive gut. The fore-gut gives rise to the posterior part of the mouth cavity, pharynx, oesophagus, stomach and
a good part of the small intestine. The hind-gut forms the rest of
the small intestine, colon and rectum. The intermediate region (midgut) is unimportant in man.
C. Pharyngeal derivatives. The endodermic lining of the pharynx
gives rise to organs which in the adult have no connection with the
digestive tract (thyroid, parathyroids and thymus).
II. The Organs of the Mouth Cavity
Since the tongue develops from the branchial arches it will be
considered in the next section.
A. The teeth. They are the homologues of the scales of the elasmobranch fishes (i.e. products of the skin) and as such arise from two
different sources: the epidermis, which forms the enamel, and a
dermal papilla which is transformed into dentine and tooth pulp
(dental papilla).
1. The dental lamina. This is a slightly curved epithelial ridge
which sinks into the substance of the primitive gum.
2. Enamel organs. They are thickenings which develop at intervals along the lamina. Early in the 3rd month the deeper side of
each organ presses against the dense accumulation of mesenchyme
of the dental papilla.
a. Number. Ten enamel organs develop in each jaw. They are
the primordia of the deciduous (milk) teeth.
b. Structure. The enamel organ or sac resembles an inverted
cup with its concavity applied against the dental papilla.
(1) Outer enamel cells. They line the convex portion of the
cup. At first cuboidal they later become flat. They do not
contribute to the formation of the tooth.
[ 121 ]
(2) Inner enamel cells (ameloblasts). They line the concavity of the sac. They are columnar and secrete the enamel
which covers the crown of the tooth.
(3) Enamel. Is laid down as a fibrillar layer which then calcifies in the form of elongated prisms, one for each ameloblast.
It appears first at the apex of the crown and extends gradually toward the region of the future root, which it does not
(4) Enamel pulp. This is derived from the epithelial elements between the outer and inner enamel cells. They are
transformed into a reticulum.
c. Nasmyth’s membrane (dental cuticle). Represents the remains of the enamel organ covering the apex of the tooth at
eruption. It soon wears off.
3. The dental papilla. This is the mesodermic or dermal portion
of the tooth.
a. Odontoblasts. The superficial cells (facing the ameloblasts)
become the columnar odontoblasts which secrete the dentine
(p- 55 )
b. Dental pulp. The remaining mesenchyme differentiates into
the dental pulp.
4. The dental sac. This is formed by the mesenchyme which surrounds the developing tooth. The inner portion produces a layer
of osteoblasts at the level of the root; they deposit the cementum.
When the tooth fills its alveolus the sac becomes the peridental
membrane (p. 55).
5. Disintegration of the dental lamina. It occurs after the first set
of enamel organs has been laid down, but its free edge gives rise
to the enamel organs of the permanent teeth. A backward growth
of the lamina produces the enamel organs of three molars not
represented in the primary dentition.
6. Eruption. This is caused by growth of the root; the crown
pushes to the surface and compresses the gum, which atrophies
at this point.
7. Permanent teeth. They develop in the same way as the deciduous and lie on the lingual side of the latter. By their rapid
growth at certain periods (6th to 18th year, according to the
tooth) they press against the deciduous teeth, the roots of which
undergo partial resorption. This causes their shedding.
[ 122]
B. The palate. Its development begins with the formation of the :
1. Lateral palatine processes. They are shelf-like folds of the maxillae which project toward the midline.
a. Fusion. They fuse with each other and with the greater portion of the nasal septum; the fusion begins anteriorly.
b. Formation of the hard palate. This is formed of bone which
arises in the anterior part of the fused processes.
c. Soft palate. The caudal part of the processes does not unite
with the nasal septum and is not ossified; this is the soft palate
and its free posterior apex, the uvula, notched at first.
2. Cleft palate. Results from total or partial failure of the fusion
of the lateral palatine processes. It may be associated with hare-lip
(p. 1 12).
III. The Pharynx
The lateral walls of the embryonic pharynx form five pairs of
outpocketings (pharyngeal or branchial pouches), the last of which
is rudimentary in man. They come into contact with the ectoderm
of corresponding branchial grooves and fuse with it, forming the
closing plates, which become perforated in human embryos only
A. The branchial arches. Their position has already been described
(p. hi). They give rise to several head structures (jaws, face and
external ear) and various muscles, cartilages and bones. On the floor
of the pharynx they contribute to the formation of the tongue and
1. The tongue. The body or apical half of the organ arises in
front of the second branchial arches, the root develops primarily
from the second arches, but receives additions from the third and
fourth. The boundary line between the body and root is the Vshaped sulcus terminalis.
a. The body. It arises from three primordial
(1) Tuberculum impar, or median primordium present in
the pharyngeal floor between the first pair of pouches. It contributes little or nothing to the formation of the human
tongue, according to some authors.
(2) Paired lateral swellings, located in the mandibular arches;
they meet at the median septum linguae.
b. The root. Arises from a median primordium (copula) which
[ 123]
is produced by the union of the second branchial arches in the
midline. The adjacent portions of the arches join the copula.
c. Foramen caecum. Between the tuberculum impar and the
copula is the point of origin of the thyroid diverticulum, represented by a pit (foramen caecum) in the adult.
d. Musculature. Arises from mesoderm of the floor of the
2. The epiglottis. The copula connects with a rounded prominence developed from the bases of the third and fourth branchial
arches : this is the epiglottis, which becomes concave on its ventral
(laryngeal) surface.
B. The pharyngeal (branchial) pouches and their derivatives. The
first and second pharyngeal pouches open into broad lateral expansions of the pharynx, while the third and fourth communicate with
the pharyngeal cavity through narrow canals. The different pouches
give rise to a number of structures:
1. The auditory (Eustachian) tube and tympanic cavity. These
arise from the first pair of pouches. Each tube is formed by a
dorsal outpocketing or wing of the pouch and opens into the
expanded tympanic cavity. The first branchial groove deepens as
the external auditory meatus, while its closing plate becomes the
ear-drum (tympanic membrane).
2. The palatine tonsils. The dorsal angle of the second pouch persists as the tonsillar fossa, which gives rise to the crypts of the
palatine tonsils. (The pharyngeal and lingual tonsils are not pharyngeal pouch derivatives.)
3. The thymus. It appears as two large ventral diverticuli of the
third pair of pharyngeal pouches. The corresponding diverticuli
of the fourth pouches have been regarded as rudimentary thymic
primordia which usually atrophy.
a. Loss of the lumina. The diverticuli become solid epithelial
masses and lose their connection with the wall of the pouch.
b. Fusion. The fusion of the epithelial masses in the midline is
superficial and produces the body of the gland, which gradually
takes its permanent position in the thorax.
c. Formation of the reticulum. The epithelium is changed into
a reticular framework through formation of large cytoplasmic
vacuoles. The Hassal bodies are supposed to arise from this endodermic reticulum.
[ 124]
d. Invasion by lymphocytes. Takes place toward the end of the
3rd month when the organ begins its differentiation into cortex
and medulla.
4. The parathyroids. They arise from the dorsal diverticuli of the
third and fourth pharyngeal pouches and, accordingly, are designated as parathyroid III and IV, respectively.
a. Migration. They leave the pouches in the 7th week and migrate caudad.
b. Permanent position. Parathyroids III are dragged downward
by the thymic primordia so that they come to lie at the caudal
thyroid border, while parathyroids IV are nearer the cranial
5. The ultimobranchial bodies. Usually regarded as derived from
the fifth pouches, they leave their site of origin and migrate
caudad with parathyroids IV, fusing with the thyroid. Their ultimate evolution varies according to the species; in man it is
claimed that they give rise to thyroid tissue.
C. The thyroid gland. It develops as a diverticulum arising from
the floor of the pharynx.
1. Thyro-glossal duct. The thyroid diverticulum is connected with
the pharyngeal epithelium through this duct which, if persistent,
opens in the foramen caecum of the tongue. It usually atrophies
during the 6th week.
2. Loss of the lumen. The body of the diverticulum becomes bilobed and through loss of its lumen is converted into a solid structure composed of epithelial plates.
3. Formation of follicles. Cavities representing the follicles begin
to appear in the epithelial plates; they soon acquire colloid. This
process ends by the end of the 4th month.
IV. The Digestive Tube
The development of its different regions is rather uniform except
for such differences as size, shape and position. The epithelial lining
is endoderm invested by splanchnic mesoderm; the latter gives rise
to the other layers.
A. Oesophagus. Its development is characterized by a gradual differentiation of the walls.
B. Stomach. In early embryos it is a spindle-shaped dilatation of the
1. Formation of the curvatures. The dorsal border grows faster
than the ventral and this unequal growth causes the formation
of the greater curvature. The fundus arises as a sacculation near
the cardia.
2. Mesenteries. The dorsal mesogastrium grows faster than the
ventral; it forms the omental bursa (p. 129).
3. Rotation. The stomach rotates about its long axis until the
greater curvature lies on the left and the lesser (primitive ventral
border) is on the right.
C. Intestine. In early embryos the intestine forms a single loop
which enters the coelomic extension of the umbilical cord. Later the
caudal limb of the loop develops a swelling which indicates the
caecum, and the loop is withdrawn into the body cavity.
1. Torsion. This takes place about the superior mesenteric artery
in such a way that the cranial limb of the loop is carried to the
right and caudad of the caudal limb; the latter shifts to the left
and cephalad.
2. Elongation and formation of loops. Rapid growth of the small
intestine causes the formation of its characteristic loops. The first
of these is the duodenum.
3. Differentiation of the colon. The formation of the three portions of the colon is a complicated process and will not be described here.
V. The Liver
It first appears as the hepatic diverticulum which is an outpocketing of the ventral floor of the fore-gut.
A. Penetration into the ventral mesentery. Ventral growth of the
hepatic diverticulum causes its penetration into the splanchnic mesoderm of the ventral mesentery; the latter is split into halves which
encapsulate the liver, forming the capsule of Glisson.
B. Formation of the hepatic cords. Soon after the formation of the
diverticulum its blind end produces solid anastomosing cellular
cords which constitute the parenchyma of the liver.
C. Formation of the sinusoids. The hepatic diverticulum lies between the vitelline (ophalomesenteric) veins, which form plexuses
in the ventral mesentery. The epithelial cords grow between these
venous plexuses which become the sinusoids.
D. Ducts. The hepatic duct and the common bile duct (ductus
choledochus) are the stem portions of the original hollow hepatic
[ 126 ]
diverticulum, while the gall bladder and cystic duct represent a
secondary, more caudally placed outpocketing.
VI. The Pancreas
The pancreas arises from two primordia (dorsal and ventral),
which are outpocketings of the endoderm lining the duodenum.
A. Dorsal pancreas. It extends into the mesentery as a solid cell
mass connected with the duodenum by a duct.
B. Ventral pancreas. This remains smaller and its short duct is
dragged away from the duodenum by the common bile duct, from
which it secondarily arises.
C. Fusion of the primordia. The primordium of the ventral pancreas is shifted to the dorsal mesentery near the dorsal pancreas with
which it fuses completely. It forms the head of the organ.
D. Ducts. The distal segment of the dorsal duct fused with the entire ventral duct form the main pancreatic duct (of Wirsung). The
proximal segment of the dorsal duct becomes the accessory duct (of
Santorini) .
The respiratory system (except the nasal passages) arises as an
outpocketing or evagination of the ventral wall of the fore-gut
I. Early Development
A. The laryngo-tracheal groove. It appears in very early embryos
along the floor of the fore-gut, caudal to the pharyngeal pouches.
It becomes the larynx and trachea.
B. Lung buds. The rounded, posterior end of the groove projects
ventrally and represents a single primordium of the lungs. It splits
caudally into two outpocketings, the lung buds, which remain connected with the future trachea.
C. Tracheo-oesophageal grooves. They form on the lateral aspects
of the fore-gut; their deepening toward the midline and subsequent
fusion cause the separation of the trachea from the oesophagus.
II. The Larynx
A. Arytenoid swellings. They bound laterally the upper end of the
laryngeal portion of the laryngo-tracheal groove.
1. Fusion with the epiglottis. Through fusion with the epiglottis
the arytenoid swellings produce a U-shaped ridge, the furcula.
The fusion is temporary, however.
2 . Bending. The parallel swellings are bent at the middle so that
their cranial positions diverge laterally, nearly at right angles to
their caudal portions.
3 . Formation of the glottis. When the arytenoid swellings lose
contact with the epiglottis the entrance to the larynx — previously
T-shaped and obliterated through fusion of the epithelial lining —
becomes oval and patent.
B. Development of the laryngeal cartilages and muscles. They arise
from condensations of mesenchyme derived from the fourth and
fifth pairs of branchial arches (p. 150) .
III. The Trachea
Its development is mainly represented by elongation and the differentiation of its walls.
IV. The Lungs
The right lung bud soon becomes larger and is directed caudad.
A. Bronchial buds. The right lung bud gives off two lateral bronchial buds, the left only one.
1. Relation to the lobes. The bronchial buds indicate the position
of the upper and middle lobes on the right side, the upper lobe
on the left. The lower lobes arise from the blind ends of the lung
2 . Eparterial bronchus. This is the apical bronchus of the right
upper lobe, so called because it alone passes dorsal to the pulmonary artery.
3 . Cardiac bronchus. This is the ventral bronchus of the right
lower lobe, which in a way compensates for the loss of a corresponding branch of the left side, eliminated so as to make room
for the heart.
4. Branching of the buds. The bronchial buds branch repeatedly,
and their epithelium becomes lower; in the terminal portions
(pulmonary alveoli) it is actually flattened. The existence of alveolar epithelium, however, is questioned by some (p. 69).
B. Development of the lobes. The respiratory tree develops in a
median mass of mesenchyme which resembles a broad mesentery
and is later called the mediastinum.
1. Invasion of the pleural cavities. The developing lungs, invested
by a layer of mesoderm, grow out laterally into the pleural cavities. The branching of the bronchial buds takes place within this
mesoderm, and the external lobation becomes apparent.
2. Differentiation of the mesoderm. The mesenchyme surrounding the bronchial tree produces the tissues of the wall of the
3. Visceral and parietal pleura. The surface of each developing
lung is covered with mesoderm lined externally by mesothelium;
this is the visceral pleura. The corresponding layer lining the
thoracic wall is the parietal pleura.
C. The lungs at birth. Until birth the lungs are small and compact
and do not fill the pleural cavities. With the onset of breathing they
gradually distend with air and the lung tissue becomes light and
I. The Mesenteries
Soon after it is formed the primitive gut is enclosed into a mesentery, which arises through fusion of the splanchnic mesoderm of the
two sides in the midline. The gut subdivides this primitive mesentery into dorsal and ventral halves.
A. The dorsal mesentery. The pharynx and upper oesophagus have
no mesentery; the lower oesophagus, like the trachea, lies in the
future mediastinum. The rest of the digestive tract is suspended
from the dorsal body wall by a continuous mesentery.
1. Regional names. The portion which attaches the stomach to
the dorsal body wall is the dorsal mesogastrium or greater omentum; then there is a mesoduodenum, mesentery of the small intestine, mesocolon and mesorectum.
2. The formation of the omental bursa. The lengthening and
bending of the dorsal mesogastrium toward the left during rotation of the stomach (p. 126) forms the omental bursa. In young
embryos (up to 10 mm.) the bursa is bounded mesially by the
dorsal mesogastrium (greater omentum) and the right wall of
the stomach, laterally by the right lobe of the liver and the mesen
[ 129]
tery in which the hepatic portion of the inferior vena cava develops (caval mesentery).
a. Epiploic foramen (of Winslow). The bursa communicates
to the right with the vestibule; the latter opens into the peritoneal cavity through this foramen situated between the liver
and the caval mesentery.
b. The inferior recess. This is due to enlargement of the bursa
to the left and caudad. Posteriorly it ends blindly.
c. Fusion with the dorsal body wall and colon. The dorsal wall
of the bursa fuses with the dorsal body wall as well as with
the colon and its mesentery (mesocolon).
d. Obliteration of the inferior recess. Its anterior and posterior
walls fuse. In the adult it is reduced to a space between the
stomach and dorsal fold of the greater omentum, the latter
being largely fused with the dorsal body wall.
3. Secondary fusions of the dorsal mesentery. They occur as the
result of the upright position in man and the higher apes. The
most important leads to the formation of the transverse mesocolon and fixes the duodenum and pancreas to the dorsal body
B. The ventral mesentery. It is associated intimately with the development of the heart and liver. The portion between the liver, stomach and duodenum is the lesser omentum. The greater part of the
ventral mesentery disappears early and the right and left peritoneal
cavities merge into a single cavity. What remains gives rise to the
falciform and coronary ligaments of the liver.
II. The Coelom (Body Cavity)
In early embryos the two halves of the coelom merge into a single
cavity in front as well as ventral to the heart, but caudal to the latter
the two coelomic cavities remain independent. The coelom can be
compared with an inverted U; the bend is occupied by the pericardial cavity, while the limbs represent the pleuro-peritoneal canals.
A. Division into separate cavities. The separation of the pericardium, pleural cavities and peritoneal cavity is effected by the development of three sets of partitions.
1. The septum transversum. This is located caudal to the heart
and fills the space between the gut, yolk stalk, and ventral body
wall, separating the pericardial and peritoneal cavities.
[ 130]
a. Pleuro-pericardial canals. Since the septum does not extend
dorsal to the gut it leaves on each side a canal through which
the pericardial and peritoneal cavities communicate.
b. Migration. The septum, at first in the cervical region, undergoes a gradual displacement caudad. The permanent location
is reached in the two-month embryo.
2. Pleuro-pericardial membranes. They separate the pleural cavities from the pericardial cavity; they develop around the common
cardinal vein of each side.
3. Pleuro-peritoneal membranes. These gradually separate the
pleural cavities and the single peritoneal cavity.
B. The diaphragm. The partitioning of the coelom results in the
formation of the diaphragm.
1. Origin. The diaphragm of the adult is derived from four
sources :
a. Its ventral portion from the septum transversum.
b. Its lateral parts from the pleuro-peritoneal membranes, and:
c. Derivatives from the body wall.
d. The median dorsal portion is contributed by the dorsal
2. Diaphragmatic hernia. Since the diaphragm arises from multiple sources, imperfect development or absence of one of them
leads to this defect, which is more common on the left side due
to failure of the formation of the pleuro-peritoneal membrane.
I. Origin of the Blood and Hemopoiesis in the Embryo
The blood and the blood vessels first appear in the splanchnic
mesoderm that invests the yolk sac.
A. Blood islands. These are solid masses of cells which are soon
changed into vesicles.
1. Formation of the endothelium. The peripheral cells of the
blood islands are arranged into a single layer of flattened cells,
which may also arise from the surrounding mesenchyme.
2. Blood cells. The other cells in the island become erythrocytes.
3. Plasma. It accumulates within the island and separates the
blood cells, which thus float in it.
[ I3 1 1
B. Area vasculosa. This is a network of primitive vessels in the wall
of the yolk sac. It arises through fusion of the blood islands.
C. Embryonic vessels. The first vessels to appear within the embryo
proper arise as clefts within the body mesenchyme.
D. Sites of hemopoiesis. The formation of blood takes place in the
following locations:
1. The yolk sac (4th week).
2 . Body mesenchyme and blood vessels (5th week) .
3 . Liver sinusoids (6th week).
4. Spleen, lymph nodes and thymus (2nd to 3rd months).
5. Bone marrow, from 3rd month on throughout postnatal life.
(For hemopoiesis in postnatal life see p. 10.)
II. The Early Vascular System
The embryonic vessels arise through coalescence of the vesicles
in which the blood develops. The first paired vessels to appear are
A. Aortae. They run anteriorly under the fore-gut (ventral aortae)
and bend dorsally in front of its blind end to become the dorsal
aortae. The latter soon fuse into a single descending aorta.
B. Cardiac tubes. The short ventral aortae are connected posteriorly
with the cardiac tubes, which later fuse into a single heart.
C. Umbilical arteries. The dorsal aorta give off caudally these two
vessels which enter the body stalk on their way to the chorion.
D. Umbilical veins. These course in the body wall and return the
blood from the chorion to the heart.
E. Vitelline vessels. They are: a pair of vitelline arteries arising
from the dorsal aorta and ending in the area vasculosa of the yolk
sac, and a pair of vitelline veins opening into the heart.
F. Embryonic veins. They arise within the body of the embryo.
1. Anterior cardinal (precardinal) veins. They drain the blood
from the head region; they course in the somatopleura.
2 . Posterior cardinal (postcardinal) veins. They return the blood
from the posterior end of the body.
3 . Common cardinals (ducts of Cuvier). Before entering the heart
the two cardinals of each side form this common trunk which
crosses the pleuro-peritoneal canal.
G. Aortic arches. They connect the ventral with the dorsal aortae.
[ 13 2 ]
The first pair is the anterior bend of the ventral aorta as it becomes
dorsal; four more pairs develop more caudally (p. 136 ).
III. Development of the Heart
The heart arises through fusion of paired primordia (cardiac
tubes) just posterior to the ventral aortae.
A. Early development.
1. Fusion into a single tube. This is caused by the process of folding which gives rise to the fore-gut. The single cardiac tube has
an endothelial lining.
a. Dorsal mesocardium. The cardiac tube is suspended from
the dorsal body wall by this double sheet, formed by fusion of
the two plates of splanchnic mesoderm in the midline. It soon
b. Ventral mesocardium. This develops in the chick embryo
but it is absent in the mammal due to the precocious splitting
of the mesoderm.
c. Epi-myocardium. The layer of thickened splanchnic mesoderm that surrounds the endothelial tube and gives rise to the
epicardium and myocardium.
2. Division into regions. The single cardiac tube soon shows the
following regions :
a. The sinus venosus, which receives the blood from the umbilical, vitelline and common cardinal veins; it develops a pair
of valves which guard the opening into:
b. The atrium, placed anteriorly to the sinus and communicating with:
c. The ventricle through a narrow atrio-ventricular canal.
d. The bulbus, continuous with the short ventral aortae.
B. External changes. They result from the bending of the single
cardiac tube, which grows in length faster than the cavity in which
it is contained.
1. Bulbo- ventricular loop. This chief early flexure is to the right,
and it has the shape of a U; one limb is the bulbus, the other the
2. Formation of. the atria. Due to growth of the bulbo-ventricular
loop the atrium and sinus venosus shift cephalad. The single
atrium forms lateral outpocketings which become the paired atria;
the furrow between them is the interatrial sulcus.
[ : 33 ]
3. Formation of the primitive ventricle. With continued growth
of the bulbo-ventricular loop its two limbs become confluent: the
single chamber is the primitive ventricle, separated from the atria
by the deep coronary sulcus.
4 . Interventricular sulcus. This is the external manifestation of
the formation of the interventricular septum, which separates the
two ventricles.
C. Internal changes. They lead to the formation of the fourchambered heart characteristic of birds and mammals.
1. Development of the atria. The partitioning of the atria is a
gradual process which is not completed until after birth.
a. Septum primum. At first it is a sickle-shaped partition that
grows from the mid-dorsal atrial wall: it advances toward the
ventricle and its free edge fuses with the endocardial cushions
(p. 136), which have split the primitive atrio-ventricular canal
into right and left halves.
(1) Foramen interatriale primum. This is the space enclosed
within the concavity of the septum: before the latter finally
reaches the endocardial cushions a secondary perforation occurs, the:
(2) Foramen interatriale secundum (ovale I), which is located near the attachment of the septum to the dorsal atrial
b. Septum secundum. It makes its appearance just to the right
of the septum primum. It arises from the caudal end of the left
valve of the sinus venosus (p. 135). It is also sickle-shaped; its
concavity is the:
(1) Foramen ovale (ovale II), which never disappears as
such and becomes the oval fossa of the adult heart.
(2) Relation of the foramen ovale II to the septum primum.
Since the foramen ovale II is placed more ventrally than the
interatriale secundum (ovale I), it is overlapped by the imperforated portion of the septum primum.
(3) Passage of the blood through the foramen ovale. The
portion of the septum primum covering the foramen ovale II
serves as a flap-like valve permitting passage of the blood
from the right to the left atrium, but not in the reverse
c. The atrial septum. It arises after birth through fusion of the
[ J 34 1
edges of the septum secundum with the septum primum. The
edge of the former becomes the limbus of the oval fossa, the
septum primum the membranous portion of the fossa. This
fusion closes the foramen.
2. The sinus venosus. The sinus venosus soon develops a large
right and a smaller left horn. The horns receive the blood returning to the heart through the primitive embryonic veins (p. 132).
a. The right horn. After their formation (p. 139), the superior
and inferior vena cava open into the right horn. Rapid atrial
growth incorporates the horn into the wall of the right atrium,
and the venae cavae open directly into the latter.
b. The left horn. It becomes the coronary sinus.
c. Transformations of the valves. The opening of the sinus
venosus into the atrium is guarded by two valves (valvulae
venosae). The left valve is incorporated into the septum. The
right forms the:
(1) Crista terminalis, which is a continuation of its cephalic
portion: and the:
(2) Eustachian and Thebesian valves, which arise from the
remainder of the valve. The former (valve of the inferior
vena cava) is larger than the Thebesian (valve of the coronary
3. The pulmonary veins. The single pulmonary vein of the early
embryo splits into right and left branches which in turn bifurcate.
During the rapid growth of the atria first the single stem, then
its two branches of bifurcation, are incorporated into the atrial
wall, and the four branches (two for each lung) come to open
directly into the atrium.
4. Origin of the aorta and pulmonary artery. They arise early in
embryonic life through division of the aortic bulb (bulbus, p.
3:33) . This is accomplished by two lateral ridges which meet and
fuse in the midline.
a. Relative position. After they are formed, the two arteries are
not parallel but arranged somewhat like an X; the more ventral
of the two is the pulmonary artery, the other the aorta (crossing
toward the right dorsal to the pulmonary).
b. Aortic and pulmonary valves. They arise from endocardial
thickenings of the aortic bulb.
5. The ventricles. The ventricles are lateral outpocketings of the
[ *35 ]
early single ventricle. Their separation is accomplished by the
formation of the:
a. Interventricular septum, which arises at the time of division
of the aortic bulb, as a median elevation extending to the ventral endocardial cushion.
b. Interventricular foramen. This is a temporary communication between the right and left ventricles.
c. Septum membranaceum. It closes the interventricular foramen and completes the partition of the single ventricle.
6. The atrio-ventricular valves. They arise from the endocardial
cushions which by fusion convert the single atrio-ventricular canal
into two canals. Endocardial folds at the margins of these canals
form the flaps of the valves, which become attached to the muscular trabeculae of the inner ventricular wall.
D. Anomalies. They are rather frequent. Among the most important are:
1. Dextrocardia, or transposition of the heart, usually associated
with general inversion of the viscera (situs inversus).
2. Incomplete ventricular septum due to deficiency of the septum
3. Persistence of the foramen ovale due to improper fusion of the
septum primum and secundum. When the blood of the two sides
mixes it causes cyanosis, seen in the “blue baby.”
IV. Development of the Arteries
The first arteries to appear in the embryo have already been mentioned, as well as the presence of aortic arches. The transformations
of the latter are of great importance.
A. Transformation of the aortic arches.
1. Number of arches. In human embryos there are five pairs of
aortic arches, which are numbered first, second, third, fourth and
sixth since the fifth, present in other animals, never develops fully.
They are not all present at any one time, due to early degeneration
of the first and second.
2. Internal carotids. They are cephalic portions of the dorsal
aortae after the disappearance of the first and second arches.
They continue growing cephalad to enter the head.
3. External carotids. Each arises from the third arch, the proximal
part of which becomes the common carotid. The distal part of
the arch joining with the dorsal aorta becomes the proximal segment of the internal carotid.
4. Fourth arches. They also persist.
a. Left side. It becomes the arch of the aorta. Proximally, the
short left ventral aorta is added to it.
b. Right side. The fourth arch arises from the enlarged right
ventral aorta (now called the innominate) and constitutes the
proximal part of the right subclavia. The middle part of the
latter is the portion of the left aorta between the fourth arch
and the vicinity of the point of fusion of the two aortae, while
the distal part is a new growth arising from the caudal end of
the middle portion at the level of the limb bud.
5. Left subclavia. This springs directly from the left dorsal aorta
at the level of the corresponding limb bud but its position is
shifted cephalad in later stages of development.
6. Sixth (pulmonary) arches. They arise from the pulmonary
artery after its separation from the aorta (p. 135) and connect
with the dorsal aortae.
a. Right side. A branch entering the corresponding lung bud
arises about the middle of the arch. The portion of the latter
between the origin of the branch and the right aorta degenerates.
b. Left side. A similar branch for the left lung bud is given off,
but the portion of the arch between the branch and the left
aorta remains as the:
c. Ductus arteriosus (Botalli) which becomes the arterial ligament of the adult.
B. Branches of the dorsal aorta. The aortae give off dorsal, lateral
and ventral branches. The most important of each are:
1. Dorsal (intersegmental) arteries.
a. Vertebral artery. It arises from the subclavia. The two vertebrals join under the brain with the basilar artery.
b. Intercostal and lumbar arteries. They are the ventral rami
of the dorsal intersegmental arteries.
2. Lateral arteries. They give rise to the renal, suprarenal, inferior
phrenic and internal spermatic and ovarian arteries.
3. Ventral branches. The most important are the vitelline, coeliac,
superior and inferior mesenteric, and the paired umbilicals.
V. Development of the Veins
The first paired veins to develop are the vitelline, umbilical and
cardinals (p. 132). They undergo a series of transformations leading
to the venous plan of the adult.
A. The vitelline (omphalomesenteric) veins. Their course is interrupted by growth of the liver which divides them into a large number of sinusoids. Each vein has a distal segment (from the yolk sac
to the liver) and a proximal (from the liver to the corresponding
horn of the sinus venosus).
1. The hepatic veins. They arise from the proximal parts of the
vitelline veins.
2. Fate of the distal segments. They communicate with each other
by three transverse anastomoses: a cranial (within the liver) and
two dorsal and ventral to the duodenum, respectively. The more
cranial portion of the left vitelline (within the liver) and the
middle portion of the right drop out. What remains is shaped
like an S.
a. Formation of the superior mesenteric. This is a new vessel
which develops in the mesentery of the intestinal loop and joins
the left vitelline vein near its middle anastomosis.
b. The portal vein. The persisting portion of the left vitelline
vein and the portion of the right between the middle and
cranial anastomosis become this vessel.
B. The umbilical veins. As the liver expands, its lateral surfaces engulf the umbilicals, which then send their blood to the heart by
the more direct route of the liver sinusoids.
1. Fate of the right umbilical. When all the umbilical blood enters the liver the entire right umbilical vein atrophies.
2 . Left umbilical. Its proximal segment also atrophies.
3. Formation of the ductus venosus. This arises through enlargement of some of the hepatic sinusoids. It communicates with the
left umbilical and opens into the common hepatic vein.
4. Fate of the ductus venosus. After birth it is obliterated and
forms the solid venous ligament.
5 . Fate of the left umbilical vein. Through a similar obliteration
its remnant, from the navel to the liver, constitutes the ligamentum teres.
C. The cardinal veins.
1. Anterior cardinals. An oblique anastomosis between these veins
gives rise to the :
a. Left innominate vein, which increases in diameter as the
proximal portion of the left cardinal atrophies.
b. Superior vena cava. The right common cardinal and anterior
cardinal as far as the oblique anastomosis become the superior
vena cava.
c. Right innominate vein. The portion of the right anterior
cardinal between the anastomosis and the right subclavian.
d. Internal jugulars. They are the distal segments of the anterior cardinals.
e. External jugulars and subclavian veins. They develop independently and later open into the anterior cardinals.
2. Posterior cardinals, subcardinals and supracardinal veins. These
three sets of veins appear successively in the order mentioned.
a. Postcardinals. They develop primarily as the veins of the
mesonephroi and disappear as these organs wane.
b. Subcardinals. They anastomose in the midline; the anastomosis forms the left renal vein.
c. Supracardinals. They unite by a transverse anastomosis and
become the azygos and hemiazygos veins.
3. The inferior vena cava. It consists of four segments arising
from different sources:
a. * An hepatic segment, derived from the hepatic vein and sinusoids; it connects with the right subcardinal through a vein in
the caval mesentery (p. 130).
b. A prerenal segment, formed from the right subcardinal.
c. A renal segment, comprising an anastomosis between the
right subcardinal and right supracardinal veins.
d. A supracardinal segment, from the lumbar portion of the
right supracardinal vein.
VI. Fetal Circulation and Changes at Birth
A. Course followed by the blood in the fetus. Contrary to formerly
held views the oxygenated blood reaching the fetus through the
umbilical vein becomes mixed with venous blood from diverse
1. Source of the oxygenated blood. This is the placenta, where the
r 139]
venous blood conveyed to the chorionic villi by the umbilical
arteries becomes arterial.
2 . Return of the oxygenated blood. By way of the left umbilical
vein it enters the ductus venosus and reaches the right atrium
through the inferior vena cava.
3 . Mixing of the blood. The venous blood of the portal vein and
inferior vena cava contaminates the oxygenated blood; a further
mixture of bloods takes place in the right atrium, which receives
venous blood through the superior vena cava.
4. Passage through the heart. The mixed blood which has entered
the right atrium follows two different courses:
a. Through the foramen ovale to left atrium, and through the
aorta to the head and body.
b. Through the right atrio-ventricular foramen to the right ventricle and hence to the aorta through left pulmonary artery
and ductus arteriosus.
B. Changes at birth. The placental circulation ceases when the lungs
become functional. The chief events following this change are:
1. Gradual closure of the foramen ovale (p. 134) resulting from
equalization of the pressures in the two atria.
2 . Obliteration of the ductus arteriosus (p. 137) following increased diversion of blood from the pulmonary trunk to the lungs.
3. Rapid obliteration of the umbilical vein, whose fate has been
indicated (p. 138). The arteries become the lateral umbilical
4. Atrophy of the ductus venosus, and its transformation into the
ligamentum venosum.
The urinary and reproductive systems are closely associated in
development. Both arise from mesoderm of the same region as a
common urogenital fold which is soon divided into nephric and
genital ridges.
I. The Urinary Organs
In the course of evolution the vertebrates have developed three
types of kidneys: the pronephros, present in Amphioxus and certain lampreys; the mesonephros, functional throughout life in fishes
[ 140]
and amphibians; and the metanephros or definitive kidney of reptiles, birds and mammals. The three types occur in a sequence during the development of the higher vertebrates.
A. Pronephros. In the human it consists of about seven pairs of
rudimentary tubules.
1. Origin. They are formed as dorsal outpocketings of the intermediate cell mass (nephrotome) of the 7th to 14th somites. The
first formed tubules degenerate before the last appear.
2. Pronephric ducts. The tubules of each side open into a longitudinal collecting tube which reaches the lateral wall of the cloaca,
in which it opens.
B. Mesonephros (Wolffian body). This is larger than the pronephros and serves as a temporary excretory organ. It is constituted
by many tubules (up to 80) which arise cranially as far as the 6th
cervical segment.
1. Differentiation of the tubules. The free end of the early Sshaped tubule is dilated, and its walls become thin. The proximal
end is united with the pronephric (now mesonephric) duct.
a. Formation of the glomeruli. A knot of looped blood vessels
pressing on one of the hemispheres of the dilated portion causes
its invagination into the other hemisphere.
b. Bowman’s capsule. This is the double-walled capsule produced by the invagination mentioned above. The capsule and
glomerulus together constitute a mesonephric (Malpighian)
c. Tubular portion. Each tubule shows a light staining secretory portion and a thinner, more deeply stained collecting part
opening into the mesonephric duct.
d. Position. The glomeruli are mesially placed, the ducts occupy
a lateral position while the tubules are largely dorsal.
C. Metanephros (permanent kidney). This arises in the pelvic
region and it has a double origin. The ureter, pelvis and collecting
tubules are outgrowths of the mesonephric duct; the secretory tubules and glomeruli develop from the caudal end of the nephrogenic
1. The ureteric bud. This arises from the mesonephric duct; it
grows at first dorsad, then turns cephalad. Its proximal, elongated
portion is the ureter, the distal expanded portion the renal pelvis.
2. Formation of collecting tubules. These grow out from the
1 141 ]
primitive renal pelvis. Through branching they give rise to secondary, tertiary, quaternary, etc., tubules until about 12 generations have been produced; the tubules of the 5th order become
papillary ducts (p. 72). The collecting tubules form a large part
of the medulla.
3 . Differentiation of the nephrogenic blastema. This forms a cap
about the primitive pelvis and is carried along with it during
the elongation of the ureteric bud.
a. Formation of the lobes. The nephrogenic blastema covers
the ends of the newly formed collecting tubules tributary to a
primary tubule; in this way the cortex is subdivided into lobes
by grooves. The external lobation gradually disappears after
b. Formation of the secretory tubules. They arise from the blastema. The first few generations degenerate; new ones are produced near the surface of the organ.
4. Union of the secretory and collecting portions of the tubules.
They unite secondarily into continuous tubules. Failure of this
union leads to congenital cystic kidney.
D. Differentiation of the cloaca. In early human embryos the cloaca
receives laterally the mesonephric ducts, dorsally the hind-gut, while
its cephalic end gives off the allantois.
1. Division. It is accomplished through the development of the
cloacal septum which, pushing caudad, separates the dorsal rectum from the ventral urogenital sinus.
2. Primitive perineum. This is the exposed tip of the septum,
after rupture of the cloacal membrane (p. 121) which it reaches.
3 . Differentiation of the urogenital sinus. By elongation and constriction the sinus is divided into:
a. The vesico-urethral portion, which receives the mesonephric
ducts and ureters and is continuous with the allantois.
b. The phallic portion, connected with the former by a narrow
constriction and extending into the genital tubercle of both
sexes (p. 147). It becomes the cavernous urethra of the male
but it is merely merged with the vaginal vestibule of the female
(P- I 47)
4. Differentiation of the vesico-urethral portion. The enlarging
bladder takes up into its walls the proximal ends of the meso
• [ 142 ]
nephric ducts to a level beyond the origin of the ureters; the four
ducts thus acquire separate openings.
a. The ureters, open more laterally into the saccular bladder.
b. The mesonephric ducts are displaced caudad and come to
open into the dorsal wall of the urethra on a hillock (Muller’s
c. Urachus. This is the apex of the bladder continuous with the
allantoic stalk at the umbilicus; after birth it constitutes the
middle umbilical ligament.
II. The Genital Organs
The early development of the genital organs is identical in the
two sexes. Each embryo develops a male and female system of ducts;
after the sex is definitely established the ducts of the opposite sex
A. The gonads. This term is applied to the primordial sex glands
during their early indifferent stage.
1. Origin. They arise from the genital fold, which separates from
the mesonephric fold in early phases of development.
2. Structure. The indifferent gonad consists of:
a. The germinal epithelium, of cuboidal cells forming one or
more layers.
b. An inner epithelial mass of anastomosed strands derived
from the germinal epithelium. The cords are separated by
mesenchyme and contain scattered germ cells.
B. Differentiation of the testis. This happens after the 6th week.
1. The testis cords. These are branched and anastomosed strands
proliferated from the germinal epithelium. They consist of indifferent cells with a few larger germ cells.
2. The albuginea. It arises from mesenchyme that penetrates between the germinal epithelium and the testis cords.
3. Fate of the germinal epithelium. It is changed into ordinary
4. The rete testis. The testis cords converge toward the mesorchium and connect with the dense primordium of the rete testis.
5. The seminiferous tubules. The cord cells gradually arrange
themselves as a stratified epithelium lining lumina continuous
with the lumina of the rete. Spermatogonia arise from indifferent
[ 143]
cells. The proximal portions of the tubules remain straight (tubuli
C. Differentiation of the ovary. The differentiation of the ovary
takes place later than in the case of the testis.
1. Division into cortex and medulla. The inner epithelial mass of
the indifferent gonad becomes less dense centrally to produce the
medulla, while near the periphery it constitutes a denser cortex.
Primordial germ cells occur in both zones, but they predominate
in the medulla.
2 . The rete ovarii. This is the homologue of the rete testis and
arises from a dense primordium.
3. The second proliferation. After the 3rd month the ovary grows
rapidly, owing to the formation of a new cortex probably derived
through proliferation of the germinal epithelium.
4. The albuginea. After the second proliferation the albuginea
differentiates beneath the germinal epithelium, which does not
become mesothelium but remains as a layer of cuboidal or low
columnar cells (p. 81).
D. Transformation of the mesonephric tubules and ducts. The involution or degeneration of the mesonephros spares a number of
mesonephric tubules, which remain connected with the sex glands
in the two sexes. They form a cranial and a caudal group.
1. In the male.
a. Cranial group. Most of the cranially placed tubules (9 to 15)
become connected with the tubules of the rete testis to form
the ductuli efferentes, but a few of the most cranially placed
form the appendix of the epididymis.
b. Caudal group. This, although composed of vestigial tubules,
persists as the coiled, blindly ending tubules of the paradidymis,
and the aberrant ductules.
c. Mesonephric duct. Its upper end coils into the duct of the
epididymis, while the caudal portion remains straight and extends from the epididymis to the urethra as the ductus deferens
and ejaculatory duct.
d. Ampulla. It develops near the opening of the ejaculatory
duct into the urethra; the seminal vesicle is an outpocketing
of the ampulla.
2 . In the female. Although the rete ovarii is vestigial it is retained
in the adult.
a. Cranial group. Most tubules of this group form the epoophoron, but a few, cranially placed, become the cystic vesicular appendages associated with the fimbria; they are the homologues
of the efferent ductules and appendix of the epididymis.
b. Caudal group. They constitute the more inconstant paroophoron, the homologue of the paradidymis and ductuli aberrantes
of the male.
c. Mesonephric duct. Its greater part atrophies; the persisting
portions are the ducts of the paroophoron (Gartner’s ducts)
present in the region of the uterus and vagina; they correspond
to the duct of the epididymis, ductus deferens, seminal vesicle
and ejaculatory duct of the male.
E. The Mullerian ducts.
1. Origin. They first appear as a ventro-lateral groove in the
thickened epithelium of each urogenital fold, near the cephalic
pole of the mesonephros.
2. Closure of the groove. The cranial end of the groove remains
open, while the rest closes into a tube which separates from the
epithelium, beneath which it comes to lie.
3. Opening into the cloaca. The solid end of the tube grows
caudad, beneath the epithelium and lateral to the mesonephric
duct. The tubes meet in the midline and penetrate the dorsal wall
of the uro-genital sinus, along with the mesonephric ducts.
4. Fate in the two sexes.
a. In the female.
(1) Uterine (Fallopian) tubes. They arise from the cranial
portions of the ducts.
(2) Uterus. This originates from the next portion of the
ducts, which fuse into a single tube. The thick muscular walls
of the uterus are foreshadowed by the presence of a thick
layer of mesenchyme around the epithelial portions of the
(3) Vagina. The upper two thirds are probably formed
through fusion of the Mullerian ducts in the midline. The
lower third arises from the uro-genital sinus.
b. In the male. Degeneration of the Mullerian ducts begins
with the third month and only the extreme cranial and caudal
ends are spared.
(1) Cranial end. It becomes the appendage of the testis.
( 2 ) Caudal end. It persists as a small pouch on the dorsal
wall of the urethra, the utriculus prostaticus or masculine
F. Descent of the testis and ovary. Their original positions gradually change during development. At first they extend caudad from
the diaphragm, but later they are shifted to a more caudal position.
1. Testes. Their caudal ends come to lie at the boundary between
abdomen and pelvis. This early migration is followed by their
descent into the scrotal sacs.
a. Formation of the vaginal processes. These arise early in the
third month. Each is an outpocketing of the abdominal cavity
which passes over the pubis, then through the inguinal canal
into the corresponding scrotal sac.
b. The gubernaculum testis. A continuous ligament extending
from the caudal end of the testis through the inguinal canal to
the scrotal integument.
c. Penetration into the scrotal sacs. During the 8th month shortening of the gubernacula draws the testes into the scrotum.
Each testis is still retro-peritoneal (i.e. it is covered by the wall
of the processus vaginalis) so it lies outside the cavity of the
latter. Failure of the testis to enter the scrotal sac causes cryptorchism.
d. Obliteration of the canal of the vaginal process. After birth
this narrow canal, connecting the vaginal process with the abdominal cavity, disappears.
e. Tunica vaginalis. The now isolated vaginal process or sac
represents the tunica vaginalis of the testis; its visceral layer
closely invests the testis whereas the parietal lines the scrotal
f. Spermatic cord. The ductus deferens and spermatic vessels
and nerves are carried down into the scrotum along with the
testis and epididymis. They are surrounded by connective tissue
and constitute the spermatic cord.
2. Ovaries. After their early migration they come to lie within the
pelvis, where each rotates until it is placed in a transverse position.
G. The external genitalia.
1. Indifferent stage. Up to the beginning of the 8th week the external genitalia are identical in the two sexes.
[ M 6]
a. Genital tubercle. This round eminence develops in the ventral body wall between the umbilical cord and the tail.
b. Urethral groove and folds. It is located on the caudal surface
of the tubercle, and is separated from the anus by the primitive
perineum. The margins of the groove are the urethral folds.
c. Phallus. This is visible by the end df the 7th week as a cylindrical prolongation ending distally as a rounded glans.
d. Labio-scrotal swellings. They occur on each side of the base
of the phallus, from which they are separated by a groove.
2 . Transformation in the two sexes. The fate of the parts just
mentioned differs according to the sex, which cannot be recognized for sure until the end of the 10th week.
a. Male.
(1) Formation of the urethra. This is accomplished through
transformation of the urethral groove into a hollow tube; the
fused edges of the groove constitute the raphe.
(2) Migration of the scrotal swellings. These shift caudad
and each becomes a half of the scrotum, separated from the
other half by the raphe and the underlying scrotal septum.
(3) Elongation of the penis. It is accompanied by a continuation of the formation of the urethra, which finally reaches the
(4) Corpora cavernosa. They arise as columns of mesenchyme
within the shaft of the penis.
b. Female. The changes are less marked and take place much
more gradually.
(1) Phallus. It lags in development and becomes the clitoris;
its distal portion is the glans clitoridis.
(2) Urethral groove. This never reaches the glans, as in the
male, but remains open as the vestibule.
(3) Urethral folds. They become the labia minora.
(4) Labio-scrotal swellings. They grow caudad and fuse in
front of the anus as the posterior commisure, while their
lateral portions are converted into the labia majora.
H. Anomalies. True hermaphroditism in man is very rare; false
hermaphroditism, characterized by the presence of the genital
glands of one sex with external genitalia and secondary sexual characteristics of the other, is much more frequent. When the lips of
the uro-genital sinus in males fail to fuse hypospadias result, a common feature in hermaphroditism of the female type.
The supporting tissues (connective tissue, cartilage and bone)
arise from mesenchyme, which consists of irregularly branched cells
separated by uneven spaces filled with a fluid resembling lymph.
I. Connective Tissue
The mesenchyme cells become fibroblasts.
A. Origin of the fibers. The characteristic connective tissue fibers
arise in the intercellular spaces rather than within the fibroblasts,
as was formerly supposed.
1. Argyrophil fibers. These are the first to appear, remaining as
such in the reticulum of certain organs (spleen, liver, lymph
2. Collagenous fibers. They arise through chemical transformation of argyrophil fibers which are aggregated into bundles.
3. Elastic fibers. Their development is not exactly known; they
are laid down amongst the collagenous fibers.
B. Adipose tissue. Certain mesenchyme cells, called lipoblasts, give
rise to fat cells. Fat droplets appear in their cytoplasm and coalesce
into a large drop which pushes the nucleus to the periphery of the
II. Cartilage
The mesenchymal cells which will give rise to cartilage lose their
processes and are aggregated into a mass of polygonal cells, known
as precartilage. The intercellular substance appearing between the
cells becomes the ground substance or matrix of the cartilage and
the cells are enclosed within lacunae.
III. Bone. Development of the Skeleton
The histogenesis of bone has already been described in the Histology (p. 18 ). From the standpoint of Embryology the skeleton
is composed of two portions, the axial and appendicular skeleton,
A. Axial skeleton. This comprises the vertebral column and ribs,
the sternum and the skull.
[ 148]
1. Vertebrae. The vertebral column and ribs originate from the
sclerotomes of the somites (p. 109), which consist of spindleshaped mesenchymal cells.
a. Blastemic stage. Each sclerotome differentiates into a caudal
dense half, and a cranial less dense portion. The dense portion
of each sclerotome mass later joins the looser cranial mass of
the sclerotome right caudal to it, to form the substance of the
(1) Formation of the body of the vertebra. The two sclerotomic portions enclose the notochord to form this part.
(2) Vertebral arch. From the dense half, dorsal extensions
grow around the neural tube.
(3) Costal processes. These are ventro-lateral outgrowths.
(4) Intervertebral disks. They arise from mesenchyme derived from the dense portion of the sclerotome.
b. Chondrification. There are six centers: two in the vertebral
body, one in each half of the vertebral arch, and one in each
costal process. They enlarge and fuse into a solid cartilaginous
c. Ossification. This occurs during the 10th week. There is a
single center for the body of the vertebra and one in each half
of its arch; the union of these parts is not completed until several years after birth.
2. Ribs. They arise from the costal processes; their original union
with the vertebra is replaced by a joint which receives the head
of the rib. The transverse process of the vertebra extends outward
and articulates with the growing tubercle of the rib. There is a
single center of ossification for each rib.
3. Sternum. Arises from paired sternal bars which unite the upper
eight or nine cartilaginous thoracic ribs of each side. At an early
period the two bars fuse together; this is followed by ossification.
4. The skull. Most of the bones of the skull arise from the chondrocranium; the flat bones of the vault and face (frontal, parietals,
nasals, lacrimals, zygomatics and vomer) are purely membranous,
whereas the occipital, sphenoid and temporals are mixed.
a. The chondrocranium. The chondrocranium is a continuous
mass of cartilage extending from the occipital to the ethmoidal
region and to a certain extent dorsally at the sides and behind.
(1) The periotic (auditory) capsules. They are formed by
[ 149]
cartilage which encloses the internal ear. They fuse with the
(2) Ossification. Most cartilage bones of the skull develop
from two or more formative centers. The ossification begins
early in the third month. The occipital has four centers of
ossification, the sphenoid five, the ethmoid four, and the
temporal probably two.
b. Membrane bones of the skull. The frontal develops from two
centers on each side of the midline, the parietals from one center each, the vomer from two, the nasal, lacrimal and zygomatic
from one center each.
c. Branchial arch skeleton. This is formed by cartilage and
membrane bones derived from the branchial arches.
(1) First (mandibular) arch. A cartilaginous bar (Meckel’s
cartilage) develops only in the mandibular process; the maxillary process has no cartilaginous skeleton but two membrane bones, the palatine and maxilla are developed in it, the
former from one center, the maxilla from two or possibly
(a) Proximal portion of Meckel’s cartilage. It extends into
the tympanic cavity, where it forms two of the bones of
the middle ‘ear, the malleus (hammer) and incus (anvil).
(b) Distal portion. This is invested by membrane bone
which forms the body and rami of the jaw; the membrane
bones are paired, and fuse in the midline in a symphysis.
The invested portion of Meckel’s cartilage degenerates.
(2) Second (hyoid) arch. Its cartilage also enters the periotic
capsule; this proximal segment gives rise to the stapes (stirrup) of the middle ear, the styloid process of the temporal,
and the lesser horn of the hyoid bone. Between the latter and
the styloid process failure of ossification produces the stylohyoid ligament.
(3) Third branchial arch. It produces the greater horns of
the hyoid, while the plate (copula) connecting the two arches
becomes the body of this bone.
(4) Fourth and fifth branchial arches. They differentiate into
the cartilages of the larynx.
B. Appendicular skeleton. It is derived from the somatic mesenchyme which forms the core of the limb bud and becomes converted
into cartilage; ossification of the latter produces all the bones of the
limbs, with the possible exception of the clavicle.
1. Pectoral girdle and arm.
a. The clavicle is the first bone of the skeleton to ossify; it has
two centers of ossification.
b. The scapula has two chief centers, one for the body and
spine, the other for the coracoid process, and several later
epiphyseal centers.
c. The humerus, radius and ulna all ossify from a single primary center in the diaphysis and an epiphyseal center at each
d. Carpals, metacarpals and phalanges. Each carpal ossifies
from a single center; the metacarpals and phalanges also have
a single primary center and an additional epiphyseal center.
2. Pelvic girdle and leg.
a. The innominate arises from three main centers of ossification, one for the ileum, one for the ischium, and another for
the pubis. The three join in the acetabulum, which receives the
head of the femur.
b. Femur, tibia and fibula. Their development is similar to that
of the corresponding bones of the arm. The patella is regarded
as a sesamoid bone.
c. Tarsal, metatarsals and phalanges. They develop as the corresponding bones of the hand.
The histogenesis of the three varieties of muscle has been already
dealt with (pp. 21, 22, 24).
I. The Visceral Musculature
The muscle (smooth) associated with the hollow viscera arises
from the splanchnic mesoderm (p. 109). The same layer originates
the cardiac (striated) variety, which develops from the thickened
epimyocardial lining on the outer surface of the cardiac tubes (p.
II. The Skeletal Musculature
Most of the skeletal muscles originate from the myotomes. The
differentiation of the skeletal muscles takes place rapidly, and at
[ 151 1
about the 8th week they are already capable of correlated movements.
A. Changes in the myotomes.
1. Migration, wholly or in part, to more or less distant regions.
2. Fusion of portions of successive myotomes into a single
muscle. With the loss of the segmental arrangement the original
innervation of each portion of a myotome is retained throughout
3. Splitting. This may be longitudinal or tangential. In the first
case the myotome gives rise to several subdivisions, in the second
there is an increase in the number of layers. With the splitting
there may be a change in the direction of the fibers.
4. Degeneration of myotomes or parts of myotomes; the degenerated portions may be changed into ligaments, fascias and aponeurosis.
B. Muscles of the trunk.
1. The lateral and abdominal muscles originate as ventral extensions of the myotomes; the somatic mesoderm gives rise only to
the intermuscular connective tissue.
2. The superficial portions of a dorsal longitudinal column of
fused myotomes on each side produce the various long muscles
of the back, innervated by the dorsal rami of the spinal nerves.
3. The intervertebral muscles develop from the deeper, non-fused
portions of the myotomes.
C. Muscles of the neck. The long muscles arise from the same longitudinal column producing the corresponding muscles of the trunk,
and are also innervated by dorsal rami. Other muscles differentiate
from ventral extensions of the cervical myotomes and from the
branchial arches.
D. Muscles of the head. The head lacks definite somites but it is
possible that the mesenchyme which gives rise to the eye muscles —
supplied by somatic motor nerves (III, IV and VI) — is of myotomic
origin. The other muscles of the head develop from the lateral mesoderm and retain their primitive branchial arch innervation.
1. First (mandibular) arch. Gives rise to the muscles of mastication (temporal, masseter, anterior belly of digastric, mylohyoid
and pterygoids) and other muscles associated with the trigeminal
(fifth) nerve (tensors of palatine velum and tympanum).
2. Second (hyoid) arch. Produces the muscles of expression (facial
[ 152]
muscles) and all other muscles supplied by the facial (seventh)
nerve (stylo-hyoid, posterior belly of digastric, stapedius, platysma,
3. Third arch. This is the source of muscles supplied by the glossopharyngeal (ninth) nerve (stylo-pharyngeus and part of the pharyngeal constrictors).
4. Fourth and fifth arches. They give rise to part of the pharyngeal constrictors, certain muscles of the palate and the muscles of
the larynx, all of which receive their supply from the vagus
(tenth) nerve. The accessory (eleventh) nerve innervates the
sterno-mastoid and trapezius, regarded as branchiomeric muscles.
E. Muscles of the limbs. The direct myotomic origin of these muscles in mammals is questionable even though there are indications
of migration of mesenchyme cells from the edges of the cervical
myotomes. The development of the upper limb muscles is more
advanced than those of the lower limb, and the proximal muscles
appear earlier than the distal in any case.
Although it is included among the ectodermic derivatives the integument has a double origin.
I. The Skin
The superficial epithelium arises from ectoderm; the derma or
corium from mesoderm.
A. Epidermis. In early embryos it is a single layer of cuboidal cells,
but it soon becomes double-layered.
1. Epitrichium (periderm). This is the most superficial layer, composed of flattened cells. The term ‘epitrichium’ alludes to the fact
that the layer is lifted off by the growing hairs. Failure of desquamation leads to ichthyosis, a condition which is frequently
2. Basal layer. Made up of cuboidal cells which will give rise to
the other layers of the epidermis. Stratification occurs after the
4th month.
3. Vernix caseosa. This is a mixture of desquamated epidermal
cells, lanugo hairs and sebaceous secretion which covers the fetal
skin and prevents its maceration by the amniotic fluid.
B. Derma. In most vertebrates it arises from the dermatomes of the
somites (p. 109), but it has been claimed that dermatomes are absent
in mammalian embryos. The derma, therefore, probably arises from
7 II. The Hair
The earliest hairs begin to appear at the end of the second month
on the eyebrows, upper lip and chin; later (4th month) the body
hair develops. The latter is at first fine and silky, and is known as
A. Development. A hair follicle begins as a cluster of basal layer
cells in the epidermis.
1. Elongation. The cluster becomes elongated and gradually sinks
into the underlying derma.
2. Differentiation. The base of the hair primordium enlarges into
the bulb, which fits as a cap on the surface of the mesenchyme
mass which will become the papilla.
3. Structure. The follicle at this stage consists of two layers :
a. An outer layer of columnar cells continuous with the basal
layer of the epidermis; they give rise to the outer sheath.
b. A core of polyhedral cells, which produces the substance of
the hair.
c. Connective tissue sheath. This arises from mesenchyme.
4. Growth of the hair. The hair is a proliferation from the basal
epidermal cells next to the papilla. They produce an axial core
which becomes the inner sheath and shaft, respectively.
III. The Sebaceous Glands
Most sebaceous glands develop in connection with the hair follicles. They arise as solid epidermal buds which become lobulated.
IV. The Sweat Glands
Some develop in connection with hair follicles, from which they
separate later, but most appear independently as solid downgrowths
from the epidermis. The simple, cylindrical downgrowths coil and
acquire lumina. The walls of the tubules consist of:
[ 154 ]
A. An inner layer of cuboidal cells, which become the glandular
B. An outer layer, whose cells become transformed into myo-epithelial, contractile elements (p. 48) .
V. The Mammary Glands
They are usually regarded as modified sweat glands and appear
early in the two sexes as longitudinal ectodermal thickenings which
extend on each side between the bases of the limb buds. Each ridge
is called the milk line.
A. Disappearance of the milk line. In man the milk line is best seen
in the pectoral region; the more caudal portion soon disappears. If
persistent it gives rise to accessory mammary glands.
B. Development of the glands. Each gland begins as a downgrowth
from the milk line in the region of the future breast. The primordium gradually grows into 15-20 cords (primary milk ducts) which
branch in the derma and give rise to acini.
C. Development of the nipple. Where the milk ducts open the epidermis is raised to form the nipple; this may not happen until after
The central nervous system develops from the neural plate, an
ectodermic band along the mid-dorsal line of the embryo (p. 108).
I. The Central Nervous System
A. Histogenesis. The neural tube gives rise to all the nervous tissues
except the cerebrospinal and sympathetic ganglia, and the olfactory
neurones (p. 95). The primitive cells of the tube differentiate into
two kinds of elements, namely, the nerve cells and the supporting
1. Early differentiation of the neural tube. Its wall shows at first
several indistinct layers; later it becomes separated into three distinct zones:
a. Inner (ependymal) zone, near the lumen of the tube from
which it is separated by a thin internal limiting membrane; its
cells send processes toward the periphery. It constitutes most
of the roof and floor of the neural canal (roof and floor plates).
b. Middle (mantle) zone, consisting of many, closely packed
cells; it becomes the gray matter, and contributes to the marked
thickening of the lateral walls of the neural tube.
c. Marginal zone, largely non-nucleated and gradually invaded
by the axons of the cells of the preceding zone; it becomes the
white matter. Externally it is bounded by the outer limiting
d. Sulcus limitans. A groove on the inner surface of each lateral
wall subdivides the latter into a dorsal alar plate (sensory) and
a ventral basal plate (motor).
2 . Differentiation of the neuroblasts. The neuroblasts are the embryonic nerve cells which become the neurones of the adult. They
arise from germinal cells which occur in the ependymal zone and
divide by mitosis.
a. Development of efferent neurones. The neuroblasts become
pear-shaped and from the narrow end of the cell a slender axon
grows. Later they acquire neurofibrils and develop dendritic
processes. Many axons leave the spinal cord as ventral roots.
b. Development of the ganglia. They arise from the :
(1) Neural (ganglion) crest, which is a longitudinal ridge of
cells on each side, where the ectoderm joins the wall of the
neural groove.
(2) Migration of the crest. After closure of the neural tube
(p. 108) the neural crests separate into right and left halves
which occupy a position between the tube and the dorsal portions of the myotomes.
(3) Segmentation into spinal ganglia. The continuous neural
crest bands caudal to the otocysts soon show swellings which
become spinal ganglia. The portions of the crest between the
ganglionic swellings disappear.
(4) Formation of the cranial ganglia. These also develop
from the crest but are not segmentally arranged. Certain
cranial ganglia receive contributions from ectodermal thickenings called placodes.
(5) Formation of sympathetic ganglia. They are believed by
some to originate from the neural crest.
c. Development of afferent neurones. The cells of the cerebrospinal ganglia differentiate into ganglion cells (afferent neurones) and supporting cells.
(1) Bipolar stage. The ganglionic neuroblasts become spindle-shaped, and are transformed into bipolar neurones (p.
24) through growth of a process at each end. They remain in
this condition in the auditory ganglia (p. 31).
(2) Dorsal roots. They are formed by processes (axons) directed toward the neural tube. They enter the latter and
bifurcate into an ascending and a descending process coursing
in the dorsal region of the tube. Collaterals from the processes
establish connection with the neurones of the mantle layer.
(3) Peripheral processes. These pass outward and join the
axons of the efferent neurones of the cord coursing into the
ventral roots; the common bundles thus constituted are the
trunks of the spinal nerves.
(4) Transformation into monopolar neurones. While some
neuroblasts remain in the bipolar stage most others are
changed into monopolar neurones chiefly by fusion, for a
variable distance from the cell body, of the two primary processes into a common stem.
3. Differentiation of the supporting elements. These arise from
spongioblasts, which originate from the undifferentiated cells of
the neural plate tissue.
a. In the neural tube. For some time the spongioblasts are elongated and radially arranged within the tube, with their nuclei
placed close to the lumen; the inner end of the cell touches the
internal limiting membrane, while the outer reaches the periphery of the tube.
(1) Ependymal cells. These are spongioblasts which retain
their primitive shape and position.
(2) Neuroglia cells. Many of the elongated spongioblasts lose
their connections with the lumen of the neural tube, and
some of them also lose their peripheral portion, to become
neuroglia cells (astroglia and oligodendroglia, p. 33).
b. In the ganglia. The supporting cells become capsule cells,
satellite cells (p. 31), and sheath cells; the latter migrate peripherally along with the growing axons and envelop them as
neurilemma or cells of Schwann (p. 27).
B. Morphogenesis. The formation of the neural tube and the subdivision of its anterior, expanded end into the three primary brain
[ 157]
vesicles have already been described (p. 108). The rest of the tube
is the spinal cord.
1. The spinal cord. The typical three zones described previously
are clearly seen by the 5th week.
a. Shape. At first cylindrical it becomes enlarged at the level of
the two nerve plexuses that supply the limbs (cervical and lumbar enlargements, respectively).
b. Decrease in growth. After the 3rd month the vertebral column grows faster than the spinal cord. Since the latter is
anchored to the brain the caudal displacement of the vertebrae
causes an elongation of the roots of the lumbo-sacral nerves,
which together constitute the cauda equina.
c. Filum terminale. Since the posterior end of the neural tube
retains its terminal connections during the period of unequal
growth the caudal portion of the tube becomes this slender,
fibrous strand which occupies the axis of the cauda equina.
d. Formation of the central canal. The neural canal is at first
quite large and roughly diamond-shaped in cross section. Later
the lateral walls fuse dorsally (i.e. above the sulcus limitans);
in this way the dorsal portion of the canal is obliterated and
the persisting ventral portion becomes the definitive central
e. Differentiation of the walls. The thickening of the lateral
walls of the early spinal cord dominates the final arrangement
of the gray and white matter and is largely responsible for the
disappearance of the roof plate and reduction in size of the
floor plate.
(1) Formation of the dorsal median septum. This arises
largely from the fused ependymal layers during obliteration
of the dorsal portion of the neural canal.
(2) Formation of the ventral median fissure. The floor plate
lags in development and since it is interposed between the
rapidly thickening ventral portions of the lateral walls these
fail to meet, giving rise to the fissure.
f. Anomalies. The spinal cord may be absent, or the neural tube
may have failed to close; this condition often accompanies cleft
spine which is really a defect of the vertebral column. A sac
may protrude through the cleft and may be formed by the cord
only (myelocele) or by the cord and its meninges (meningomyelocele).
2 . The brain. Of the three primary vesicles (p. 108) the first and
last are subdivided into two secondary vesicles each: the telencephalon and diencephalon, in the case of the fore-brain (prosencephalon), and the metencephalon and myelencephalon, in the
case of the hind-brain (rhombencephalon).
a. Cavities. With the subdivision of the primary vesicles the
number of cavities is increased to four.
(1) Lateral ventricles. The cavity of the telencephalon extends into the paired hemispheres as the lateral (first and
second) ventricles.
(2) Third ventricle. This is the cavity of the median portion
of the telencephalon plus the cavity of the diencephalon.
(3) Fourth ventricle. It includes the merged lumina of the
metencephalon and myelencephalon; it is continuous caudally
with the central canal of the spinal cord.
(4) Cerebral aqueduct (of Sylvius). The mid-brain (mesencephalon) remains undivided; its primitive cavity becomes
this narrow canal connecting the third and fourth ventricles.
b. Myelencephalon (medulla oblongata). It is the transition
between the spinal cord and the brain. Its walls undergo certain
(1) Roof -plate. This, instead of disappearing, as in the spinal
cord, becomes the thin ependymal roof of the 4th ventricle.
(2) Chorioid plexus. Blood vessels grow into the layer of
mesenchyme (tela chorioidea) which covers the outer surface
of the ependymal roof and upon invagination of the latter
they form this plexus.
(3) Lateral walls. The sulcus limitans persists, separating
each wall into an alar and a basal plate.
(4) Floor plate. It persists and its ependymal cells elongate
as the ventral wall of the myelencephalon thickens; the processes of the ependymal cells extend from the floor of the
fourth ventricle to the ventral surface as the raphe.
(5) Nuclei of the alar plate. Neuroblasts arrange themselves
into the terminal (receptive) nuclei of nerves V, VII, VIII,
IX and X.
(6) Nuclei of the basal plate. Efferent neuroblasts form the
motor nuclei of origin of nerves V, VI, VII, IX, X, XI, and
c. Metencephalon. In general structure is similar to the preceding but it is the site of a marked embryonic flexure, and it also
develops two specialized parts, the pons and cerebellum, respectively.
(1) Pontine flexure. This, although temporary, is highly
characteristic of the embryo. Its convexity is ventrally directed.
It disappears completely during fetal life.
(2) Roof-plate. Part of it is transformed into a thin plate of
white matter both in front and behind the cerebellum, known
as the anterior and posterior medullary velum, respectively;
the other part merges with the cerebellum.
(3) Lateral walls. The still present sulcus limitans divides
them into alar and basal plates. In the latter develop the
motor nuclei of nerves V, VI, and VII, along with the reticular formation, present also in the myelencephalon. The alar
plates contribute to the formation of the:
(4) Cerebellum. The plates assume a transverse position as
the pontine flexure develops. Paired swellings near the midline foreshadow the vermis, while the lateral portions become
the cerebellar hemispheres. The latter connect with the pons
by means of the brachium pontis (middle cerebellar peduncle) .
(5) Pons. The pons develops as a thickening of the anterior
wall of the pontine flexure.
(6) Floor plate. This forms the portion of the raphe within
this region, where it is said to end.
d. Mesencephalon. This is the least-modified portion of the
primitive brain tube.
(1) Roof plate. It becomes very narrow and finally disappears.
(2) Alar plates. They develop into the lamina that bears the
corpora quadrigemina (superior and inferior colliculi), composed of stratified layers of neuroblasts.
(3) Basal plates. Their efferent neuroblasts form the motor
nuclei of nerves III and IV. The tegmentum is to be regarded
as an anterior extension of the reticular formation of the
mete- and myelencephalon.
(4) Floor plate. It is absent in the mesencephalon and anterior to it.
(5) Cerebral peduncles. They occur on each side of the midline in the floor of the mesencephalon, and are composed of
nerve fibers from the fore-brain and of sensory tracts coursing
in the opposite direction.
e. Diencephalon. It lacks floor and basal plates, and nerves do
not arise from it. It is connected ventrally with the hypophysis
(pituitary body). Its cavity is the third ventricle.
(1) Roof plate. It becomes a thin plate in which the chorioid
plexus of the third ventricle develops.
(2) Alar plates. Each of these is subdivided into three main
regions :
(a) Epithalamus, at the junction of the caudal portion of
the roof plate with the alar plate. The pineal gland (epiphysis cerebri) arises from this region.
(b) Thalamus. This is a marked swelling on the lateral
wall; the two thalami may join each other in the midline
through the massa intermedia. Ventrally is the:
(c) Hypothalamus, containing the infundibulum, tuber
cinereum, and mammillary bodies.
(3) The hypophysis. This important endocrine has a double
(a) Anterior lobe. It develops from an invagination of the
roof of the stomodaeum just in front of the pharyngeal
membrane (Rathke’s pouch). The invaginated ectodermic
sac sinks beneath the epithelium, and becomes a hollow
vesicle whose cavity is the residual lumen of the adult
gland (p. 90).
(b) Posterior lobe (pars nervosa). This is the enlarged tip
of the infundibular process, which grows as an invagination
of the floor of the diencephalon and meets the hollow vesicle arising from Rathke’s pouch.
(4) Optic stalks. They are connected with each side of the
diencephalon (p. 166).
f. Telencephalon. It becomes the most specialized and complex
region of the mammalian brain. It consists of a median portion,
continuous posteriorly with the diencephalon and containing
[ 161 ]
the cephalic part of the third ventricle, and two lateral outpocketings, the cerebral hemispheres.
(1) Roof plate. It gives rise to the chorioid plexus.
(2) Alar plates. They produce practically the whole cerebral
(3) Cerebral hemispheres. They arise between the 5th and
6th week, and grow beyond the original rostral end of the
neural tube, the wall of which is the lamina terminalis.
(a) Lateral ventricles. These are the cavities of the hemispheres; they communicate with the third ventricle through
the paired interventricular foramina (of Monro).
(b) Corpus striatum. This is a thickening of the floor of
the hemisphere. The groove separating it from the thalamus disappears and the two portions merge into a continuous mass.
(c) Internal capsule. Nerve fibers coursing between the
striate body and the thalamus are gathered into a V-shaped
lamina, open laterally. This is the internal capsule, which
partly subdivides the corpus striatum into secondary masses
(caudate and lenticular nuclei).
(d) Rhinencephalon. It is represented by the olfactory
lobes which arise as swellings of the ventral surfaces of the
hemispheres. The rostral or anterior part of each develops
into the olfactory bulb and tract. Connected with the olfactory apparatus there is also a portion of the brain cortex
(hippocampal system).
II. The Peripheral Nervous System
The formation and early differentiations of the neural crests have
already been considered (pp. 108, 156).
A. The spinal nerves. Each nerve is attached to the cord by two
roots: dorsal and ventral.
1. Dorsal root. It has a spinal ganglion associated with it. The
neuroblasts send their axons into the marginal zone of the cord
as dorsal root fibers; their peripheral processes join the ventral
root fibers.
2 . Ventral root. This carries efferent fibers (axons of cells within
the cord).
3. Nerve trunks. The mixed nerve trunks give off:
a. A dorsal ramus, supplying the dorsal skin and musculature;
it continues as:
b. The ventral ramus which in turn sends a:
c. Ramus communicans, to the sympathetic; the ramus communicans carries efferent fibers (preganglionics).
d. The lateral and ventral terminal rami arise through division
of the ventral ramus.
4. Plexuses. They are produced through anastomoses between the
spinal nerves. The brachial and lumbo-sacral plexuses arise in this
R. The cranial nerves. The cranial nerves are not segmentally arranged. There are twelve pairs of which three are purely (special)
sensory, four are purely (somatic) motor while the other five are
mixed (except the spinal accessory, purely motor in the adult but
intimately associated with the vagus).
1. Special sensory:
a. Olfactory (I). It has no ganglion. For its termination see p.
b. Optic (II). Consists of axons of neurones in the retina (p.
c. Auditory (VIII). Axons growing from the auditory ganglia
(vestibular and cochlear).
2. Somatic motor:
a. Oculomotor (III). Nucleus of origin in the basal plate of the
b. Trochlear (IV). Nucleus of origin as in the preceding, but
more caudally placed.
c. Abducens (VI). Nucleus of origin in the pontine region of
the metencephalon.
d. Hypoglossal (XII). Nucleus of origin in the basal plate of
the myelencephalon; associated in embryonic life with rudimentary dorsal ganglia (of Froriep) which later disappear.
3. Visceral sensory and motor:
a. Trigeminal (V). Chiefly sensory; its main ganglion (semilunar or Gasserian ganglion) gives off three branches: ophthalmic, maxillary and mandibular. The motor nucleus sends fibers
to the muscles of mastication.
b. Facial (VII). Chiefly motor; its sensory fibers are prolongations of neurones in the geniculate ganglion and end in the
sensory organs of the tongue.
c. Glossopharyngeal (IX). Chiefly sensory; its motor fibers arise
from the nucleus ambiguus, which it shares with the vagus.
They innervate some of the pharyngeal muscles (p. 153). The
sensory fibers are the peripheral processes of the superior and
petrosal ganglia.
d. Vagus (X) and spinal accessory (XI). They occur as a complex.
(1) The motor fibers arise from nuclei in the spinal cord
(spinal portion of accessory) and myelencephalon (bulbar
portion of accessory; motor portion of the vagus). The accessory fibers soon separate from the vagus, which supplies
motor fibers for the pharynx and larynx.
(2) The sensory fibers are processes of neurones residing in
the jugular and nodose ganglia, respectively.
C. The sympathetic nervous system. Its origin is still a matter of
discussion. The ganglia of the trunk develop before those of the
head and neck region.
1. Formation of the sympathetic chains. The sympathetic primordia are first continuous neuroblastic bands; later the neuroblasts
concentrate into segmentally arranged ganglia, connected by a
longitudinal nerve cord (sympathetic trunk).
2. The collateral ganglia. The collateral ganglia (ganglia of the
pre vertebral plexuses) develop later.
3 . Cranial sympathetic ganglia. The ciliary, spheno-palatine, and
otic ganglia (parasympathetic) are not segmental and are derived
mainly from the primitive semilunar (Gasserian) ganglion.
4. Chromaffin bodies. These, arising from cells in the primitive
sympathetic ganglia which give the chromaffin reaction (p. 94),
occur in close proximity to the ganglia (paraganglia) and in the
abdominal sympathetic plexus. They gradually degenerate after
birth, except the largest which is the:
5 . Suprarenal gland. It has a double origin:
a. Medulla. Its chromaffin cells (ectodermic) are derived from
the coeliac plexus. They arise as masses of cells which invade
the median side of the primordium of the:
[ 164]
b. Cortex. The cortex is of mesodermic origin and arises as proliferations of the peritoneal lining, on each side of the root of
the mesentery. The early suprarenals are quite large and project from the dorsal coelomic wall, between the mesonephros
and mesentery.
6. Carotid body (glomus caroticum). Although usually included
among the paraganglia, its paraganglionic nature is doubtful since
it has been shown to be a chemoreceptor (p. 30).
Only the specialized sense organs will be considered here.
I. The Organ of Taste
The taste buds arise as local thickenings of the tongue epithelium
as well as the epithelium of the oral mucosa, pharynx and epiglottis.
The cells of the thickening become the characteristic elements of
the bud (p. 95). In late fetal life many of the taste buds degenerate
and the adult distribution is attained.
II. The Nose
The early development of the nose has been considered in the
section dealing with the formation of the face (p. 112).
A. Formation of the primitive choanae. The epithelial plates which
separate the nasal fossae from the mouth cavity rupture caudally
to produce these internal nasal openings. The nasal fossae now have
outer (nostrils) and inner openings (choanae).
B. Formation of the lip and premaxillary palate. The front part of
each epithelial plate is invaded by mesoderm, and becomes these
C. The nasal septum. This arises from the medial fronto-nasal process (p. 1 12), which becomes narrower between the nasal fossae.
D. Separation of the nasal passages from the mouth cavity. It takes
place after fusion of the palatine processes in the midline (p. 123).
Fusion of the ventral border of the septum with the palate completes the separation of the nasal passages.
E. The permanent choanae. Their formation is finally accomplished
by the fusion mentioned above. The permanent nasal passages con
sist, therefore, of the nasal fossae plus a portion of the primitive
mouth cavity.
F. Vomero-nasal organs (of Jacobson). They are rudimentary epithelial sacs which open toward the front of the nasal septum. They
usually degenerate in late fetal stages.
G. The conchae. They arise as folds on the lateral and medial walls
of the nasal fossae; first cartilage, then bone develops in them.
H. The sinuses. They arise through absorption of bone. The spaces
thus formed are soon lined by epithelium which evaginates from
the nasal passages.
III. The Eye
Its development is complex. The sensory portion (retina) arises
from the brain as optic vesicles, while the lens is an invagination
of the ectoderm in front of each vesicle.
A. The optic vesicles. These are outpocketings of the fore-brain to
which they are attached by narrower optic stalks.
1. The optic cups. They arise through invagination of the lateral
hemisphere of the vesicle into the mesial hemisphere. A doublewalled cup is thus produced, connected with the diencephalon by
the optic stalk.
a. The chorioid fissure. The invaginated portion of the vesicle
is notched ventrally, the notch extending along the ventral surface of the optic stalk as a groove (chorioid fissure), through
which the central artery of the retina reaches the optic cup.
(1) Closure of the fissure. It becomes a tube through approximation and fusion of its edges. Incomplete closure supposedly gives rise to absence of a sector of the iris, ciliary body
or chorioid; this is known as coloboma.
(2) Obliteration of the optic stalk. This takes place upon
growth of axons from the retina. The axons fill the lumen
of the stalk on their way to the brain and the stalk is transformed into the optic nerve.
b. Pigment layer. The outer, thinner layer of the optic cup becomes this portion of the adult retina. Pigment appears very
c. Nervous layer. The internal, thicker layer of the cup becomes
this retinal portion.
(1) Pars caeca. This is the zone bordering the rim, which
later is subdivided into the pars ciliaris and pars iridica, respectively (pp. 97, 98).
(2) Pars optica (visualis). The more centrally located portion, separated from the former by the ora serrata. In it develop the rods and cones and the other layers of the adult
retina (p. 98).
B. The lens. The ectoderm in front of the optic vesicle thickens to
form the lens placode, which is soon changed into a vesicle.
1. Position. After invagination of the placode has ended the lens
vesicle occupies the concavity of the optic cup.
2. Differentiation of the walls. From its early formation the lateral
wall of the vesicle is thinner than the medial wall.
a. Lens epithelium. The cells of the lateral wall remain as low
columnar elements and form this part of the lens.
b. Lens fibers. The cells of the medial wall become much elongated, their nuclei degenerate and they become the transparent
long prisms or fibers.
c. Disappearance of the lens cavity. This is gradually obliterated
when the rapidly elongating fibers come to be in contact with
the posterior surface of the lens epithelium.
3. Capsule. It is apparently formed by the cells of the lens vesicle,
but it lacks a definite structure.
C. The vitreous body. Fills the space between the lens and retina;
it is produced by the latter, and is secondarily invaded by mesenchyme, some of which enters with the central artery.
1. The hyaloid artery. This is the branch of the central artery
that crosses the developing vitreous body and spreads over the
posterior surface of the lens. It degenerates, leaving the hyaloid
canal (p. 100).
2. Pupillary membrane. This contains small vessels supplying the
rest of the lens; they are derived from the peripheral rim of the
D. The fibrous and vascular coats. They arise from mesenchyme
which forms a double layer around the developing eye. The outer
layer gives rise to the sclera and cornea, while the inner produces
the iris, ciliary body and choroid. The anterior chamber arises as
the result of degeneration of the mesenchyme between the lens and
the surface ectoderm. Its continuous peripheral extension separates
the iris from the cornea.
IV. The Ear
The formation of the external auditory meatus and the tympanic
cavity has been already indicated (p. 124), as well as the origin of
the ossicles of the middle ear (p. 150). Only the internal ear will be
considered here. Its epithelial lining is of ectodermic origin.
A. The auditory placode. This is an area of thickened ectoderm located on each side of the hind-brain. The placodes appear very early,
when a few somites are present in the embryo.
B. The otocyst, or auditory vesicle results from invagination of the
placode. It loses all connection with the outside, but near the point
where the otocyst joined the ectoderm there appears the:
1. Endolymphatic duct, which is a tubular outpocketing ending
blindly distally; the blind end is the endolymphatic sac.
2. Division into regions. The otocyst elongates dorso-ventrally; its
narrow ventral part becomes the:
a. Cochlear duct, which will soon coil to form the cochlea.
b. Vestibular portion. This is the more expanded, dorsal portion
of the otocyst.
3. Subdivision of the vestibular portion. This will produce dorsally the:
a. Semicircular canals. The anterior and posterior arise from a
single pouch at the dorsal border while the lateral begins as a
horizontal outpocketing placed a little more ventrally.
b. Formation of the ampullae. The anterior and posterior semicircular canals have a common opening dorsally into the vestibule, but their opposite ends and the rostral end of the lateral
canal are dilated into the ampullae (p. 102).
c. Utriculus and sacculus. They develop from the more ventral
part of the vestibular portion through the formation of a constriction. The semicircular canals are attached to the utriculus,
while the cochlea is connected with the sacculus.
d. Further development. The general shape of the inner ear of
the adult is attained during the 3rd month. The utriculus and
sacculus become separated from each other, but they keep their
connection with the endolymphatic duct.
C. The bony labyrinth.
1. Production of cartilage. The mesenchyme surrounding the de
veloping otocyst (membranous labyrinth) produces cartilage
which completely encloses the labyrinth.
2. Formation of the perilymphatic space. Later on the cartilage
next to the labyrinth undergoes regression and the space thus
formed becomes the perilymphatic space.
3. Ossification. The bony labyrinth is produced during the 5 th
month by replacement of the cartilage capsule by bone. The
modiolus of the cochlea develops directly from mesenchyme as a
membrane bone.
Bailey’s Textbook of Histology, edited by P. E. Smith, ioth edit.
Williams and Wilkins, Baltimore, 1940.
Bremer, J. L. A Textbook of Histology arranged upon an Embryological basis. 5th edit. Blakiston, Philadelphia, 1936.
Cowdry, E. V. A Textbook of Histology. 2nd edit. Lea and Febiger,
Philadelphia, 1938.
Cowdry, E. V., editor. Special Cytology. 2nd edit., 3 vols. Hoeber,
New York, 1932.
Dahlgren, U., and W. Kepner. Textbook of the Principles of Animal
Histology. Macmillan, New York, 1908.
Hill, C. A Manual of Normal Histology and Organography. 7th
edit. Saunders, 1937.
Jordan, H. E. A Textbook of Histology. 8th edit. D. AppletonCentury, New York, 1940.
Krause, R. A Course in Normal Histology. Rebman, New York,
Lambert, A. E. Introduction and Guide to the Study of Histology.
Blakiston, Philadelphia, 1938.
Maximow, A., and Bloom, W. A Textbook of Histology. 3rd edit.
Saunders, Philadelphia, 1938.
Mollendorf, W. von, editor: Handbuch der mikroskopischen
Anatomie des Menschen. Springer, Berlin.
Piersol’s Normal Histology, edited by W. H. F. Addison. 15th edit.
Philadelphia, Lippincott, 1932.
Ramon y Cajal, S. Histology. W. Wood and Co., Baltimore, 1933.
Schafer, E. S. The Essentials of Histology. 13th edit. Lea and
Febiger, Philadelphia, 1934.
Sobotta, J. Textbook and Atlas of Human Histology and Microscopic Anatomy. 2nd edit. Stechert, New York, 1930.
[ 17 1 ]
Arey, L. B. Developmental Anatomy. A Textbook and Laboratory
Manual of Embryology. 4th edit. Saunders, Philadelphia, 1940.
Boyden, E. A. A Laboratory Atlas of the 13mm. Pig Embryo. 3rd
edit. Wistar Institute, Philadelphia, 1936.
Frazer, }. E. A Manual of Embryology. The Development of the
Human Body. 2nd edit. Bailliere, Tindall and Cox, London,
Jordan, H. E., and Kindred, J. E. A Textbook of Embryology. 3rd
edit. Appleton-Century, New York, 1937.
Keibel, F., and Mall, F. P. Manual of Human Embryology. 2 vols.
Lippincott, Philadelphia, 1910, 1912.
Keith, A. Human Embryology and Morphology. 5th edit. Arnold,
London, 1933.
Lillie, F. R. The Development of the Chick. 2nd edit. Holt, New
York, 1927.
McMurrich, J. P. The Development of the Human Body. 7th edit.
Blakiston, Philadelphia, 1923.
Patten, B. M. The Early Embryology of the Chick. 3rd edit.
Blakiston, Philadelphia, 1929.
Patten, B. M. The Embryology of the Pig. 2nd edit. Blakiston,
Philadelphia, 1931.

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Nonidez JF. Histology and Embryology. (1941) Oxford University Press, London.

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   Histology and Embryology 1941: Histology - 1 The Cell | 2 The Tissues | 3 The Organs     Embryology - 1 General Development | 2 Organogenesis | Bibliography



Embryology deals with the development of the individual from the moment of fertilization of the ovum to the attainment of the adult form. Used in this broad sense it comprises some of the events of early postnatal life. In the present outline only the development of the human will be considered.

Part One - General Development

   Histology and Embryology 1941: Histology - 1 The Cell | 2 The Tissues | 3 The Organs     Embryology - 1 General Development | 2 Organogenesis | Bibliography

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