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==Part IV Anatomy Of Vertebrate Embryos==
+
=Part IV Anatomy Of Vertebrate Embryos=
==text==
 
Chapter XI The Anatomy Of Frog Embryos
 
  
In earlier chapters we have discussed the fertilization of the
+
==Chapter XI The Anatomy Of Frog Embryos==
frog’s egg (page 57), its cleavage (pages 97, 103), and germ-layer
 
formation (pages 109, 118), and have observed that while the germ
 
layers are being laid down the process is complicated by the early
 
localization of some of the organ systems, notably the sensorynervous complex (page 129). In this account of later organogeny, three stages of development seem especially significant:
 
first, an early embryo of about 3 mm. body length in which the
 
visceral grooves are apparent, a stage attained in Rana pipiens
 
about the second day after the eggs are laid; second, the newly
 
hatched larva of about 6 mm. with external gills developing,
 
about two weeks old; third, a young “ tadpole ” stage of about
 
11 mm. with the opercula covering the internal gills, about the
 
age of one month.
 
  
These stages are easily identified even though the lengths and
+
In earlier chapters we have discussed the fertilization of the frog’s egg (page 57), its cleavage (pages 97, 103), and germ-layer formation (pages 109, 118), and have observed that while the germ layers are being laid down the process is complicated by the early localization of some of the organ systems, notably the sensorynervous complex (page 129). In this account of later organogeny, three stages of development seem especially significant: first, an early embryo of about 3 mm. body length in which the visceral grooves are apparent, a stage attained in Rana pipiens about the second day after the eggs are laid; second, the newly hatched larva of about 6 mm. with external gills developing, about two weeks old; third, a young “ tadpole ” stage of about 11 mm. with the opercula covering the internal gills, about the age of one month.
ages can be given only approximately, for the rate of development is greatly influenced by the prevailing temperature, and
 
the size of the tadpole is determined largely by external factors,
 
such as the amount of food available.
 
  
The student must bear in mind that the sections illustrated in
+
These stages are easily identified even though the lengths and ages can be given only approximately, for the rate of development is greatly influenced by the prevailing temperature, and the size of the tadpole is determined largely by external factors, such as the amount of food available.
this and the two chapters following are for the sole purpose of giving
+
 
him starting points from which he 1s expected to study all the sections
+
The student must bear in mind that the sections illustrated in this and the two chapters following are for the sole purpose of giving him starting points from which he 1s expected to study all the sections in the series furnished him. He will probably never encounter sections exactly like those selected for these illustrations, but he will discover sections very like them from which he can commence his own observations.
in the series furnished him. He will probably never encounter
 
sections exactly like those selected for these illustrations, but he will
 
discover sections very like them from which he can commence his
 
own observations.
 
  
 
A. THE EARLY EMBRYO (3 MM.)
 
A. THE EARLY EMBRYO (3 MM.)
  
External form. — This stage corresponds approximately to the
+
External form. — This stage corresponds approximately to the embryo of 33 mm. described by Marshall. The head region, through its more rapid growth, has become easily distinguishable from the trunk, which bulges ventrally on account of the large
embryo of 33 mm. described by Marshall. The head region,
 
through its more rapid growth, has become easily distinguishable
 
from the trunk, which bulges ventrally on account of the large
 
  
amount of contained yolk, and a well-marked tail bud is present.
+
amount of contained yolk, and a well-marked tail bud is present. 275 276 THE ANATOMY OF FROG EMBRYOS
275
 
276 THE ANATOMY OF FROG EMBRYOS
 
  
The neural folds have fused throughout their length, and enclosed the blastopore. In the head the stomodeum appears
+
The neural folds have fused throughout their length, and enclosed the blastopore. In the head the stomodeum appears as an antero-posterior slit on the anterior ventral surface, and is enclosed by ridges identifiable as the maxillary processes and mandibular arches. On either side and slightly ventral to the stomodeum, are the primordia of the sucker or oral gland. At the dorso-lateral margins the olfactory placodes have begun to evaginate. Lateral bulges on either side of the head are due to the developing optic vesicles. The ear is now in the otic vesicle stage. The gill region shows five visceral grooves. Immediately behind the last arch, a swelling is caused by the developing pronephros. Dorsally, slight furrows indicate the boundaries of thirteen soEpiphysie mites. Beneath the tail Optic vesicle bud, the proctodeum Prosencephalon has united with the
as an antero-posterior slit on the anterior ventral surface, and is
 
enclosed by ridges identifiable as the maxillary processes and
 
mandibular arches. On either side and slightly ventral to the
 
stomodeum, are the primordia of the sucker or oral gland. At the
 
dorso-lateral margins the olfactory placodes have begun to evaginate. Lateral bulges on either side of the head are due to the
 
developing optic vesicles. The ear is now in the otic vesicle stage.
 
The gill region shows five visceral grooves. Immediately behind
 
the last arch, a swelling is caused by the developing pronephros.
 
Dorsally, slight furrows indicate the boundaries of thirteen soEpiphysie mites. Beneath the tail
 
Optic vesicle bud, the proctodeum
 
Prosencephalon has united with the
 
  
Oral gland hind-gut to form the
+
Oral gland hind-gut to form the Visceral pouch eloacal aperture.
Visceral pouch eloacal aperture.
 
  
Fore gut Endodermal derivaay Liver tives. - The anterior
+
Fore gut Endodermal derivaay Liver tives. - The anterior portion of the gastrocoel is now a large fore-gut with a thin-walled lining. From this, on a . either side, the begin eurenteric . : canal nings of three visceral Fig. 182. —3 mm. frog embryo, viewed from right pouches can be seen.
portion of the gastrocoel
 
is now a large fore-gut
 
with a thin-walled
 
lining. From this, on
 
a . either side, the begin
 
eurenteric . :
 
canal nings of three visceral
 
Fig. 182. —3 mm. frog embryo, viewed from right pouches can be seen.
 
  
 
side as a transparent object. X15.
 
side as a transparent object. X15.
  
From the fore-gut a narrow evagination grows backward into the floor of the mid-gut as
+
From the fore-gut a narrow evagination grows backward into the floor of the mid-gut as the primordium of the liver. The mid-gut is distinguishable by its relatively narrow lumen and thick yolk-laden floor. The small but thin-walled hind-gut opens above into the neurenteric canal by which it is connected with the neurocoel, and opens ventrally to the exterior by way of the proctodeum. An axial rod, the hypochord, is found beneath the notochord. It originates from the roof of the gastrocoel and disappears soon after hatching.
the primordium of the liver. The mid-gut is distinguishable by
+
 
its relatively narrow lumen and thick yolk-laden floor. The
+
Mesodermal derivatives. — The notochord is large and vacuolated and enclosed by two sheaths. The somites have now attained their maximum number (13) in the trunk, but are not
small but thin-walled hind-gut opens above into the neurenteric
+
 
canal by which it is connected with the neurocoel, and opens
+
 
ventrally to the exterior by way of the proctodeum. An axial
 
rod, the hypochord, is found beneath the notochord. It originates from the roof of the gastrocoel and disappears soon after
 
hatching.
 
  
Mesodermal derivatives. — The notochord is large and vacuolated and enclosed by two sheaths. The somites have now
 
attained their maximum number (13) in the trunk, but are not
 
  
   
 
   
 
 
  
 
Mesencephalon
 
Mesencephalon
  
Otic vesicle
+
Otic vesicle Rhombencephalon
Rhombencephalon
+
 
 +
Somite I Notochord + THE EARLY EMBRYO 277
 +
 
 +
yet distinguishable in the tail region. The intermediate mesoderm, after a temporary division into nephrotomes, is now reunited into a nephrotomal band in which spaces have appeared opposite the second, third, and fourth somites, indicative of the pronephric tubules which are to develop. A thickening along the
  
Somite I
 
Notochord +
 
THE EARLY EMBRYO 277
 
  
yet distinguishable in the tail region. The intermediate mesoderm, after a temporary division into nephrotomes, is now reunited into a nephrotomal band in which spaces have appeared
 
opposite the second, third, and fourth somites, indicative of the
 
pronephric tubules which are to develop. A thickening along the
 
  
 
 
 
  
 
Mesencephalon PD, Prosencephalon
 
Mesencephalon PD, Prosencephalon
  
 
 
  
Neurenteric
+
 
canal
+
Neurenteric canal
  
 
Fia. 1838. —3 mm. frog embryo. Sagittal section: 50.
 
Fia. 1838. —3 mm. frog embryo. Sagittal section: 50.
  
nephrotomal band immediately below the ventro-lateral margins
+
nephrotomal band immediately below the ventro-lateral margins of the somites is the primordium of the pronephric duct. Immediately below the floor of the fore-gut, the lateral mesoderm has separated into dorsal splanchnic and ventral somatic layers, while the contained space is the beginning of the pericardial cavity, the only region of the coelom yet apparent.
of the somites is the primordium of the pronephric duct. Immediately below the floor of the fore-gut, the lateral mesoderm has
+
 
separated into dorsal splanchnic and ventral somatic layers, while
+
Ectodermal derivatives. — The epidermis at this stage is ciliated. The neurocoel, as has been remarked above, is con278 , THE ANATOMY OF FROG EMBRYOS
the contained space is the beginning of the pericardial cavity, the
+
 
only region of the coelom yet apparent.
+
nected with the hind-gut by the neurenteric canal. At the anterior end, the brain is distinguishable by its relatively larger lumen and by the cranial flexure over the anterior end of the notochord. The divisions between the three primary vesicles are not marked by the constrictions characteristic of many vertebrates, but are distinguished by the following points of reference: the prosencephalon extends to a Optic line projected from a thickvesicle ening on the floor known as the tuberculum posterius to a point just in front of a similar thickening on the Hypophysis — dorsal wall; the mesencephOral gland = aon, from the boundary of the prosencephalon to a line Fia. 184. —3 mm. frog embryo. Transverse connecting the tuberculum section through optic vesicle. 50. anda point just behind the dorsal thickening; the rhombencephalon merges imperceptibly into the spinal cord. From the prosencephalon, the optic vesicles extend on either side and cause the external bulges already noted. From the ventral side of the prosencephalon, a depression, the infundibulum, extends towards the hypophysis, which in the frog grows inward as a solid wedge of ectodermal cells anterior to the stomodeum. Dorsally, the epiphysis appears as a median evagination.
  
Ectodermal derivatives. — The epidermis at this stage is
 
ciliated. The neurocoel, as has been remarked above, is con278 , THE ANATOMY OF FROG EMBRYOS
 
  
nected with the hind-gut by the neurenteric canal. At the
 
anterior end, the brain is distinguishable by its relatively larger
 
lumen and by the cranial flexure over the anterior end of the
 
notochord. The divisions between the three primary vesicles
 
are not marked by the constrictions characteristic of
 
many vertebrates, but are
 
distinguished by the following points of reference: the
 
prosencephalon extends to a
 
Optic line projected from a thickvesicle ening on the floor known as
 
the tuberculum posterius to
 
a point just in front of a
 
similar thickening on the
 
Hypophysis — dorsal wall; the mesencephOral gland = aon, from the boundary of
 
the prosencephalon to a line
 
Fia. 184. —3 mm. frog embryo. Transverse connecting the tuberculum
 
section through optic vesicle. 50. anda point just behind the
 
dorsal thickening; the rhombencephalon merges imperceptibly
 
into the spinal cord. From the prosencephalon, the optic vesicles
 
extend on either side and cause the external bulges already noted.
 
From the ventral side of the prosencephalon, a depression, the
 
infundibulum, extends towards the hypophysis, which in the frog
 
grows inward as a solid wedge of ectodermal cells anterior to
 
the stomodeum. Dorsally, the epiphysis appears as a median
 
evagination.
 
  
   
 
 
  
 
Fore-gut
 
Fore-gut
Line 151: Line 64:
 
B. THE LARVA AT HATCHING (6 MM.)
 
B. THE LARVA AT HATCHING (6 MM.)
  
External form. — Although the larva, if it may be so called,
+
External form. — Although the larva, if it may be so called, has emerged from the protecting membranes of egg jelly, the mouth has not yet opened and for several days the yolk is still the sole source of food. The head region is still easily distinguishable from the trunk, while the tail has increased greatly in length and has become bilaterally compressed. In the head, the stomodeal pit has deepened at the anterior end, and the maxillary processes and mandibular arches are more sharply sculptured. The invagination of the nasal (olfactory) placodes has THE LARVA AT HATCHING 279
has emerged from the protecting membranes of egg jelly, the
 
mouth has not yet opened and for several days the yolk is still
 
the sole source of food. The head region is still easily distinguishable from the trunk, while the tail has increased greatly in
 
length and has become bilaterally compressed. In the head, the
 
stomodeal pit has deepened at the anterior end, and the maxillary processes and mandibular arches are more sharply sculptured. The invagination of the nasal (olfactory) placodes has
 
THE LARVA AT HATCHING 279
 
  
 
  
Fig. 185.—3 mm. frog embryo. Transverse section through otic (auditory)
 
vesicle. 50.
 
  
+
Fig. 185.—3 mm. frog embryo. Transverse section through otic (auditory) vesicle. 50.
  
Fig. 186. —8 mm. frog embryo. Transverse section through mid-gut and liver.
 
X50.
 
280 THE ANATOMY OF FROG EMBRYOS
 
  
 
  
Fig. 187. — 3 mm. frog embryo. Frontal section through optic stalks, liver, and
+
Fig. 186. —8 mm. frog embryo. Transverse section through mid-gut and liver. X50. 280 THE ANATOMY OF FROG EMBRYOS
hind-gut. 50.
 
THE LARVA AT HATCHING 281
 
  
continued to the point where they may be called pits, connected
+
 
to the anterior margins of the stomodeal pit by oro-nasal grooves.
+
 
The bulge of the eye is still prominent. The primordia of the oral
+
Fig. 187. — 3 mm. frog embryo. Frontal section through optic stalks, liver, and hind-gut. 50. THE LARVA AT HATCHING 281
glands have fused to form a U-shaped sucker ventral and posterior to the stomodeum. The visceral grooves are still separated from the visceral pouches by closing membranes, while on
+
 
the third and fourth arches external gills have appeared. Behind
+
continued to the point where they may be called pits, connected to the anterior margins of the stomodeal pit by oro-nasal grooves. The bulge of the eye is still prominent. The primordia of the oral glands have fused to form a U-shaped sucker ventral and posterior to the stomodeum. The visceral grooves are still separated from the visceral pouches by closing membranes, while on the third and fourth arches external gills have appeared. Behind them the pronephric elevation is well marked, and continues backward as a slight ridge marking the pronephric duct. Intersomitic grooves are still apparent. On the ventral side at the base of the tail is the cloacal aperture.
them the pronephric elevation is well marked, and continues
 
backward as a slight ridge marking the pronephric duct.
 
Intersomitic grooves are still apparent. On the ventral side
 
at the base of the tail is the cloacal aperture.
 
  
 
Prosencephalon
 
Prosencephalon
  
   
+
 
 
+
 
 
+
 
 
+
 
  
 
Optic cup
 
Optic cup
  
Mesencephalon
+
Mesencephalon é
é
 
  
 
Otic vesicle a
 
Otic vesicle a
  
Heart
+
Heart Rhombencephalon S External gills Pronephros i a— Liver
Rhombencephalon S External gills
+
 
Pronephros i a— Liver
 
  
 
  
 
Yolk
 
Yolk
Line 210: Line 101:
 
Myotomes
 
Myotomes
  
A
+
A Fig. 188. — 6 mm. frog larva (just hatched). Transparent preparation, viewed from right side. X15.
Fig. 188. — 6 mm. frog larva (just hatched). Transparent preparation, viewed from
 
right side. X15.
 
  
Endodermal derivatives. — On either side of the fore-gut are
+
Endodermal derivatives. — On either side of the fore-gut are to be seen five visceral pouches, although they would hardly be recognized as such since they are so compressed. A groove on the ventral side of the pharyngeal cavity is the primordium of the thyroid gland. At this stage, also, the dorsal epithelial 282 THE ANATOMY OF FROG EMBRYOS
to be seen five visceral pouches, although they would hardly be
 
recognized as such since they are so compressed. A groove on
 
the ventral side of the pharyngeal cavity is the primordium of
 
the thyroid gland. At this stage, also, the dorsal epithelial
 
282 THE ANATOMY OF FROG EMBRYOS
 
  
bodies of the first two visceral pouches (hyomandibular and first
+
bodies of the first two visceral pouches (hyomandibular and first branchial) may be distinguished. The liver diverticulum has increased in length. The hind-gut has lost its connection with the neurocoel through the occlusion of the neurenteric canal, but now receives the posterior ends of the pronephric ducts.
branchial) may be distinguished. The liver diverticulum has
 
increased in length. The hind-gut has lost its connection with
 
the neurocoel through the occlusion of the neurenteric canal, but
 
now receives the posterior ends of the pronephric ducts.
 
  
Mesodermal derivatives. — The notochord has grown back
+
Mesodermal derivatives. — The notochord has grown back into the tail. The somites have now become differentiated into the myotomes, dermatomes, and sclerotomes, while from the myotomes muscle cells have been formed. The pronephros is now established. There are three pronephric tubules, each opening into the coelom by means of a ciliated nephrostome. Opposite these, a mass of capillaries, connected with the dorsal aorta, forms the so-called glomus, equivalent to the separate glomeruli of other vertebrates. The pronephric tubules grow backward into the pronephric ducts, which have acquired lumina. At the time of hatching, the primordia of the heart have fused to form a tube, twisted slightly and almost S-shaped, suspended in the pericardial cavity by a dorsal mesocardium. ‘Two regions may be distinguished, the posterior atrium and anterior ventricle. From the ventricle leads the bulbus, arising from the fusion of paired primordia. This connects with the dorsal aorta, also the result of fusion, by means of aortic arches in the third and fourth visceral arches (vestiges of the first and second aortic arches have already appeared and disappeared). At a slightly later stage, loops from these arches will grow out into the external gills to form a branchial circulation. The anterior ends of the dorsal aortae are prolonged to form the internal carotids, while the posterior ends unite directly above the heart, and just after uniting give off the glomi on either side. Both the somatic and splanchnic venous systems are represented at this stage. Two vitelline veins unite to enter the heart at the sinus venosus. The cardinal veins at this time are represented by irregular lacunar spaces in the head and near the pronephros.
into the tail. The somites have now become differentiated into
 
the myotomes, dermatomes, and sclerotomes, while from the
 
myotomes muscle cells have been formed. The pronephros is
 
now established. There are three pronephric tubules, each
 
opening into the coelom by means of a ciliated nephrostome.
 
Opposite these, a mass of capillaries, connected with the dorsal
 
aorta, forms the so-called glomus, equivalent to the separate
 
glomeruli of other vertebrates. The pronephric tubules grow
 
backward into the pronephric ducts, which have acquired lumina.
 
At the time of hatching, the primordia of the heart have fused
 
to form a tube, twisted slightly and almost S-shaped, suspended
 
in the pericardial cavity by a dorsal mesocardium. ‘Two regions
 
may be distinguished, the posterior atrium and anterior ventricle. From the ventricle leads the bulbus, arising from the
 
fusion of paired primordia. This connects with the dorsal aorta,
 
also the result of fusion, by means of aortic arches in the third
 
and fourth visceral arches (vestiges of the first and second aortic
 
arches have already appeared and disappeared). At a slightly
 
later stage, loops from these arches will grow out into the external gills to form a branchial circulation. The anterior ends of
 
the dorsal aortae are prolonged to form the internal carotids,
 
while the posterior ends unite directly above the heart, and just
 
after uniting give off the glomi on either side. Both the somatic
 
and splanchnic venous systems are represented at this stage.
 
Two vitelline veins unite to enter the heart at the sinus venosus.
 
The cardinal veins at this time are represented by irregular
 
lacunar spaces in the head and near the pronephros.
 
  
Ectodermal derivatives.— The epidermis is still ciliated.
+
Ectodermal derivatives.— The epidermis is still ciliated. From the prosencephalon the thin-walled cerebral vesicle has appeared. The epiphysis is well marked, and the infundibulum is in contact with the hypophysis. At this time the primordia of cerebrospinal nerves may be distinguished. In the spinal nerves, dorsal roots arise from the ganglia produced by the segTHE LARVA AT HATCHING
From the prosencephalon the thin-walled cerebral vesicle has
 
appeared. The epiphysis is well marked, and the infundibulum
 
is in contact with the hypophysis. At this time the primordia
 
of cerebrospinal nerves may be distinguished. In the spinal
 
nerves, dorsal roots arise from the ganglia produced by the segTHE LARVA AT HATCHING
 
  
   
 
  
283
+
 
Infundibulum
+
283 Infundibulum Epiphysi Mesencephalan. piphysis Prosencephalon Rhombencephalon Fore gut Oral gland
Epiphysi
 
Mesencephalan. piphysis
 
Prosencephalon
 
Rhombencephalon
 
Fore gut
 
Oral gland
 
  
 
Heart
 
Heart
Line 278: Line 121:
 
Liver
 
Liver
  
Sittegretee SEN
+
Sittegretee SEN Bengawere ee
Bengawere ee
 
  
 
ee
 
ee
  
pear
+
pear = a 2
=
 
a
 
2
 
  
 
os
 
os
  
a ‘5-3 eo
+
a ‘5-3 eo Re Od SA eGo
Re Od
 
SA eGo
 
  
a Hoe Raa pide
+
a Hoe Raa pide Pina SRL
Pina SRL
 
  
eC
+
eC PRES ee
PRES ee
 
  
 
BJ
 
BJ
  
eS
+
eS Fs “
Fs
 
 
  
i)
+
i) ce ‘ ret OKs Ve on
ce ‘
 
ret
 
OKs Ve
 
on
 
  
 
a
 
a
Line 320: Line 149:
 
Yolk
 
Yolk
  
Ps
+
Ps ae oe
ae
 
oe
 
  
 
Fig. 189. — 6 mm. frog larva.
 
Fig. 189. — 6 mm. frog larva.
  
Sagittal section, anterior portion. 50.
+
Sagittal section, anterior portion. 50. 284 THE ANATOMY OF FROG EMBRYOS
284 THE ANATOMY OF FROG EMBRYOS
+
 
 +
 
 +
 
  
 
 
 
 
 
 
  
 
Prosencephalon
 
Prosencephalon
  
Optic
+
Optic cup Lens Optic . ZH stalk FF 5 —_ Notochord
cup
 
Lens
 
Optic . ZH
 
stalk FF 5 —_ Notochord
 
  
 
Fore-gut
 
Fore-gut
Line 351: Line 173:
 
cavity
 
cavity
  
Fig. 191. — 6 mm. frog larva. Transverse section through otic vesicle. 50.
+
Fig. 191. — 6 mm. frog larva. Transverse section through otic vesicle. 50. THE LARVA AT HATCHING 285
THE LARVA AT HATCHING 285
 
  
mentation of the neural crest while the ventral roots arise from
+
mentation of the neural crest while the ventral roots arise from neuroblasts in the spinal cord. In the head, four ganglia arise and with each is associated a placode of nervous ectoderm. From the first ganglion and placode, the trigeminal (V) nerve arises. The second combination gives rise to the facial (VII) and acoustic (VIII) cranial nerves, while the remainder of this placode invaginates to form the otic vesicle. The third ganglion and placode produce the glossopharyngeal (IX) cranial nerve, and the
neuroblasts in the spinal cord. In the head, four ganglia arise
+
 
and with each is associated a placode of nervous ectoderm. From
+
Pronephric tubules
the first ganglion and placode, the trigeminal (V) nerve arises.
 
The second combination gives rise to the facial (VII) and acoustic
 
(VIII) cranial nerves, while the remainder of this placode invaginates to form the otic vesicle. The third ganglion and placode produce the glossopharyngeal (IX) cranial nerve, and the
 
  
Pronephric
 
tubules
 
  
 
  
 
Fig. 192. — 6 mm. frog larva. Transverse section through pronephros. 50.
 
Fig. 192. — 6 mm. frog larva. Transverse section through pronephros. 50.
  
fourth gives rise to the vagus (X). The fourth placode grows
+
fourth gives rise to the vagus (X). The fourth placode grows back as far as the tail, giving off as it goes small groups of cells which later become the lateral line organs of the trunk. Those of the head arise from the second and third placodes. At this time, also, ganglion cells are migrating toward the dorsal aorta to aggregate as the ganglia of the autonomic nervous system. The eye is well advanced in development, as the optic vesicles have invaginated to form the optic cup and the lens placode has separated from the epidermis and acquired a cavity. The ear is in the otic vesicle stage with an endolymphatic duct. The nose is still represented by the nasal pits. From the prolongation of the fourth placode referred to above, the lateral line system is in process of formation. 286 THE ANATOMY OF FROG EMBRYOS
back as far as the tail, giving off as it goes small groups of cells
 
which later become the lateral line organs of the trunk. Those
 
of the head arise from the second and third placodes. At this
 
time, also, ganglion cells are migrating toward the dorsal aorta to
 
aggregate as the ganglia of the autonomic nervous system. The
 
eye is well advanced in development, as the optic vesicles have
 
invaginated to form the optic cup and the lens placode has separated from the epidermis and acquired a cavity. The ear is in
 
the otic vesicle stage with an endolymphatic duct. The nose is
 
still represented by the nasal pits. From the prolongation of
 
the fourth placode referred to above, the lateral line system is
 
in process of formation.
 
286 THE ANATOMY OF FROG EMBRYOS
 
  
   
 
 
 
 
 
 
  
Visceral 1
 
pouch
 
  
I Visceral
 
arch
 
  
Pronephric
 
tubules
 
  
Segmental
+
 
muscles
+
Visceral 1 pouch
 +
 
 +
I Visceral arch
 +
 
 +
Pronephric tubules
 +
 
 +
Segmental muscles
  
 
Fig. 193. —6 mm. frog larva. Frontal section through nasal pit and visceral
 
Fig. 193. —6 mm. frog larva. Frontal section through nasal pit and visceral
  
pouches. 450.
+
pouches. 450. THE YOUNG TADPOLE 287
THE YOUNG TADPOLE 287
 
  
 
C. THE YOUNG TADPOLE (11 MM.)
 
C. THE YOUNG TADPOLE (11 MM.)
  
External form. — The head and trunk are now fused into a
+
External form. — The head and trunk are now fused into a common ovoid mass, sharply distinguished from the long bilaterally compressed tail. The mouth is open and equipped with horny raspers, while the oral gland is reduced to two vestiges on the ventral side of the head. On the dorsal surface, the large eyes, now functional, protrude slightly. Anterior to these are the external openings of the nasal tubes (external nares). The external gills, which were developing at hatching, have now degenerated and been replaced by internal gills concealed from view by the opercula. On the left side, the opercular aperture serves as a means of egress for the water from which the gills obtain their oxygen. The tail, now two-thirds the length of the tadpole, has a dorsal and a ventral fin. Close to the margin of the latter, at the base of the tail, is the cloacal opening.
common ovoid mass, sharply distinguished from the long bilaterally compressed tail. The mouth is open and equipped with
+
 
horny raspers, while the oral gland is reduced to two vestiges
+
Endodermal derivatives. — The mouth has been formed by the breaking through of the oral membrane. From the pharynx, all the visceral pouches except the hyomandibular and the vestigial sixth pouch open to the exterior as visceral clefts; and demibranchs have arisen on the anterior and posterior margins of the third, fourth, and fifth visceral arches and on the anterior margin of the sixth. These are the internal gills which hang down into the opercular cavity. The epithelial bodies from the hyomandibular pouch have disappeared. Those from the second pouch form the thymus gland, while similar buds arise from the third and fourth but presently disappear. The ventral epithelial bodies of the second pouch are said to give rise to the carotid gland, and those of the third and fourth to “ parathyroids.” The fifth pouch never gains communication with the exterior but gives rise to the ultimobranchial bodies. The thyroid is now separated from the pharynx. In the tadpole the pulmonary organs consist of a pair of thin-walled sacs, the lungs, arising from a laryngeal cavity connected with the pharynx by a narrow opening, the glottis. Posterior to the pharynx comes the esophagus, which was occluded just before the opening of the mouth but now possesses a narrow lumen opening into the stomach, which is not greatly dilated. The vesicle, which formerly represented the liver, persists as the gall bladder and common bile duct, rela288 THE ANATOMY OF FROG EMBRYOS
on the ventral side of the head. On the dorsal surface, the large
 
eyes, now functional, protrude slightly. Anterior to these are
 
the external openings of the nasal tubes (external nares). The
 
external gills, which were developing at hatching, have now degenerated and been replaced by internal gills concealed from view
 
by the opercula. On the left side, the opercular aperture serves
 
as a means of egress for the water from which the gills obtain
 
their oxygen. The tail, now two-thirds the length of the tadpole, has a dorsal and a ventral fin. Close to the margin of the
 
latter, at the base of the tail, is the cloacal opening.
 
  
Endodermal derivatives. — The mouth has been formed by
 
the breaking through of the oral membrane. From the pharynx,
 
all the visceral pouches except the hyomandibular and the
 
vestigial sixth pouch open to the exterior as visceral clefts; and
 
demibranchs have arisen on the anterior and posterior margins of
 
the third, fourth, and fifth visceral arches and on the anterior
 
margin of the sixth. These are the internal gills which hang down
 
into the opercular cavity. The epithelial bodies from the hyomandibular pouch have disappeared. Those from the second pouch
 
form the thymus gland, while similar buds arise from the third
 
and fourth but presently disappear. The ventral epithelial
 
bodies of the second pouch are said to give rise to the carotid
 
gland, and those of the third and fourth to “ parathyroids.”
 
The fifth pouch never gains communication with the exterior
 
but gives rise to the ultimobranchial bodies. The thyroid is
 
now separated from the pharynx. In the tadpole the pulmonary
 
organs consist of a pair of thin-walled sacs, the lungs, arising
 
from a laryngeal cavity connected with the pharynx by a narrow
 
opening, the glottis. Posterior to the pharynx comes the esophagus, which was occluded just before the opening of the mouth but
 
now possesses a narrow lumen opening into the stomach, which is
 
not greatly dilated. The vesicle, which formerly represented the
 
liver, persists as the gall bladder and common bile duct, rela288 THE ANATOMY OF FROG EMBRYOS
 
  
   
 
  
Internal
+
Internal gills
gills
 
  
 
Intestine
 
Intestine
  
Fig. 194. — 11 mm. frog larva.1_ Transparent preparation viewed from right side.
+
Fig. 194. — 11 mm. frog larva.1_ Transparent preparation viewed from right side. X15.
X15.
+
 
 +
1 Figs. 194-198 inclusive are from preparations loaned me by Dr. A. R. Cahn. In earlier editions they were labelled 9 mm., as measured after preservation. THE YOUNG TADPOLE 289
  
1 Figs. 194-198 inclusive are from preparations loaned me by Dr. A. R. Cahn.
 
In earlier editions they were labelled 9 mm., as measured after preservation.
 
THE YOUNG TADPOLE 289
 
  
   
 
  
 
Infundibulum
 
Infundibulum
  
Stomach
+
Stomach Notochord Intestine Dorsal aorta Yolk Muscles of tail
Notochord
+
 
Intestine
+
ie Fig. 195. — 11 mm. frog larva. Sagittal section, anterior part. 40. 290 THE ANATOMY OF FROG EMBRYOS
Dorsal aorta Yolk
 
Muscles
 
of tail
 
  
ie
+
tively small in comparison with the great glandular mass of the liver. Although the pancreas arose from paired primordia of the duodenum, these have now shifted their position so that their ducts open into the common bile duct. The intestine is extremely long and coiled into a double spiral. It terminates in a slightly dilated rectum, opening into the cloacal cavity which also receives the pronephric ducts and opens to the exterior by the cloacal aperture.
Fig. 195. — 11 mm. frog larva. Sagittal section, anterior part. 40.
 
290 THE ANATOMY OF FROG EMBRYOS
 
  
tively small in comparison with the great glandular mass of the
+
Mesodermal derivatives. — The notochord has elongated toward the posterior end, accompanying the growth of the tail. The two most anterior somites have disappeared, leaving eleven in the trunk region and a much larger and variable number in
liver. Although the pancreas arose from paired primordia of the
 
duodenum, these have now shifted their position so that their
 
ducts open into the common bile duct. The intestine is extremely long and coiled into a double spiral. It terminates in a
 
slightly dilated rectum, opening into the cloacal cavity which also
 
receives the pronephric ducts and opens to the exterior by the
 
cloacal aperture.
 
  
Mesodermal derivatives. — The notochord has elongated toward the posterior end, accompanying the growth of the tail.
 
The two most anterior somites have disappeared, leaving eleven
 
in the trunk region and a much larger and variable number in
 
  
 
  
 
Fig. 196. — 11 mm. frog larva. Transverse section, through eye. X40.
 
Fig. 196. — 11 mm. frog larva. Transverse section, through eye. X40.
  
the tail. In the tail the myotomes have given rise to the dorsal
+
the tail. In the tail the myotomes have given rise to the dorsal and ventral musculature. The pronephros has become larger and more complicated through the branching of the pronephric tubules. The coelom consists of a pericardial cavity containing the heart, whose myocardia have disappeared, and an abdominal cavity in which the gut is suspended by the dorsal mesentery. These cavities are still continuous up to the time of metamorphosis. In the heart the sinus venosus is now a large transverse sac; the atrium is partially divided by the interatrial septum; the ventricle has thick muscular walls; and the short bulbus opens into the ventral aorta which is divided into proximal and distal portions by a pair of valves. The ventral aorta is divided into THE YOUNG TADPOLE 291
and ventral musculature. The pronephros has become larger
 
and more complicated through the branching of the pronephric
 
tubules. The coelom consists of a pericardial cavity containing
 
the heart, whose myocardia have disappeared, and an abdominal
 
cavity in which the gut is suspended by the dorsal mesentery.
 
These cavities are still continuous up to the time of metamorphosis. In the heart the sinus venosus is now a large transverse sac;
 
the atrium is partially divided by the interatrial septum; the
 
ventricle has thick muscular walls; and the short bulbus opens
 
into the ventral aorta which is divided into proximal and distal
 
portions by a pair of valves. The ventral aorta is divided into
 
THE YOUNG TADPOLE 291
 
  
four afferent branchial arteries, the ventral portions of aortic
+
four afferent branchial arteries, the ventral portions of aortic arches III-VI. From these the blood passes through the internal gills by means of capillaries and is conveyed to four efferent branchial arteries, the dorsal portions of the aortic arches referred to above, which in turn lead to the dorsal aortae. The carotid arteries are connected in front of and behind the infundibulum by commissural vessels, and continue forward as the anterior cerebral arteries. From the anterior commissure the basilars run backward and the anterior palatines forward. The pharyngeal
arches III-VI. From these the blood passes through the internal
 
gills by means of capillaries and is conveyed to four efferent
 
branchial arteries, the dorsal portions of the aortic arches referred
 
to above, which in turn lead to the dorsal aortae. The carotid
 
arteries are connected in front of and behind the infundibulum by
 
commissural vessels, and continue forward as the anterior cerebral arteries. From the anterior commissure the basilars run
 
backward and the anterior palatines forward. The pharyngeal
 
  
 
Myelencephalon
 
Myelencephalon
  
 
+
 
  
 
Otic vesicle
 
Otic vesicle
  
 
+
 
  
 
Horizontal
 
Horizontal
Line 519: Line 250:
 
Fig. 197. — 11 mm. frog larva. Transverse section through ear. X40.
 
Fig. 197. — 11 mm. frog larva. Transverse section through ear. X40.
  
artery, running forward from the dorsal aorta, at a point just
+
artery, running forward from the dorsal aorta, at a point just posterior to the anterior commissure, represents the dorsal portion of the mandibular arch; the lingual artery arises independently and unites with the first efferent branchial. From the efferent branchial arteries of the sixth arch, the pulmonary arteries grow backward to the lungs. The vitelline veins have been broken up, by their inclusion in the developing liver, into hepatic veins, opening into the sinus venosus, and hepatic-portal veins from the intestine. The anterior cardinal veins are formed by the union of the superior jugular and facial veins and empty into the common cardinals. From the ventral side of the head the inferior jugulars drain into the common cardinals. The posterior somatic veins are the posterior cartlinals, which return the blood from the 292 THE ANATOMY OF FROG EMBRYOS
posterior to the anterior commissure, represents the dorsal portion
 
of the mandibular arch; the lingual artery arises independently
 
and unites with the first efferent branchial. From the efferent
 
branchial arteries of the sixth arch, the pulmonary arteries grow
 
backward to the lungs. The vitelline veins have been broken
 
up, by their inclusion in the developing liver, into hepatic veins,
 
opening into the sinus venosus, and hepatic-portal veins from the
 
intestine. The anterior cardinal veins are formed by the union
 
of the superior jugular and facial veins and empty into the common cardinals. From the ventral side of the head the inferior
 
jugulars drain into the common cardinals. The posterior somatic
 
veins are the posterior cartlinals, which return the blood from the
 
292 THE ANATOMY OF FROG EMBRYOS
 
  
region of the pronephros into the common cardinals. The lymphatic vessels of the tadpole have arisen from the confluence of
+
region of the pronephros into the common cardinals. The lymphatic vessels of the tadpole have arisen from the confluence of numerous, small intercellular spaces in the mesenchyme. Ectodermal derivatives. — The epidermis is no longer ciliated. The cerebral vesicle is now subdivided into right and left portions, while immediately behind this is the choroid plexus of the diencephalon. The pineal gland is connected with the diencephalon by:a small stalk; the pituitary gland has lost all connection with the exterior. In the mesencephalon the optic
numerous, small intercellular spaces in the mesenchyme.
 
Ectodermal derivatives. — The epidermis is no longer ciliated.
 
The cerebral vesicle is now subdivided into right and left portions, while immediately behind this is the choroid plexus of
 
the diencephalon. The pineal gland is connected with the
 
diencephalon by:a small stalk; the pituitary gland has lost all
 
connection with the exterior. In the mesencephalon the optic
 
  
 
Neural tube
 
Neural tube
  
   
 
 
  
Notochord
 
  
 
 
  
Intestine
+
Notochord
  
Fig. 198. — 11 mm. frog larva. Transverse section through pronephros. X40.
 
  
lobes are just apparent. The metencephalon is distinguishable
 
by the thickness of its walls as compared with the choroid plexus
 
of the myelencephalon. All cranial nerves and spinal nerves
 
are now established. The eye now contains all elements necessary for functioning; rods and cones of the sensory layer connect
 
with the neurons of the optic nerve; pigment is deposited in
 
the pigment layer; the choroid and sclerotic layers have been
 
formed from mesenchyme; the lens is transparent, as is the
 
cornea formed from the ectoderm. The otocyst is partially
 
divided by a dorsal partition into an outer saccule and inner
 
utricle. The nasal pits have grown backward as solid rods
 
which by now have acquired lumina and will soon open into the
 
  
pharynx.
 
REFERENCES 293
 
  
Diencephalon
+
Intestine
  
Ear
+
Fig. 198. — 11 mm. frog larva. Transverse section through pronephros. X40.
  
Notochord
+
lobes are just apparent. The metencephalon is distinguishable by the thickness of its walls as compared with the choroid plexus of the myelencephalon. All cranial nerves and spinal nerves are now established. The eye now contains all elements necessary for functioning; rods and cones of the sensory layer connect with the neurons of the optic nerve; pigment is deposited in the pigment layer; the choroid and sclerotic layers have been formed from mesenchyme; the lens is transparent, as is the cornea formed from the ectoderm. The otocyst is partially divided by a dorsal partition into an outer saccule and inner utricle. The nasal pits have grown backward as solid rods which by now have acquired lumina and will soon open into the
  
Pronephros
 
 
   
 
 
Wall of
 
Intestine
 
  
 
Fig. 199. — 11 mm. frog larva. Trontal section through nose, eye, and ear. 40.
 
Fig. 199. — 11 mm. frog larva. Trontal section through nose, eye, and ear. 40.
  
REFERENCES
+
===References===
  
 
Huxley, J. S., and de Beer, G. R. 1934. The Elements of Experimental Embryology, Chap. 2.
 
Huxley, J. S., and de Beer, G. R. 1934. The Elements of Experimental Embryology, Chap. 2.
Line 599: Line 287:
 
Morgan, T. H. 1897. The Development of the Frog’s Egg.
 
Morgan, T. H. 1897. The Development of the Frog’s Egg.
  
Zeigler, H. E. 1902. Lehrbuch der vergleichenden Entwickelungsgeschichte der
+
Zeigler, H. E. 1902. Lehrbuch der vergleichenden Entwickelungsgeschichte der niederen Wirbeltiere.  
niederen Wirbeltiere. .
+
 
CHAPTER XII
+
==Chapter XII The Anatomy Of Chick Embryos==
THE ANATOMY OF CHICK EMBRYOS
 
  
The traditional stages in the development of the chick Gallus
+
The traditional stages in the development of the chick Gallus domesticus) for laboratory practice are those at the end of each of the first three days of incubation. So many important changes take place during the second day, however, that it is advisable to study an additional stage intermediate between twenty-four and forty-eight hours in age. The chick of thirty-three hours is selected because the form of the embryo is not yet affected by torsion or flexure, and the headfold of the amnion has not yet slipped over the head of the chick.
domesticus) for laboratory practice are those at the end of each
 
of the first three days of incubation. So many important changes
 
take place during the second day, however, that it is advisable
 
to study an additional stage intermediate between twenty-four
 
and forty-eight hours in age. The chick of thirty-three hours
 
is selected because the form of the embryo is not yet affected by
 
torsion or flexure, and the headfold of the amnion has not yet
 
slipped over the head of the chick.
 
  
As it is a well-known fact that, in these first few days of incubation, embryos of the same age have attained varying degrees of
+
As it is a well-known fact that, in these first few days of incubation, embryos of the same age have attained varying degrees of development, the length of the embryo has been proposed as a mark of identification. The flexures of the body, however, make this standard impracticable, and the remaining alternative is to select the specific development of some particular structure as a basis of arrangement. For this purpose the number of somites, suggested by Lillie, is admirable. Still, it must be remembered that on account of the effect of temperature upon the rate of development, the number of somites is not correlated exactly with the number of hours of incubation, as may be seen from the following table.
development, the length of the embryo has been proposed as a
 
mark of identification. The flexures of the body, however,
 
make this standard impracticable, and the remaining alternative
 
is to select the specific development of some particular structure
 
as a basis of arrangement. For this purpose the number of
 
somites, suggested by Lillie, is admirable. Still, it must be remembered that on account of the effect of temperature upon the
 
rate of development, the number of somites is not correlated
 
exactly with the number of hours of incubation, as may be seen
 
from the following table.
 
  
 
  
 
  
TABLE 12
+
 
Duval Keibel Lillie Patten
+
 
About 24 hours Fig. 76 Vig. 9, 9A Vig. 59 Fig. 36
+
TABLE 12 Duval Keibel Lillie Patten About 24 hours Fig. 76 Vig. 9, 9A Vig. 59 Fig. 36
  
 
(24 hrs. 6S) (24 hrs. 7-88) (25 hrs. 7S) (27 hrs. 8S)
 
(24 hrs. 6S) (24 hrs. 7-88) (25 hrs. 7S) (27 hrs. 8S)
  
 
  
About 33 hours Fig. 93 Fig. 10, 10A Fig. 68 Fig. 39
 
(33 hrs. 168) (32 hrs. 9 S) (33 hrs. 128) (33 hrs. 128)
 
  
+
About 33 hours Fig. 93 Fig. 10, 10A Fig. 68 Fig. 39 (33 hrs. 168) (32 hrs. 9 S) (33 hrs. 128) (33 hrs. 128)
  
About 48 hours Fig. 109 Fig. 16, 16A Fig. 93 Fig.
 
(48 hrs. 27-28S)| (52 hrs. 278) a8 hrs. 278) (55 hrs. 2 S)
 
  
 
  
About 72 hours Fig. 115 Fig. 18, 18A g. 117 g. 63
+
About 48 hours Fig. 109 Fig. 16, 16A Fig. 93 Fig. (48 hrs. 27-28S)| (52 hrs. 278) a8 hrs. 278) (55 hrs. 2 S)
(68 hrs. 37S) {(67 hrs. 35-37 S) (ak 18s, 35S) ah ee 368)
 
  
 
  
 
  
+
About 72 hours Fig. 115 Fig. 18, 18A g. 117 g. 63 (68 hrs. 37S) {(67 hrs. 35-37 S) (ak 18s, 35S) ah ee 368)
  
 
  
 
  
° 294
 
TWENTY-FOUR HOURS 295
 
  
A. THE TWENTY-FOUR HOUR STAGE
 
  
At the end of the first day of incubation, the chick embryo has
 
completed the period of cleavage (pages 98, 105) and germ-layer
 
formation (pages 111, 121), and is in the early stages of organogeny.
 
  
Anterior neuropore
 
  
Head fold Proamnionrn t hy
 
  
 
  
     
 
   
 
 
 
 
 
 
 
 
 
 
 
 
 
  
Amnio te Anteriorcardiac ntestinal portal
 
vesicle Neural fold
 
  
Neural
+
° 294 TWENTY-FOUR HOURS 295
groove
 
  
4th
+
A. THE TWENTY-FOUR HOUR STAGE
Somite
 
  
Area &
+
At the end of the first day of incubation, the chick embryo has completed the period of cleavage (pages 98, 105) and germ-layer formation (pages 111, 121), and is in the early stages of organogeny.
pellucida
 
Primitive
 
knot
 
  
Primitive
 
streak
 
  
Area
+
Fig. 200. — 24 hour chick embryo. Cleared preparation from dorsal side. X25.
vasculosa
 
  
Fie. 200. — 24 hour chick embryo. Cleared preparation from dorsal side. X25.
+
External form. — The embryo, 3.3 mm. in length, lies along the axial line of the slipper-shaped area pellucida which in turn is surrounded by the crescent-shaped area vasculosa, whose anterior horns, separated by the proamnion, reach about to the level of 296 THE ANATOMY OF CHICK EMBRYOS
  
External form. — The embryo, 3.3 mm. in length, lies along
+
tip of the head. At the anterior end, the head fold of the embryo is lifted above the proamnion from which it is separated by the subcephalic pocket. In the head fold is contained the fore-gut,
the axial line of the slipper-shaped area pellucida which in turn is
 
surrounded by the crescent-shaped area vasculosa, whose anterior
 
horns, separated by the proamnion, reach about to the level of
 
296 THE ANATOMY OF CHICK EMBRYOS
 
 
 
tip of the head. At the anterior end, the head fold of the embryo
 
is lifted above the proamnion from which it is separated by the
 
subcephalic pocket. In the head fold is contained the fore-gut,
 
  
 
0.59 mm. in length, which opens at its
 
0.59 mm. in length, which opens at its
Line 724: Line 344:
 
means of the anterior intestinal portal.
 
means of the anterior intestinal portal.
  
On either margin of the portal the pri
+
On either margin of the portal the pri mordia of the vitelline veins are to be
mordia of the vitelline veins are to be
 
  
recognized in thick bands of splanchnic
+
recognized in thick bands of splanchnic mesoderm. The neural plate has already given rise to the neural folds which extend back as far as the first somite. They have united just posterior to the region where the optic vesicles are _ to appear and thus have given rise to a neural tube 0.3 mm. in length, which is widely open in front and behind as the anterior and posterior neuropores, respectively. Behind the head fold the axial mesoderm is segmented into six somites. Between the neural folds the notochord can be recognized as a faint line which joins, at its posterior end, the
mesoderm. The neural plate has already given rise to the neural folds
 
which extend back as far as the first
 
somite. They have united just posterior
 
to the region where the optic vesicles are
 
_ to appear and thus have given rise to a
 
neural tube 0.3 mm. in length, which is
 
widely open in front and behind as the
 
anterior and posterior neuropores, respectively. Behind the head fold the
 
axial mesoderm is segmented into six
 
somites. Between the neural folds the
 
notochord can be recognized as a faint
 
line which joins, at its posterior end, the
 
  
 
primitive streak, now reduced to 0.83
 
primitive streak, now reduced to 0.83
Line 745: Line 352:
 
mm. in length.
 
mm. in length.
  
Endodermal derivatives. — The only
+
Endodermal derivatives. — The only differentiation which has taken place in the endoderm consists of the establishment of the fore-gut by means of the folding off of the head from the proamnion. As this process continues the fore gut will be lengthened at the expense of
differentiation which has taken place in
 
the endoderm consists of the establishment of the fore-gut by means of the
 
folding off of the head from the proamnion. As this process continues the fore
 
gut will be lengthened at the expense of
 
  
Fia. 201. — 24 hour chick em- the widely open mid-gut, and the an
+
Fia. 201. — 24 hour chick em- the widely open mid-gut, and the an ean Sagittal section. terior intestinal portal: will progress steadily backward.
ean Sagittal section. terior intestinal portal: will progress
 
steadily backward.
 
  
Mesodermal derivatives. — The mesoderm proper does not
+
Mesodermal derivatives. — The mesoderm proper does not extend into the head, but a loose aggregate of mesenchyme derived from it is present. Posterior to the head the axial mesoderm is divided into six somites. Transverse sections show that TWENTY-FOUR HOURS 297
extend into the head, but a loose aggregate of mesenchyme
 
derived from it is present. Posterior to the head the axial mesoderm is divided into six somites. Transverse sections show that
 
TWENTY-FOUR HOURS 297
 
  
 
Epidermis , , Brain
 
Epidermis , , Brain
  
+
 
 
+
 
   
+
 
+
 
  
 
Mesenchyme
 
Mesenchyme
  
Splanchnic
+
Splanchnic mesoderm
mesoderm
 
  
 
Proamnion
 
Proamnion
  
 
  
Somatic mesoderm
 
Fore-gut
 
  
 
+
Somatic mesoderm Fore-gut
 +
 
 +
 
  
 
Ectoderm Endoderm.
 
Ectoderm Endoderm.
  
Fig. 202. — 24 hour chick embryo. Transverse section through brain region. The
+
Fig. 202. — 24 hour chick embryo. Transverse section through brain region. The neural folds have met but are not yet fused together. X50.
neural folds have met but are not yet fused together. X50.
 
  
   
 
  
Axial mesoderm Notochord
 
ge ie
 
See
 
SO ete Ry
 
oe ayer
 
Vitelline vein Amnio-cardiac Splanchnopleure
 
vesicle
 
  
Fig. 203. — 24 hour chick embryo. Transverse section through region of intestinal
+
Axial mesoderm Notochord ge ie See SO ete Ry oe ayer Vitelline vein Amnio-cardiac Splanchnopleure vesicle
portal. X50.
+
 
 +
Fig. 203. — 24 hour chick embryo. Transverse section through region of intestinal portal. X50.
  
 
Neural groove
 
Neural groove
  
| Somite IY
+
| Somite IY Ectoderm
Ectoderm
 
  
 
Exocoel
 
Exocoel
  
 
 
   
 
 
   
 
 
 
  
PO Blood island
 
nom i ram) Canes.
 
  
Notochord So
 
Endoderm ~
 
  
Fig. 204. — 24 hour chick embryo. Transverse section through fourth somite.
+
 
X50.
+
 
 +
 
 +
 
 +
PO Blood island nom i ram) Canes.
 +
 
 +
Notochord So Endoderm ~
 +
 
 +
Fig. 204. — 24 hour chick embryo. Transverse section through fourth somite. X50.
  
 
Intermediate mesoderm
 
Intermediate mesoderm
Line 828: Line 412:
 
Endoderm
 
Endoderm
  
HH, BY
+
HH, BY Fig. 205. — 24 hour chick embryo. Transverse section through primitive streak. x50. 298 THE ANATOMY OF CHICK EMBRYOS
Fig. 205. — 24 hour chick embryo. Transverse section through primitive streak.
 
x50.
 
298 THE ANATOMY OF CHICK EMBRYOS
 
  
each has a minute cavity, or myocoel. The intermediate mesoderm does not divide into nephrotomes as in the frog. The
+
each has a minute cavity, or myocoel. The intermediate mesoderm does not divide into nephrotomes as in the frog. The lateral mesoderm is divided into the somatic and splanchnic layers. In the latter, numerous blood islands appear and give the characteristic mottled appearance to the area vasculosa. The coelom of the embryo is continuous with that of the extra-embryonic regions, or exocoel. In the region on either side of the head, between the proamnion and the intestinal portal, the coelom is distended into an amniocardiac vesicle, so called because the somatopleure will contribute to the head fold of the amnion, while the splanchnic mesoderm will give rise to the primordia of the heart, and the cavities of the vesicles will unite to form the pericardial cavity. The notochord, from its point of origin, the primitive streak, extends forward into the head.
lateral mesoderm is divided into the somatic and splanchnic
 
layers. In the latter, numerous blood islands appear and give
 
the characteristic mottled appearance to the area vasculosa. The
 
coelom of the embryo is continuous with that of the extra-embryonic regions, or exocoel. In the region on either side of the head,
 
between the proamnion and the intestinal portal, the coelom is
 
distended into an amniocardiac vesicle, so called because the somatopleure will contribute to the head fold of the amnion, while
 
the splanchnic mesoderm will give rise to the primordia of the
 
heart, and the cavities of the vesicles will unite to form the
 
pericardial cavity. The notochord, from its point of origin, the
 
primitive streak, extends forward into the head.
 
  
Ectodermal derivatives. — The ectoderm at this stage consists of the elongate neural plate, with its groove and folds which
+
Ectodermal derivatives. — The ectoderm at this stage consists of the elongate neural plate, with its groove and folds which are already in process of fusion, and the epidermis or non-nervous ectoderm.
are already in process of fusion, and the epidermis or non-nervous
 
ectoderm.
 
  
 
B. THE THIRTY-THREE HOUR STAGE
 
B. THE THIRTY-THREE HOUR STAGE
  
External form. — In the chick embryo, after thirty-three hours’
+
External form. — In the chick embryo, after thirty-three hours’ incubation, the length has increased to 4.3 mm. There is a slight bending of the head downward over the end of the notochord, foreshadowing the cranial flexure. The area vasculosa, in which the blood islands are being converted into capillaries, now has grown in toward the embryo, so that the area pellucida persists only around the head and tail regions. The anterior horns of the area vasculosa have met in front, completely inclosing the proamnion. The head has increased in length not only by actual forward growth but also by the backward extension of the lateral margins of the head fold, so that the enclosed foregut is now 1 mm. long. The vitelline veins are prominent at the margins of the intestinal portal and continue on the ventral side of the fore-gut to meet at the posterior end of the heart, which is now a single tube, slightly bent toward the right. The neural folds are fused as far back as the eleventh somite, where the posterior neuropore is now known as the rhomboidal sinus. The anterior neuropore is about to close, and in the head the neural tube shows three regions of dilation which represent the THIRTY-THREE HOURS 299
incubation, the length has increased to 4.3 mm. There is a
+
 
slight bending of the head downward over the end of the notochord, foreshadowing the cranial flexure. The area vasculosa,
+
Head fold |. of amnion
in which the blood islands are being converted into capillaries,
+
 
now has grown in toward the embryo, so that the area pellucida
+
 
persists only around the head and tail regions. The anterior
 
horns of the area vasculosa have met in front, completely inclosing the proamnion. The head has increased in length not
 
only by actual forward growth but also by the backward extension
 
of the lateral margins of the head fold, so that the enclosed foregut is now 1 mm. long. The vitelline veins are prominent at
 
the margins of the intestinal portal and continue on the ventral
 
side of the fore-gut to meet at the posterior end of the heart,
 
which is now a single tube, slightly bent toward the right. The
 
neural folds are fused as far back as the eleventh somite, where
 
the posterior neuropore is now known as the rhomboidal sinus.
 
The anterior neuropore is about to close, and in the head the
 
neural tube shows three regions of dilation which represent the
 
THIRTY-THREE HOURS 299
 
  
Head fold |.
 
of amnion
 
  
 
+
4 neuropore Prosencephalon
 
 
  
4 neuropore
+
Optic
Prosencephalon
+
7 vesicle Mesencephalon Foregut Rhombencephalon & Heart
: Optic
 
7 vesicle
 
Mesencephalon
 
Foregut
 
Rhombencephalon
 
& Heart
 
  
- Vitelline
+
- Vitelline vein
vein
 
  
 
Somite 6
 
Somite 6
  
Sinus
+
Sinus rhomboidialig
rhomboidialig
+
 
 +
Primitive streak
  
Primitive
+
Fig. 206. — 33 hour chick embryo. Cleared preparation from dorsal view. X25. 300 THE ANATOMY OF CHICK EMBRYOS
streak
 
  
Fig. 206. — 33 hour chick embryo. Cleared preparation from dorsal view. X25.
+
fore-brain, mid-brain, and hind-brain, respectively. The sides of the fore-brain are evaginating to produce the optic vesicles.
300 THE ANATOMY OF CHICK EMBRYOS
 
  
fore-brain, mid-brain, and hind-brain, respectively. The sides
+
Head fold . of amnion
of the fore-brain are evaginating to produce the optic vesicles.
 
  
Head fold .
+
Prosencephalon— Subcephalic F pocket Mesencephalon Fore-gut Pericardial cavity
of amnion
 
  
Prosencephalon—
 
Subcephalic F
 
pocket
 
Mesencephalon
 
Fore-gut
 
Pericardial
 
cavity
 
  
 
  
 
Rhombencephalon# 6.4. /"
 
Rhombencephalon# 6.4. /"
Line 924: Line 460:
 
are ~ Anterior
 
are ~ Anterior
  
intestinal
+
intestinal portal
portal
 
  
i Primitive
+
i Primitive cm, streak
cm, streak
 
  
 
aN
 
aN
Line 934: Line 468:
 
X25.
 
X25.
  
 
 
 
 
 
 
 
 
 
 
 
 
  
In the hind-brain, five neuromeres
 
can be identified. Twelve somites
 
may be counted. The notochord
 
extends forward to the fore-brain
 
from the primitive streak which is
 
now reduced to 0.3 mm.
 
  
Endodermal derivatives. — The
 
anterior end of the fore-gut is in
 
contact ventrally with the stomodeum separated only by the oral
 
plate, composed of ectoderm and
 
endoderm. At the sides, the walls
 
of the fore-gut are fused to the ectoderm at points where the first visceral pouches (hyomandibular) will
 
be located.
 
  
Mesodermal derivatives. — The
 
somites now number twelve, and
 
myocoels are still apparent. The
 
mesomere is still unsegmented, but
 
pronephric tubules have appeared
 
in the region corresponding to somites 5-12. The four posterior
 
tubules are growing back to form
 
the pronephric duct. In the
 
splanchnic mesoderm the blood
 
islands are being converted into
 
capillaries. The vitelline veins are
 
prominent and continue forward
 
into the heart, of which the endo
 
-cardium and myocardium are dis
 
tinct. The heart is supported by
 
the dorsal mesocardium, the ventral
 
mesocardium having disappeared.
 
The primordial tubes, from the
 
  
fusion of which the heart arose, continue forward as the ventral
 
aortae which bend around the pharynx (first aortic arches) and
 
THIRTY-THREE HOURS 301
 
  
continue backward along the dorsal surface of the pharynx as the
 
dorsal aortae. At the level of the primitive streak they are lost
 
in a capillary nexus which foreshadows the vitelline arteries.
 
From a point immediately in front of the optic vesicle, the anterior
 
cardinals course backward on either side of the neural tube, bending down ventrally to enter the heart with the vitelline veins.
 
The notochord is slightly bent at the anterior end.
 
  
Ectodermal derivatives. The ncural folds now extend to
+
 
the eleventh somite and have fused throughout the length of the
+
In the hind-brain, five neuromeres can be identified. Twelve somites may be counted. The notochord extends forward to the fore-brain from the primitive streak which is now reduced to 0.3 mm.
head. The anterior neuropore is almost closed. The three
+
 
 +
Endodermal derivatives. — The anterior end of the fore-gut is in contact ventrally with the stomodeum separated only by the oral plate, composed of ectoderm and endoderm. At the sides, the walls of the fore-gut are fused to the ectoderm at points where the first visceral pouches (hyomandibular) will be located.
 +
 
 +
Mesodermal derivatives. — The somites now number twelve, and myocoels are still apparent. The mesomere is still unsegmented, but pronephric tubules have appeared in the region corresponding to somites 5-12. The four posterior tubules are growing back to form the pronephric duct. In the splanchnic mesoderm the blood islands are being converted into capillaries. The vitelline veins are prominent and continue forward into the heart, of which the endo -cardium and myocardium are dis tinct. The heart is supported by the dorsal mesocardium, the ventral mesocardium having disappeared. The primordial tubes, from the
 +
 
 +
fusion of which the heart arose, continue forward as the ventral aortae which bend around the pharynx (first aortic arches) and THIRTY-THREE HOURS 301
 +
 
 +
continue backward along the dorsal surface of the pharynx as the dorsal aortae. At the level of the primitive streak they are lost in a capillary nexus which foreshadows the vitelline arteries. From a point immediately in front of the optic vesicle, the anterior cardinals course backward on either side of the neural tube, bending down ventrally to enter the heart with the vitelline veins. The notochord is slightly bent at the anterior end.
 +
 
 +
Ectodermal derivatives. The ncural folds now extend to the eleventh somite and have fused throughout the length of the head. The anterior neuropore is almost closed. The three
  
 
Prosencephalon
 
Prosencephalon
Line 998: Line 495:
 
=» _
 
=» _
  
ts wa
+
ts wa P Exocoel é OOF
P Exocoel é OOF
 
  
 
  
Splanchnopleure Sub-cephalic
 
pocket
 
  
Fia. 208. — 33 hour chick embryo. Transverse section through optic vesicles.
+
Splanchnopleure Sub-cephalic pocket
X50.
 
  
dilations which represent the prosencephalon, mesencephalon,
+
Fia. 208. — 33 hour chick embryo. Transverse section through optic vesicles. X50.
and rhombencephalon are distinct. From the prosencephalon
+
 
the two optic vesicles extend to the ectoderm of the sides of the
+
dilations which represent the prosencephalon, mesencephalon, and rhombencephalon are distinct. From the prosencephalon the two optic vesicles extend to the ectoderm of the sides of the head. Five neuromeres may be identified in the rhombencephalon. It has been asserted that in earlier stages three neuromeres may be identified in the prosencephalon and two in the mesencephalon, while the first of the five noted above has resulted from the fusion of two original neuromeres destined to give rise to the metencephalon. At about this time a shallow depression in the floor of the prosencephalon, just in front of the tip of the notochord, marks the appearance of the infundibulum. The auditory placodes may sometimes be seen in sections as thickenings at the level of the constriction separating the last two neuromeres on either side. 302 THE ANATOMY OF CHICK EMBRYOS
head. Five neuromeres may be identified in the rhombencephalon. It has been asserted that in earlier stages three neuromeres may be identified in the prosencephalon and two in the
 
mesencephalon, while the first of the five noted above has resulted from the fusion of two original neuromeres destined to
 
give rise to the metencephalon. At about this time a shallow
 
depression in the floor of the prosencephalon, just in front of the
 
tip of the notochord, marks the appearance of the infundibulum.
 
The auditory placodes may sometimes be seen in sections as
 
thickenings at the level of the constriction separating the last two
 
neuromeres on either side.
 
302 THE ANATOMY OF CHICK EMBRYOS
 
  
 
Rhombencephalon
 
Rhombencephalon
  
 
 
 
 
 
 
 
 
 
  
 
 
     
 
 
  
Notochord
+
 
Fore-gut Otic ( auditory) placode
+
 
Somatopleure TET Dorsal aorta
+
 
Dorsal [Bi soy 7S uateral sulcus
+
 
mesocardium B Feria
+
 
: 5 Way, Oe
+
 
ee ae
+
 
ox ed fhe & wou
+
 
ee se
+
Notochord Fore-gut Otic ( auditory) placode Somatopleure TET Dorsal aorta Dorsal [Bi soy 7S uateral sulcus mesocardium B Feria
Os a lamest ~
+
 
Endocardium Splanchnopleure
+
5 Way, Oe
Fig. 209. — 33 hour chick embryo. Transverse section through otic placodes.
+
ee ae ox ed fhe & wou ee se Os a lamest ~ Endocardium Splanchnopleure Fig. 209. — 33 hour chick embryo. Transverse section through otic placodes.
  
 
x50.
 
x50.
Line 1,051: Line 526:
 
Spinal cord
 
Spinal cord
  
 
+
 
+
 
 
+
 
+
 
  
 
Dorsal aorta Somite
 
Dorsal aorta Somite
  
Intermediate mesoderm
+
Intermediate mesoderm Exocoel
Exocoel
 
  
 
Se
 
Se
Line 1,065: Line 539:
 
Vitelline vein
 
Vitelline vein
  
Fie. 210. — 33 hour chick embryo. Transverse section through vitelline veins.
+
Fie. 210. — 33 hour chick embryo. Transverse section through vitelline veins. x50.
x50.
 
  
 
Spinal cord
 
Spinal cord
Line 1,072: Line 545:
 
Neural crest Somite
 
Neural crest Somite
  
   
 
 
 
 
  
Intermediate mesoderm
+
 
Somatic layer
+
 
 +
 
 +
Intermediate mesoderm Somatic layer
  
 
NAPRY
 
NAPRY
  
Fig. 211. — 33 hour chick embryo. Transverse section through sixth somite. 50.
+
Fig. 211. — 33 hour chick embryo. Transverse section through sixth somite. 50. FORTY-EIGHT HOURS . 803
FORTY-EIGHT HOURS . 803
+
 
 +
Cc. THE FORTY-EIGHT HOUR STAGE — External form. — The chick at the end of the second day of incubation has usually attained a length of 7 mm., but the form
 +
 
 +
of the body has been altered profoundly. As the head has been lifted away from the blastoderm, it has increased greatly in size,
 +
 
 +
 
 +
 
 +
 
 +
 
 +
 
 +
 
 +
 
 +
 
 +
 
  
Cc. THE FORTY-EIGHT HOUR STAGE —
 
External form. — The chick at the end of the second day of
 
incubation has usually attained a length of 7 mm., but the form
 
  
of the body has been altered profoundly. As the head has been
 
lifted away from the blastoderm, it has increased greatly in size,
 
  
 
 
 
 
 
 
 
 
 
 
 
   
 
 
 
 
 
  
 
  
Ww ir ot
+
 
y
+
Ww ir ot y h h, Mesencephalon ‘Rhombencephalon
h h, Mesencephalon
 
‘Rhombencephalon
 
  
 
Otic vesicle Optic cup
 
Otic vesicle Optic cup
  
Lens vesicle
+
Lens vesicle Visceral cleft I Prosencephalon
Visceral cleft I Prosencephalon
 
  
0
+
0 i
i
 
  
Sinus venosus—
+
Sinus venosus— Vitelline vein—
Vitelline vein—
 
  
Atrium
+
Atrium Bulbus arteriosus Ventricle
Bulbus arteriosus
 
Ventricle
 
  
Amniotic fold
+
Amniotic fold Somite XIV
Somite XIV
 
  
 
Vitelline artery
 
Vitelline artery
Line 1,135: Line 594:
 
Tail fold
 
Tail fold
  
aed cent
+
aed cent Fia. 212.—48 hour chick embryo. Transparent preparation from dorsal view (head from right side). X15.
Fia. 212.—48 hour chick embryo. Transparent preparation from dorsal view
 
(head from right side). X15.
 
  
and the cranial flexure, which was just appearing in the thirtythree hour chick, has become so pronounced that the anterior
+
and the cranial flexure, which was just appearing in the thirtythree hour chick, has become so pronounced that the anterior end of the head is directed backwards. With this growth and flexure the head is twisted normally to the right, until it lies on one side, a phenomenon known as torsion. At forty-eight hours, this torsion involves the chick as far back as the seventeenth somite. The posterior end of the chick lies in its original position, and at the extreme caudal end a tail fold is being formed. In the Fig. 213. — 48 hour chick embryo. (304)
end of the head is directed backwards. With this growth and
 
flexure the head is twisted normally to the right, until it lies on
 
one side, a phenomenon known as torsion. At forty-eight hours,
 
this torsion involves the chick as far back as the seventeenth
 
somite. The posterior end of the chick lies in its original position,
 
and at the extreme caudal end a tail fold is being formed. In the
 
Fig. 213. — 48 hour chick embryo.
 
(304)
 
  
 
frontal section due to torsion.
 
frontal section due to torsion.
Line 1,155: Line 604:
 
Head in sagittal section, somite region in
 
Head in sagittal section, somite region in
  
Stomodaeal
+
Stomodaeal plate Telencephalon.
plate
+
 
Telencephalon.
 
  
 
  
 
Diencephalon
 
Diencephalon
Line 1,165: Line 612:
 
Myelencephalon
 
Myelencephalon
  
+
 
  
 
Metencephalon
 
Metencephalon
  
 
  
 
  
 
  
Mesencephalon
 
FORTY-EIGHT HOURS 305
 
  
area vasculosa the capillaries have formed attachments with the
+
 
vitelline arteries and veins, and at the border of this area is a circular vessel, the sinus terminalis. The fore-gut is now 1.4 mm.
+
 
in length, and the first of the three visceral pouches now communicates to the exterior following the rupture of the closing
+
Mesencephalon FORTY-EIGHT HOURS 305
plate which separated it from the corresponding visceral groove.
+
 
The second and third visceral grooves are apparent, but their closing plates are still unperforated. In the visceral arches the first
+
area vasculosa the capillaries have formed attachments with the vitelline arteries and veins, and at the border of this area is a circular vessel, the sinus terminalis. The fore-gut is now 1.4 mm. in length, and the first of the three visceral pouches now communicates to the exterior following the rupture of the closing plate which separated it from the corresponding visceral groove. The second and third visceral grooves are apparent, but their closing plates are still unperforated. In the visceral arches the first three aortic arches are apparent, arising from the ventral aorta. The heart is now twisted so that the ventricular loop is upper Anterior cardinal vein Dorsal aorta
three aortic arches are apparent, arising from the ventral aorta.
 
The heart is now twisted so that the ventricular loop is upper
 
Anterior cardinal vein Dorsal aorta
 
  
 
Otic pit
 
Otic pit
  
 
 
 
 
   
 
  
 
 
  
eB
+
 
5
+
 
 +
 
 +
 
 +
 
 +
 
 +
eB 5
  
 
Chorion
 
Chorion
  
 
 
  
Yolk sac tent
+
 
Notochord eet Blood island
+
 
Visceral groove I a Pigment layer
+
Yolk sac tent Notochord eet Blood island Visceral groove I a Pigment layer
  
 
Visceral pouch I Hypophysis Sensory layer
 
Visceral pouch I Hypophysis Sensory layer
  
   
 
  
Fia. 214. — 48 hour chick embryo. Transverse section through otic pit and optic
 
cup. 650.
 
  
most. The vitelline veins are large and conspicuous, as are the
+
Fia. 214. — 48 hour chick embryo. Transverse section through otic pit and optic cup. 650.
vitelline arteries which leave the body at the level of the twentysecond somites. The neural tube is completely closed. In the
 
head the five definitive regions of the brain are outlined, the
 
prosencephalon having given rise to the telencephalon and diencephalon, and the rhombencephalon to the metencephalon and
 
myelencephalon. The eye is now in the optic cup stage, and the
 
invagination of the optic vesicle continues down the stalk to form
 
the choroid fissure. The lens is in the form of a pit which has
 
almost attained the vesicle stage. The ear is represented by an
 
otic pit which, owing to the cervical flexure, is about on a level
 
with the eye. There are twenty-seven somites at this stage.
 
The primitive streak is found only in the tail fold. At this time
 
306 THE ANATOMY OF CHICK EMBRYOS
 
  
the head fold of the amnion has grown back over the chick as
+
most. The vitelline veins are large and conspicuous, as are the vitelline arteries which leave the body at the level of the twentysecond somites. The neural tube is completely closed. In the head the five definitive regions of the brain are outlined, the prosencephalon having given rise to the telencephalon and diencephalon, and the rhombencephalon to the metencephalon and myelencephalon. The eye is now in the optic cup stage, and the invagination of the optic vesicle continues down the stalk to form the choroid fissure. The lens is in the form of a pit which has almost attained the vesicle stage. The ear is represented by an otic pit which, owing to the cervical flexure, is about on a level with the eye. There are twenty-seven somites at this stage. The primitive streak is found only in the tail fold. At this time 306 THE ANATOMY OF CHICK EMBRYOS
far as the sixteenth somite.
 
  
Endodermal derivatives. — The stomodeum, an ectodermal
+
the head fold of the amnion has grown back over the chick as far as the sixteenth somite.
invagination from the ventral surface of the head fold, has formed
 
the oral membrane by contact with the fore-gut a little back of its
 
most anterior point. Hence there is a blind pocket in front of
 
the oral plate, known as the preoral gut. Three visceral pouches
 
are present, the first of which opens into the corresponding visceral
 
furrow following the rupture of its closing membrane. The
 
primordium of the thyroid is represented by a ventral depression
 
in the floor of the pharynx at the level of the second visceral
 
pouches. The primordia of the lungs (sometimes difficult to
 
distinguish) extend to the level of the sinus venosus. The liver
 
arises at the level of the anterior intestinal portal from two
 
evaginations of the endoderm, one below and one above the
 
meatus venosus. ‘The mid-gut now has two shifting boundaries,
 
the anterior intestinal portal and the posterior intestinal portal.
 
The latter is barely apparent as the opening of a shallow endodermal pocket or hind-gut in the tail fold.
 
  
Mesodermal derivatives.—-The somites, twenty-seven in
+
Endodermal derivatives. — The stomodeum, an ectodermal invagination from the ventral surface of the head fold, has formed the oral membrane by contact with the fore-gut a little back of its most anterior point. Hence there is a blind pocket in front of the oral plate, known as the preoral gut. Three visceral pouches are present, the first of which opens into the corresponding visceral furrow following the rupture of its closing membrane. The primordium of the thyroid is represented by a ventral depression in the floor of the pharynx at the level of the second visceral pouches. The primordia of the lungs (sometimes difficult to distinguish) extend to the level of the sinus venosus. The liver arises at the level of the anterior intestinal portal from two evaginations of the endoderm, one below and one above the meatus venosus. ‘The mid-gut now has two shifting boundaries, the anterior intestinal portal and the posterior intestinal portal. The latter is barely apparent as the opening of a shallow endodermal pocket or hind-gut in the tail fold.
number, show a varying degree of specialization, with the most
 
advanced at the anterior end. In these two regions can be distinguished: a loose aggregate of cells at the median ventral angle
 
(the sclerotome); and a cap of epithelial cells at the lateral dorsal
 
angle. The cells of this cap nearest the epidermis will form the
 
dermatome, while those nearest the neural tube will form the
 
myotome.
 
  
The pronephric tubules in the more anterior somites have disappeared and mesonephric tubules are appearing in the mesomere posterior to the thirteenth somite. The pronephric (now
+
Mesodermal derivatives.—-The somites, twenty-seven in number, show a varying degree of specialization, with the most advanced at the anterior end. In these two regions can be distinguished: a loose aggregate of cells at the median ventral angle (the sclerotome); and a cap of epithelial cells at the lateral dorsal angle. The cells of this cap nearest the epidermis will form the dermatome, while those nearest the neural tube will form the myotome.
the mesonephric) duct has acquired a lumen but has not yet
 
attained its complete backward growth.
 
  
The heart is still tubular, but the ventricular limb of the cardiac
+
The pronephric tubules in the more anterior somites have disappeared and mesonephric tubules are appearing in the mesomere posterior to the thirteenth somite. The pronephric (now the mesonephric) duct has acquired a lumen but has not yet attained its complete backward growth.
loop has grown back and over the atrial limb so that the ventricular region is now caudal and dorsal with relation to the
 
atrial region. Three aortic arches are present as a rule, but infrequently the third has not developed. From the first aortic arch
 
a network of capillaries extends into the head. From these the
 
carotid arteries will be formed. The dorsal aortae have fused
 
FORTY-FIGHT HOURS 307
 
  
from a point back of the sixth somite as far as the level of the
+
The heart is still tubular, but the ventricular limb of the cardiac loop has grown back and over the atrial limb so that the ventricular region is now caudal and dorsal with relation to the atrial region. Three aortic arches are present as a rule, but infrequently the third has not developed. From the first aortic arch a network of capillaries extends into the head. From these the carotid arteries will be formed. The dorsal aortae have fused FORTY-FIGHT HOURS 307
fifteenth somite. The vitelline arteries leave the dorsal aortae at
 
the level of the twenty-second somite but the aortae continue
 
  
Common cardinal Bulbus
+
from a point back of the sixth somite as far as the level of the fifteenth somite. The vitelline arteries leave the dorsal aortae at the level of the twenty-second somite but the aortae continue
vein arteriosus Chorion
+
 
: I.- poem 2s Cra, /
+
Common cardinal Bulbus vein arteriosus Chorion
Dorsal aorta Fore-gut er ,
+
 
bAY
+
I.- poem 2s Cra, /
 +
Dorsal aorta Fore-gut er , bAY
  
 
Notochord
 
Notochord
  
   
+
 
+
 
+
 
 
+
 
+
 
  
 
Spinal cord
 
Spinal cord
Line 1,292: Line 684:
 
as
 
as
  
Epidermis 5
+
Epidermis 5 J
J
 
  
 
Amnion
 
Amnion
  
Somiter
+
“ Somiter Coelom
Coelom
 
  
‘Dorsal mesocardium
+
‘Dorsal mesocardium Fia. 215. — 48 hour chick embryo. ‘Transverse section through heart. X50.
Fia. 215. — 48 hour chick embryo. ‘Transverse section through heart. X50.
 
  
backward as the caudal arteries to the last somite. The vitelline
+
backward as the caudal arteries to the last somite. The vitelline veins are fused at their point of entrance into the heart as the sinus venosus. The anterior cardinals are prominent and extend from a capillary plexus in the head back toward the heart, where they
veins are fused at their point of entrance into the heart as the sinus
 
venosus. The anterior cardinals are prominent and extend from
 
a capillary plexus in the head back toward the heart, where they
 
  
 
Somite
 
Somite
Line 1,312: Line 698:
 
Notochord | Dorsal aorta
 
Notochord | Dorsal aorta
  
 
+
 
 
+
 
 
+
 
 
+
 
 
+
 
   
+
 
  
 
Vitelline vein,
 
Vitelline vein,
Line 1,323: Line 709:
 
Choria
 
Choria
  
- Armnion— Gi
+
- Armnion— Gi N Coelom —
N
 
Coelom —
 
  
Posterior cardinal
+
Posterior cardinal vein
vein
 
  
 
Ventricle
 
Ventricle
  
 
+
 
  
 
Meatus Mid-gut: *
 
Meatus Mid-gut: *
Line 1,340: Line 723:
 
Fig. 216. — 48 hour chick embryo. Transverse section through liver. 50.
 
Fig. 216. — 48 hour chick embryo. Transverse section through liver. 50.
  
are joined by the posterior cardinals and proceed as the common
+
are joined by the posterior cardinals and proceed as the common cardinals to: enter the heart in the angles between the sinus venosus and the vitelline veins. The posterior cardinals may be traced back to the last somite. The heart of the chick commenced 308 THE ANATOMY OF CHICK EMBRYOS
cardinals to: enter the heart in the angles between the sinus
 
venosus and the vitelline veins. The posterior cardinals may be
 
traced back to the last somite. The heart of the chick commenced
 
308 THE ANATOMY OF CHICK EMBRYOS
 
  
beating at the forty-fourth hour of incubation, so that the course
+
beating at the forty-fourth hour of incubation, so that the course of the blood is through the ventral aorta to the aortic arches and thence to the dorsal aorta. From the first aortic arch a network of capillaries supplies the head with blood (which is returned by way of the anterior cardinals). The main current of the stream passes down the dorsal aortae to the point where these fuse to form the median dorsal aorta. From the dorsal aorta, the somites are supplied by capillaries, which will later become the intersegmental arteries. This blood is returned through the posterior cardinals. Leaving the dorsal aorta by way of the vitelline arteries, the blood passes through the capillaries of the area vasculosa to the sinus terminalis, and thence to the capillary drainage of the vitclline veins which return it to the heart.
of the blood is through the ventral aorta to the aortic arches and
 
thence to the dorsal aorta. From the first aortic arch a network
 
of capillaries supplies the head with blood (which is returned by
 
way of the anterior cardinals). The main current of the stream
 
passes down the dorsal aortae to the point where these fuse to
 
form the median dorsal aorta. From the dorsal aorta, the somites
 
are supplied by capillaries, which will later become the intersegmental arteries. This blood is returned through the posterior
 
cardinals. Leaving the dorsal aorta by way of the vitelline
 
arteries, the blood passes through the capillaries of the area
 
vasculosa to the sinus terminalis, and thence to the capillary
 
drainage of the vitclline veins which return it to the heart.
 
  
The notochord is bent, not only at its tip (cranial flexure) but
+
The notochord is bent, not only at its tip (cranial flexure) but also at the point where the myelencephalon merges with the spinal cord (cervical flexure).
also at the point where the myelencephalon merges with the spinal
 
cord (cervical flexure).
 
  
Ectodermal derivatives. — The brain now has acquired its
+
Ectodermal derivatives. — The brain now has acquired its five definitive vesicles. The telencephalon is enlarged but shows
five definitive vesicles. The telencephalon is enlarged but shows
 
  
 
Amniotic raphe
 
Amniotic raphe
  
 
 
 
 
  
+
 
 +
 
 +
 
 +
 
  
 
— Posterior cardinal vein
 
— Posterior cardinal vein
Line 1,380: Line 745:
 
Beast
 
Beast
  
Lateral sulcus
+
Lateral sulcus Fig. 217. —48 hour chick embryo. Transverse section through mesonephros. 50.
Fig. 217. —48 hour chick embryo. Transverse section through mesonephros.
 
50.
 
  
no particular differentiation. From the diencephalon project the
+
no particular differentiation. From the diencephalon project the constricted optic stalks which bear the optic cups with their inner sensory layer and outer pigmented layer. (The pigment will not arise until later.) The invagination by which the cups were formed continues down the stalk as the choroid groove. On the ventral surface of the diencephalon the infundibulum has deepened. Growing in toward it from the stomodeum is an ectodermal invagination, the hypophysis, which will fuse with the infundibulum to form the pituitary gland. The lens of the eye SEVENTY-TWO HOURS 309
constricted optic stalks which bear the optic cups with their inner
 
sensory layer and outer pigmented layer. (The pigment will
 
not arise until later.) The invagination by which the cups were
 
formed continues down the stalk as the choroid groove. On the
 
ventral surface of the diencephalon the infundibulum has deepened. Growing in toward it from the stomodeum is an ectodermal invagination, the hypophysis, which will fuse with the
 
infundibulum to form the pituitary gland. The lens of the eye
 
SEVENTY-TWO HOURS 309
 
  
is in the pit stage, resulting from the invagination of a sensory
+
is in the pit stage, resulting from the invagination of a sensory placode. When the process is complete, the lens will be a vesicle completely withdrawn beneath the surface of the ectoderm, as will the otic vesicle, the primordium of the inner ear. Along the rhombencephalon and cord, the neural crest is to be seen as a narrow band of cells on each dorso-lateral angle.
placode. When the process is complete, the lens will be a vesicle
 
completely withdrawn beneath the surface of the ectoderm, as
 
will the otic vesicle, the primordium of the inner ear. Along the
 
rhombencephalon and cord, the neural crest is to be seen as a
 
narrow band of cells on each dorso-lateral angle.
 
  
 
Myelencephalon
 
Myelencephalon
  
       
+
 
  
 
Otic vesicle %, Metencephalon
 
Otic vesicle %, Metencephalon
Line 1,408: Line 759:
 
he 4 te
 
he 4 te
  
Visceral
+
Visceral cleft I * q Mesencephalon wy Choroid fissure Optic cup Atrium and lens
cleft I *
+
 
q Mesencephalon
+
Diencephalon
wy Choroid fissure
+
Nasal pit - Epiphysis Telencephalon Ventricle Anterior limb bud 3 Somite 26 Vitelline 2 artery Vitelline ; . . : Posterior vem limb bud
Optic cup
+
 
Atrium and lens
+
Fig. 218. — 72 hour chick embryo. Transparent preparation from dorsal view, head seen from right side. X15.
: Diencephalon
+
 
Nasal pit - Epiphysis
+
D. THE SEVENTY-TWO HOUR STAGE External form. — At the end of the third day of incubation, the total length of the embryo is 9.5 mm., but the curvature of the body is so great, on account of the cranial and cervical flexures in addition to the newly developed caudal flexure, that the greatest length, from neck to tail, is 7 mm. Torsion involves the 310 THE ANATOMY OF CHICK EMBRYOS
Telencephalon
+
 
Ventricle
+
body as far back as the vitelline arteries and will become complete during the fourth day. Anterior and posterior limb buds are now apparent at the levels of somites 17-19 and 26-32 respectively. The tail is curved forward. The fore-gut is still 1.4 mm. in length but has undergone further differentiation, indicated externally by the fact that the first three visceral clefts are open while the fourth is still interrupted by its closing plate. In the branchial arches four aortic arches may be seen. The telencephalon has given rise to the primordia of the cerebral hemispheres, and from the roof of the diencephalon, a small evagination represents the epiphysis or primordium of the pineal gland. The eye and ear, which were formerly in the same transverse section, are now nearly in an antero-posterior relationship. The olfactory pits have made their appearance in the head. The semilunar (fifth cranial nerve), geniculo-acoustic (seventh and eighth), and petrosal (ninth) ganglia may be seen. There are approximately thirty-five somites. The primitive streak has disappeared. The amnion is completed by the fusion of head and tail folds. The allantois, a small sac-like evagination, protrudes ventrally between the posterior limb buds.
Anterior
+
 
limb bud
+
Endodermal derivatives. — At the end of the third day the oral aperture has been formed by the rupture of the oral membrane separating the stomodeum and the fore-gut. Immediately anterior to this opening the preoral gut persists. The fore-gut is still the same length as in the chick of forty-cight hours, but is more complex in structure. The thyroid gland, which appeared during the second day, has now become differentiated into the distal dilation which will give rise to the gland proper and the thyroglossal duct. The first three visceral pouches are open to the exterior, but the epithelial buds destined to give rise to the thymus and parathyroids are not yet apparent. The fourth visceral pouch is still separated from the corresponding groove by the closing plate. The laryngeo-tracheal groove has developed in the floor of the pharynx just posterior to the fourth visceral pouches. At its posterior end the dorsal margins of this groove have closed together to form the primordium of the trachea which is thus set free from the esophagus above. The trachea is bifurcated at the posterior end, thus giving rise to the two bronchial buds which are the primordia of the lungs. SEVENTY-TWO HOURS 311
3 Somite 26
+
 
Vitelline 2
+
The esophagus, which is relatively narrow, is followed by a dilation which is to become the stomach. Posterior to this, the primordium of the liver may be seen as an evagination from the
artery
+
 
Vitelline ; .
+
 
. : Posterior
+
 
vem limb bud
+
 
 +
 
 +
 
 +
 
 +
 
  
Fig. 218. — 72 hour chick embryo. Transparent preparation from dorsal view,
 
head seen from right side. X15.
 
  
D. THE SEVENTY-TWO HOUR STAGE
 
External form. — At the end of the third day of incubation,
 
the total length of the embryo is 9.5 mm., but the curvature of
 
the body is so great, on account of the cranial and cervical flexures in addition to the newly developed caudal flexure, that the
 
greatest length, from neck to tail, is 7 mm. Torsion involves the
 
310 THE ANATOMY OF CHICK EMBRYOS
 
  
body as far back as the vitelline arteries and will become complete during the fourth day. Anterior and posterior limb buds
 
are now apparent at the levels of somites 17-19 and 26-32 respectively. The tail is curved forward. The fore-gut is still
 
1.4 mm. in length but has undergone further differentiation,
 
indicated externally by the fact that the first three visceral
 
clefts are open while the fourth is still interrupted by its closing
 
plate. In the branchial arches four aortic arches may be seen.
 
The telencephalon has given rise to the primordia of the cerebral
 
hemispheres, and from the roof of the diencephalon, a small
 
evagination represents the epiphysis or primordium of the pineal
 
gland. The eye and ear, which were formerly in the same
 
transverse section, are now nearly in an antero-posterior relationship. The olfactory pits have made their appearance in the
 
head. The semilunar (fifth cranial nerve), geniculo-acoustic
 
(seventh and eighth), and petrosal (ninth) ganglia may be seen.
 
There are approximately thirty-five somites. The primitive
 
streak has disappeared. The amnion is completed by the fusion
 
of head and tail folds. The allantois, a small sac-like evagination,
 
protrudes ventrally between the posterior limb buds.
 
  
Endodermal derivatives. — At the end of the third day the oral
 
aperture has been formed by the rupture of the oral membrane
 
separating the stomodeum and the fore-gut. Immediately anterior to this opening the preoral gut persists. The fore-gut is
 
still the same length as in the chick of forty-cight hours, but is
 
more complex in structure. The thyroid gland, which appeared
 
during the second day, has now become differentiated into the
 
distal dilation which will give rise to the gland proper and the
 
thyroglossal duct. The first three visceral pouches are open
 
to the exterior, but the epithelial buds destined to give rise to the
 
thymus and parathyroids are not yet apparent. The fourth
 
visceral pouch is still separated from the corresponding groove by
 
the closing plate. The laryngeo-tracheal groove has developed
 
in the floor of the pharynx just posterior to the fourth visceral
 
pouches. At its posterior end the dorsal margins of this groove
 
have closed together to form the primordium of the trachea which
 
is thus set free from the esophagus above. The trachea is bifurcated at the posterior end, thus giving rise to the two bronchial
 
buds which are the primordia of the lungs.
 
SEVENTY-TWO HOURS 311
 
  
The esophagus, which is relatively narrow, is followed by a
 
dilation which is to become the stomach. Posterior to this, the
 
primordium of the liver may be seen as an evagination from the
 
  
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
  
 
Aortic arches
 
Aortic arches
Line 1,498: Line 794:
 
aorta Myelencephalon
 
aorta Myelencephalon
  
Metencephalon
+
Metencephalon . Roy fandibatam
. Roy fandibatam
 
  
 
Sinus
 
Sinus
Line 1,505: Line 800:
 
venosus Isthmus
 
venosus Isthmus
  
Atrium
+
Atrium Spinal cord , Notochord ++—%
Spinal
 
cord ,
 
Notochord ++—%
 
  
>Amnion
+
>Amnion Mesencephalon
Mesencephalon
 
  
 
Diencephalon
 
Diencephalon
  
Telencephalon Epiphysis
+
Telencephalon Epiphysis Mesonephros
Mesonephros
 
  
 
Notochord
 
Notochord
Line 1,524: Line 814:
 
Fiq. 219. — 72 hour chick embryo. Sagittal section. X25.
 
Fiq. 219. — 72 hour chick embryo. Sagittal section. X25.
  
ventral floor of the duodenal region of the gut. The dorsal
+
ventral floor of the duodenal region of the gut. The dorsal pancreas arises from the duodenal region just dorsal to the liver at the end of the third day. The ventral primordia will not appear for another day. 312 THE ANATOMY OF CHICK EMBRYOS
pancreas arises from the duodenal region just dorsal to the liver
+
 
at the end of the third day. The ventral primordia will not
+
 
appear for another day.
+
 
312 THE ANATOMY OF CHICK EMBRYOS
 
  
   
 
 
   
 
  
 
halon
 
halon
Line 1,538: Line 824:
 
fe
 
fe
  
 
+
 
  
 
Fig. 220. — 72 hour chick embryo. ‘Transverse section through otic vesicle. X25.
 
Fig. 220. — 72 hour chick embryo. ‘Transverse section through otic vesicle. X25.
  
L esicle
+
L esicle Dorsal aorta Optic ome
Dorsal aorta Optic ome
 
  
 
Aortic arches
 
Aortic arches
  
d cup Sensory layer
+
d cup Sensory layer ii
ii
+
 
  
 
 
  
 
Pharynx yyy
 
Pharynx yyy
  
Visceral arches
+
Visceral arches Fia. 221. — 72 hour chick embryo. Transverse section through optic cup. X25.
Fia. 221. — 72 hour chick embryo. Transverse section through optic cup. X25.
+
 
 +
 
 +
 
 +
 
 +
 
 +
 
 +
 
 +
 
 +
 
 +
 
 +
 
  
 
 
 
 
  
 
 
 
  
     
 
 
 
 
 
  
 
  
/
+
/ Esophagus Primary ‘Common cardinal Bulbus arteriosus “ . : i Chorion Amnion bronchus yon polite eee! oa 3 A ang ots + ~ Somite AfSead- m,
Esophagus Primary ‘Common cardinal Bulbus arteriosus “
 
. : i
 
Chorion Amnion bronchus yon polite eee! oa
 
3 A ang ots + ~
 
Somite AfSead- m,
 
  
\
+
\ rst; Epidermis ip
rst; Epidermis
 
ip
 
  
 
3
 
3
Line 1,587: Line 863:
 
‘Yolk sac
 
‘Yolk sac
  
 
 
 
  
Pleural groove
 
Sinus Atrium Nasal pit
 
“venosus Pericardial
 
cavity
 
Fig. 222. — 72 hour chick embryo. Transverse section through heart and lung.
 
X25.
 
  
 
SEVENTY-TWO HOURS 313
 
  
The mid-gut region is gradually lessened by the advancing
+
Pleural groove Sinus Atrium Nasal pit “venosus Pericardial cavity Fig. 222. — 72 hour chick embryo. Transverse section through heart and lung. X25.
sulci which are cutting off the body of the embryo from the yolk.
+
 
This region opens into the yolk stalk which is still quite wide.
+
‘ SEVENTY-TWO HOURS 313
 +
 
 +
The mid-gut region is gradually lessened by the advancing sulci which are cutting off the body of the embryo from the yolk. This region opens into the yolk stalk which is still quite wide.
 +
 
 +
The hind-gut contained in the tail fold has not yet acquired its cloacal aperture nor has the proctodeum appeared. The floor of the hind-gut between the tail bud and the posterior intestinal portal evaginates to give rise to the allantoic primordium.
 +
 
 +
Mesodermal derivatives. — The somites, typically thirty-five in number, still show a varying degree of differentiation which is carried to its furthest point in the more anterior somites. The dermatome is now a thin sheet of cells along the dorso-lateral
 +
 
 +
Posterior Dorsal cardinal} Dorsal Li _Amnion aorta vein mesentery iver
 +
 
 +
 
 +
 
 +
 
  
The hind-gut contained in the tail fold has not yet acquired
 
its cloacal aperture nor has the proctodeum appeared. The
 
floor of the hind-gut between the tail bud and the posterior
 
intestinal portal evaginates to give rise to the allantoic primordium.
 
  
Mesodermal derivatives. — The somites, typically thirty-five
 
in number, still show a varying degree of differentiation which is
 
carried to its furthest point in the more anterior somites. The
 
dermatome is now a thin sheet of cells along the dorso-lateral
 
  
Posterior
 
Dorsal cardinal} Dorsal Li
 
_Amnion aorta vein mesentery iver
 
  
 
 
 
 
 
 
 
 
  
 
 
  
 
  
 
ao
 
ao
  
Spinal
+
Spinal cord
cord
 
  
 
Notochord
 
Notochord
Line 1,639: Line 898:
 
Allantoic vein
 
Allantoic vein
  
   
+
 
  
 
Ventral mesentery
 
Ventral mesentery
  
Meatus venosus
+
Meatus venosus Fig. 223. — 72 hour chick embryo. Transverse section through liver. X25.
Fig. 223. — 72 hour chick embryo. Transverse section through liver. X25.
 
  
angle of the embryo, with the myotome parallel and internal;
+
angle of the embryo, with the myotome parallel and internal; the sclerotome in these anterior segments is a large and loose aggregate of cells investing the neural tube, notochord, and aortae.
the sclerotome in these anterior segments is a large and loose
 
aggregate of cells investing the neural tube, notochord, and
 
aortae.
 
  
The pronephric tubules have degenerated to a considerable
+
The pronephric tubules have degenerated to a considerable extent, but the nephrostomes opening into the coelom may persist. The mesonephric tubules are now in process of development, with those in the more anterior segments most highly differentiated. The tubules between the thirteenth and _ thirtieth somites have progressed from the vesicle stage characteristic of those behind the twentieth somite, and some have acquired a lumen and joined the pronephric duct which henceforward is known as the mesonephric duct. A few of the more anterior tubules develop nephrostomes, but these soon disappear. 314 THE ANATOMY OF CHICK EMBRYOS
extent, but the nephrostomes opening into the coelom may persist. The mesonephric tubules are now in process of development, with those in the more anterior segments most highly
 
differentiated. The tubules between the thirteenth and _ thirtieth somites have progressed from the vesicle stage characteristic of those behind the twentieth somite, and some have acquired a lumen and joined the pronephric duct which henceforward is known as the mesonephric duct. A few of the more
 
anterior tubules develop nephrostomes, but these soon disappear.
 
314 THE ANATOMY OF CHICK EMBRYOS
 
  
Behind the twentieth somite, as far back as the thirtieth, only
+
Behind the twentieth somite, as far back as the thirtieth, only vesicles are formed. The mesonephric ducts have grown back and united with the cloaca.
vesicles are formed. The mesonephric ducts have grown back
 
and united with the cloaca.
 
  
The heart now shows a constriction between the atrial and
+
The heart now shows a constriction between the atrial and ventricular region. Four aortic arches are developed, of which
ventricular region. Four aortic arches are developed, of which
 
  
Amniotic
+
Amniotic raphe
raphe
 
  
 
Dermatome, Sclerotome \ Spinal cord
 
Dermatome, Sclerotome \ Spinal cord
  
 
 
  
Mesonephric
 
Vitelline tubule
 
artery
 
  
Lateral sulcus Dorsal
+
Mesonephric Vitelline tubule artery
aorta
 
  
Fig. 224. — 72 hour chick embryo. Transverse section through vitelline arteries
+
Lateral sulcus Dorsal aorta
leaving body. X25.
 
  
the first is becoming smaller, and somctimes has disappeared at
+
Fig. 224. — 72 hour chick embryo. Transverse section through vitelline arteries leaving body. X25.
this stage. The internal carotid arteries are now well developed,
+
 
growing forward into the head from the point of union between
+
the first is becoming smaller, and somctimes has disappeared at this stage. The internal carotid arteries are now well developed, growing forward into the head from the point of union between the first arches and the dorsal aortae. From the ventral end of the first aortic arch the external carotid takes its origin. The
the first arches and the dorsal aortae. From the ventral end of
+
 
the first aortic arch the external carotid takes its origin. The
+
Chorion Mesonephric . Amnion duct Somite Dorsal ao
  
Chorion Mesonephric .
 
Amnion duct  Somite Dorsal ao
 
  
   
 
  
 
Fig. 225. 72 hour chick embryo. Transverse section through allantois. 25.
 
Fig. 225. 72 hour chick embryo. Transverse section through allantois. 25.
  
pulmonary is sometimes apparent as a posterior prolongation of
+
pulmonary is sometimes apparent as a posterior prolongation of the ventral aorta at the point where the fifth arches will appear during the next twenty-four hours. The intersegmental arteries are now apparent as dorsal diverticula from the aorta between each pair of somites. The vitelline veins have fused for a short distance behind the sinus, thus giving rise to the meatus venosus.  
the ventral aorta at the point where the fifth arches will appear
 
during the next twenty-four hours. The intersegmental arteries
 
are now apparent as dorsal diverticula from the aorta between
 
each pair of somites. The vitelline veins have fused for a short
 
distance behind the sinus, thus giving rise to the meatus venosus.
 
REFERENCES 315
 
  
The anterior cardinal vein now possesses many branches from
+
The anterior cardinal vein now possesses many branches from the head, among which are three intersegmental veins. The posterior cardinal has continued its backward growth dorsal to the mesonephric duct as far as the thirty-third somite. It receives the intersegmental veins of this region. Where the posterior cardinals unite with the common cardinals, a capillary network indicates the beginnings of the allantoic veins.
the head, among which are three intersegmental veins. The
 
posterior cardinal has continued its backward growth dorsal
 
to the mesonephric duct as far as the thirty-third somite. It
 
receives the intersegmental veins of this region. Where the
 
posterior cardinals unite with the common cardinals, a capillary
 
network indicates the beginnings of the allantoic veins.
 
  
Ectodermal derivatives. —-'The brain at the end of the third
+
Ectodermal derivatives. —-'The brain at the end of the third day has its five definitive vesicles even more sharply demarcated. From the telencephalon two lateral vesicles have evaginated to form the primordia of the cerebral hemispheres. In the diencephalon the epiphysis has appeared as a dorsal evagination. On the floor of this vesicle the infundibulum is almost in contact with the hypophysis. The mesencephalon is separated from the metencephalon by a deep constriction known as the isthmus. Along the sides of the myelencephalon may be distinguished the following cerebral ganglia: the semilunar of the fifth cranial nerve; the acoustico-facialis which will later separate into the geniculate ganglion of the seventh and the acoustic of the eighth; and the petrosal ganglion of the ninth. The eye has increased in size, and the lens is now free from the epidermal ectoderm. The ear, too, is in the vesicle stage and possesses a short endolymphatic duct, which has lost its connection with the epidermis. On the third day the primordium of the nose is represented by two olfactory pits anterior to the mouth.
day has its five definitive vesicles even more sharply demarcated.
 
From the telencephalon two lateral vesicles have evaginated to
 
form the primordia of the cerebral hemispheres. In the diencephalon the epiphysis has appeared as a dorsal evagination.
 
On the floor of this vesicle the infundibulum is almost in contact
 
with the hypophysis. The mesencephalon is separated from the
 
metencephalon by a deep constriction known as the isthmus.
 
Along the sides of the myelencephalon may be distinguished the
 
following cerebral ganglia: the semilunar of the fifth cranial
 
nerve; the acoustico-facialis which will later separate into the
 
geniculate ganglion of the seventh and the acoustic of the eighth;
 
and the petrosal ganglion of the ninth. The eye has increased in
 
size, and the lens is now free from the epidermal ectoderm. The
 
ear, too, is in the vesicle stage and possesses a short endolymphatic duct, which has lost its connection with the epidermis.
 
On the third day the primordium of the nose is represented by
 
two olfactory pits anterior to the mouth.
 
  
REFERENCES
+
===References===
  
 
Arey, L. B. 1934. Developmental Anatomy, 3rd Ed., Chap. 18.
 
Arey, L. B. 1934. Developmental Anatomy, 3rd Ed., Chap. 18.
Line 1,733: Line 944:
 
Duval, M. 1889. Atlas d’embryologie.
 
Duval, M. 1889. Atlas d’embryologie.
  
Keibel and Abraham. 1900. Normaltafeln II, des Huhnes (Gallus domesticus).
+
Keibel and Abraham. 1900. Normaltafeln II, des Huhnes (Gallus domesticus). Lillie, F. R. 1919. The Development of the Chick, 2nd Ed.
Lillie, F. R. 1919. The Development of the Chick, 2nd Ed.
 
  
 
McEwen, R. 8S. 1931. Vertebrate Embryology, 2nd Ed., Part 4.
 
McEwen, R. 8S. 1931. Vertebrate Embryology, 2nd Ed., Part 4.
  
Patten, B. M. 1929. The Early Embryology of the Chick, 3rd Ed.
+
Patten, B. M. 1929. The Early Embryology of the Chick, 3rd Ed. CHAPTER XIII THE ANATOMY OF THE 10 MM. PIG EMBRYO
CHAPTER XIII
 
THE ANATOMY OF THE 10 MM. PIG EMBRYO
 
  
Pig embryos of 10 to 12 mm. body length are particularly
+
Pig embryos of 10 to 12 mm. body length are particularly instructive for laboratory work in mammalian embryology as they
instructive for laboratory work in mammalian embryology as they
 
  
 
Myelencephalon Metencephalon
 
Myelencephalon Metencephalon
Line 1,749: Line 956:
 
Ear,
 
Ear,
  
   
+
 
+
 
  
 
Mesencephalon
 
Mesencephalon
Line 1,758: Line 965:
 
Anterior __| a —Body stalk
 
Anterior __| a —Body stalk
  
limb bud
+
limb bud Roots of spinal nerves Posterior
Roots of
 
spinal nerves Posterior
 
  
 
Fig. 226. — 10 mm. pig embryo. Transparent preparation from right side. X11.
 
Fig. 226. — 10 mm. pig embryo. Transparent preparation from right side. X11.
  
are sufficiently large for the study of external structures and yet
+
are sufficiently large for the study of external structures and yet small enough to afford serial sections for a detailed study of the anatomy. The primordia of practically all the organ systems, excepting the skeleton and musculature, are present. In comparing the accounts given by different authors of this particular stage, it should be remembered that a large amount of shrinkage
small enough to afford serial sections for a detailed study of the
+
 
anatomy. The primordia of practically all the organ systems,
+
takes place during the preparation of fresh sections, so that, as 316 ENDODERMAL DERIVATIVES 317
excepting the skeleton and musculature, are present. In comparing the accounts given by different authors of this particular
+
 
stage, it should be remembered that a large amount of shrinkage
+
pointed out by Patten, an embryo of 12 mm. will not measure more than 93} mm. when prepared for sectioning. The account which follows corresponds in general to the pig (Sus scrofa) of 10 mm. described by Keibel, of 12 mm. (Minot), 10 mm. (Prentiss) and 9.4 mm. (Patten), but is not so advanced as the 13.5 mm. pig (Boyden).
  
takes place during the preparation of fresh sections, so that, as
+
External form. — The pig embryo at this stage is relatively ‘more advanced than the chick of seventy-two hours. The body is sharply flexed, owing to the presence of the cranial, cervical, dorsal, and caudal flexures. In the head region the olfactory pits are well developed and are connected by the naso-lachrymal groove to a depression which surrounds the bulging eyeball. The five divisions of the brain are apparent through the relatively thin overlying epidermis. Four visceral grooves can be seen, the first of which, or hyomandibular, is the primordium of the external auditory meatus. The third and fourth grooves are compressed by the cervical flexure into a deeper depression known as the cervical sinus. A frontal view of the head shows the oral cavity bounded above by the frontal process in the middle, the maxillary processes at the side, while the lower jaw is represented by the mandibular arch.
316
 
ENDODERMAL DERIVATIVES 317
 
  
pointed out by Patten, an embryo of 12 mm. will not measure
+
In the trunk region, the buds of the pectoral and pelvic appendages are large but show no further differentiation. The contours of the somites, now forty-four in number, are apparent along the back, and ventral to these can be seen the outlines of the heart, liver, and mesonephros. In some specimens there appears between the limb buds a thickened ridge from which the mammary glands develop and which is therefore known as the milk line. | The umbilical cord projects from the ventral side of the embryo. Between this and the base of the slender tail is a small protuberance, the genital tubercle, or primordium of the external genitalia.
more than 93} mm. when prepared for sectioning. The account
 
which follows corresponds in general to the pig (Sus scrofa) of
 
10 mm. described by Keibel, of 12 mm. (Minot), 10 mm. (Prentiss)
 
and 9.4 mm. (Patten), but is not so advanced as the 13.5 mm.
 
pig (Boyden).
 
  
External form. — The pig embryo at this stage is relatively
+
Endodermal derivatives. — The preoral gut still persists anterior to the oral aperture. Ventral to this, and seen best in sagittal section, is the long and slender hypophysis, now in contact with the infundibulum of the diencephalon. Both the hypophysis and infundibulum, it should be remembered, are of ectodermal origin. The pharynx is dorso-ventrally compressed, and from its floor the tongue is arising. Four visceral pouches 318 THE ANATOMY OF THE 10 MM. PIG EMBRYO
‘more advanced than the chick of seventy-two hours. The body
 
is sharply flexed, owing to the presence of the cranial, cervical,
 
dorsal, and caudal flexures. In the head region the olfactory
 
pits are well developed and are connected by the naso-lachrymal
 
groove to a depression which surrounds the bulging eyeball.
 
The five divisions of the brain are apparent through the relatively thin overlying epidermis. Four visceral grooves can be
 
seen, the first of which, or hyomandibular, is the primordium of
 
the external auditory meatus. The third and fourth grooves are
 
compressed by the cervical flexure into a deeper depression known
 
as the cervical sinus. A frontal view of the head shows the oral
 
cavity bounded above by the frontal process in the middle, the
 
maxillary processes at the side, while the lower jaw is represented
 
by the mandibular arch.
 
  
In the trunk region, the buds of the pectoral and pelvic appendages are large but show no further differentiation. The
+
are present, corresponding to the visceral grooves already noted. These do not unite to become visceral clefts but remain separated by their closing membranes. Between the second and third
contours of the somites, now forty-four in number, are apparent
 
along the back, and ventral to these can be seen the outlines of the
 
heart, liver, and mesonephros. In some specimens there appears
 
between the limb buds a thickened ridge from which the mammary
 
glands develop and which is therefore known as the milk line. |
 
The umbilical cord projects from the ventral side of the embryo.
 
Between this and the base of the slender tail is a small protuberance, the genital tubercle, or primordium of the external
 
genitalia.
 
  
Endodermal derivatives. — The preoral gut still persists anterior to the oral aperture. Ventral to this, and seen best in
+
Metencephalon Myelencephalon
sagittal section, is the long and slender hypophysis, now in contact with the infundibulum of the diencephalon. Both the
 
hypophysis and infundibulum, it should be remembered, are of
 
ectodermal origin. The pharynx is dorso-ventrally compressed,
 
and from its floor the tongue is arising. Four visceral pouches
 
318 THE ANATOMY OF THE 10 MM. PIG EMBRYO
 
  
are present, corresponding to the visceral grooves already noted.
 
These do not unite to become visceral clefts but remain separated
 
by their closing membranes. Between the second and third
 
  
Metencephalon
 
Myelencephalon
 
  
   
 
 
 
  
 
Pharynx
 
Pharynx
  
 
  
Posterior
 
vena cava
 
  
+
Posterior vena cava
+
 
   
+
 
+
 
   
+
 
 
+
 
+
 
 +
 
 +
 
  
 
//_ . Ductus venosus
 
//_ . Ductus venosus
  
i Liver
+
i Liver Duct of ventral pancreas
Duct of ventral pancreas
 
  
Spinal { nw ;
+
Spinal { nw ; artery fDuodenum___Vitelline vein on—__ Body Dorsal stalk pancreas i ses mbilical SSS ) rte Vitelline 4 7 SN d Af ty (ant. mesenteric)’ oN i R artery cl Notochord oaca Metanephros Aorta
artery fDuodenum___Vitelline vein
 
on—__ Body
 
Dorsal stalk
 
pancreas i ses mbilical
 
SSS ) rte
 
Vitelline 4 7 SN d Af ty
 
(ant. mesenteric)’ oN i R
 
artery cl
 
Notochord oaca
 
Metanephros
 
Aorta
 
  
 
Mesonephros
 
Mesonephros
Line 1,860: Line 1,012:
 
Fia. 227.— 10 mm. pig embryo. Sagittal section. 164.
 
Fia. 227.— 10 mm. pig embryo. Sagittal section. 164.
  
pouches the thyroid gland appears. From the level of the fourth
+
pouches the thyroid gland appears. From the level of the fourth pouch a short laryngeal groove is prolonged into the trachea which has given rise to the bronchial buds, three in number. Two of these, the primary bronchi, have arisen by the bifurcation of ENDODERMAL DERIVATIVES 319
pouch a short laryngeal groove is prolonged into the trachea
+
 
which has given rise to the bronchial buds, three in number. Two
+
the trachea; the third or apical bud, which will give rise to the eparterial bronchus, develops anterior to the right primary bronchus. The esophagus is relatively long and narrow and, just posterior to the level of the lung buds, passes into the stomach which is dilated and shows a slight dorsal curvature. Posterior to the stomach the duodenal glands, liver, and pancreas are well developed. The liver, now a large glandular mass traversed by
of these, the primary bronchi, have arisen by the bifurcation of
+
 
ENDODERMAL DERIVATIVES 319
+
 
 +
 
  
the trachea; the third or apical bud, which will give rise to the
 
eparterial bronchus, develops anterior to the right primary
 
bronchus. The esophagus is relatively long and narrow and,
 
just posterior to the level of the lung buds, passes into the stomach
 
which is dilated and shows a slight dorsal curvature. Posterior
 
to the stomach the duodenal glands, liver, and pancreas are well
 
developed. The liver, now a large glandular mass traversed by
 
  
 
 
 
 
 
 
 
 
  
 
W- Nerve XI
 
W- Nerve XI
  
Nerve X
+
Nerve X and jugular F Ganglion IX ganglion (superior)
and jugular F Ganglion IX
 
ganglion (superior)
 
  
 
Otic vesicle
 
Otic vesicle
  
~Ganglion VOI
+
~Ganglion VOI Myelencephalon ¥} (acoustic) Ganglion Y~}(semilunar) P Pog IT a1 Basilar Fee eof artery Nerve III Internal
Myelencephalon ¥} (acoustic)
 
Ganglion Y~}(semilunar) P
 
Pog IT a1 Basilar
 
Fee eof artery
 
Nerve III
 
Internal
 
  
 
carotid artery
 
carotid artery
Line 1,899: Line 1,033:
 
Mesencephalon
 
Mesencephalon
  
+
 
  
 
Fig. 228. — 10 mm. pig embryo. Transverse section through otic vesicles. 163}.
 
Fig. 228. — 10 mm. pig embryo. Transverse section through otic vesicles. 163}.
  
the capillaries of the hepato-portal veins, retains its original connection with the duodenum as the common bile duct from the
+
the capillaries of the hepato-portal veins, retains its original connection with the duodenum as the common bile duct from the distal end of which the gall bladder is forming. Both dorsal and ventral primordia of the pancreas are present, the duct of the latter arising from the common bile duct. The long and slender intestine extends into the umbilical cord as the intestinal loop, to which the yolk stalk is still attached. Just posterior to this, a slight enlargement may sometimes be observed which indicates the boundary between the large and small intestine. The hind-gut is dividing into a dorsal rectum and ventral urogenital 320 THE ANATOMY OF THE 10 MM. PIG EMBRYO
distal end of which the gall bladder is forming. Both dorsal
 
and ventral primordia of the pancreas are present, the duct of
 
the latter arising from the common bile duct. The long and
 
slender intestine extends into the umbilical cord as the intestinal
 
loop, to which the yolk stalk is still attached. Just posterior to
 
this, a slight enlargement may sometimes be observed which indicates the boundary between the large and small intestine. The
 
hind-gut is dividing into a dorsal rectum and ventral urogenital
 
320 THE ANATOMY OF THE 10 MM. PIG EMBRYO
 
  
sinus, prolonged into the allantoic stalk. The sinus and rectum
+
sinus, prolonged into the allantoic stalk. The sinus and rectum unite in a common cloaca which has not yet established connection with the proctodeum. Immediately posterior to the cloacal plate, a small blind pocket represents the postcloacal gut.
unite in a common cloaca which has not yet established connection
 
with the proctodeum. Immediately posterior to the cloacal plate,
 
a small blind pocket represents the postcloacal gut.
 
  
 
Spinal cord.
 
Spinal cord.
  
 
 
  
it Dorsal root
 
co Spinal ganglion
 
  
re Ventral root
+
it Dorsal root co Spinal ganglion
Dorsal ramus
+
 
 +
re Ventral root Dorsal ramus
 +
 
 +
 
 +
 
 +
 
 +
 
 +
 
 +
 
 +
 
 +
 
 +
 
 +
 
 +
 
  
   
 
     
 
 
 
 
 
 
 
 
 
 
  
 
 
 
 
 
 
  
 
Notochord
 
Notochord
  
Anterior
+
Anterior cardinal vein Ganglion X Aortic ( nodosum ) arch OT Radix aortae Visceral
cardinal vein Ganglion X
 
Aortic ( nodosum )
 
arch OT Radix aortae
 
Visceral
 
  
 
Visceral arch
 
Visceral arch
  
a Hypophysis
+
a Hypophysis Anterior cardinal vein
Anterior
 
cardinal vein
 
  
Sensory layer
+
Sensory layer Pigment layer
Pigment layer
 
  
 
Diencephalon
 
Diencephalon
Line 1,963: Line 1,077:
 
Fig. 229.10 mm. pig embryo. Transverse section through optic cup. 164.
 
Fig. 229.10 mm. pig embryo. Transverse section through optic cup. 164.
  
Mesodermal derivatives. — The notochord extends from the
+
Mesodermal derivatives. — The notochord extends from the vicinity of the floor of the mesencephalon into the tail, following the flexures of the body.
vicinity of the floor of the mesencephalon into the tail, following
+
 
the flexures of the body.
+
The somites have long since become differentiated into the myotome, dermatome, and sclerotome. In the tail region, the sclerotomes are separated into the cranial and caudal arcualia from which the vertebrae will originate.
 +
 
 +
In the pig of 10 mm., the pronephric stage has been passed; the mesonephros is at the height of its development, forming a great “Wolffian” body with a complicated network of interwoven tubules; while the mesonephric duct (originally the pronephric duct) may be recognized along the ventral margin. Emerging MESODERMAL DERIVATIVES 321
 +
 
 +
from the mesonephros, each duct enters the urogenital sinus at the same level as the allantoic stalk. From each duct a narrow stalk runs dorsally and forward as the metanephric duct, or ureter, which at its distal end is enlarged to form the pelvis of the metanephros. Around the pelvis the posterior portion of the nephrotomal band will produce the secretory tubules of the definitive kidney at a later stage. On the median ventral margin of each
 +
 
 +
 
  
The somites have long since become differentiated into the
 
myotome, dermatome, and sclerotome. In the tail region, the
 
sclerotomes are separated into the cranial and caudal arcualia
 
from which the vertebrae will originate.
 
  
In the pig of 10 mm., the pronephric stage has been passed;
 
the mesonephros is at the height of its development, forming a
 
great “Wolffian” body with a complicated network of interwoven
 
tubules; while the mesonephric duct (originally the pronephric
 
duct) may be recognized along the ventral margin. Emerging
 
MESODERMAL DERIVATIVES 321
 
  
from the mesonephros, each duct enters the urogenital sinus at the
 
same level as the allantoic stalk. From each duct a narrow stalk
 
runs dorsally and forward as the metanephric duct, or ureter,
 
which at its distal end is enlarged to form the pelvis of the metanephros. Around the pelvis the posterior portion of the nephrotomal band will produce the secretory tubules of the definitive
 
kidney at a later stage. On the median ventral margin of each
 
  
 
 
 
 
 
 
 
 
  
 
Notochord
 
Notochord
Line 1,999: Line 1,099:
 
ardinal vein
 
ardinal vein
  
Left
+
Left atrium
atrium
 
  
 
Telencephalon
 
Telencephalon
  
Fig. 230. — 10 mm. pig embryo. Transverse section through nasal (olfactory) pit.
+
Fig. 230. — 10 mm. pig embryo. Transverse section through nasal (olfactory) pit. X 163.
X 163.
 
  
mesonephros are slight swellings which will later become the
+
mesonephros are slight swellings which will later become the genital ridges, primordia of the gonads. The coelom is partially divided into the pericardial and abdominal cavities by the septum transversum. The mesenteries of the principal viscera are in evidence. The liver is still suspended in the ventral mesentery. A dorsal mesocardium is present.
genital ridges, primordia of the gonads. The coelom is partially
 
divided into the pericardial and abdominal cavities by the septum
 
transversum. The mesenteries of the principal viscera are in
 
evidence. The liver is still suspended in the ventral mesentery.
 
A dorsal mesocardium is present.
 
  
The heart of the 10 mm. pig has the four main chambers established, although not yet completely separated into right and left
+
The heart of the 10 mm. pig has the four main chambers established, although not yet completely separated into right and left halves. The sinus venosus now enters the right atrium through 322 THE ANATOMY OF THE 10 MM. PIG EMBRYO
halves. The sinus venosus now enters the right atrium through
 
322 THE ANATOMY OF THE 10 MM. PIG EMBRYO
 
  
a slit guarded by the valves of the sinus. The right and left
+
a slit guarded by the valves of the sinus. The right and left atria are partially separated by the interatrial septum in which can be seen an opening, the foramen ovale. The atrio-ventricular canal leading to the ventricle is partially separated into right and left halves by the endocardial cushion. The ventricle is partially divided by the interventricular septum. From the ventral aorta three aortic arches curve around the pharynx to unite with the dorsal aorta. These are the third, fourth, and sixth aortic arches; the first and second have degenerated, while the fifth
atria are partially separated by the interatrial septum in which
 
can be seen an opening, the foramen ovale. The atrio-ventricular canal leading to the ventricle is partially separated into right
 
and left halves by the endocardial cushion. The ventricle is partially divided by the interventricular septum. From the ventral
 
aorta three aortic arches curve around the pharynx to unite with
 
the dorsal aorta. These are the third, fourth, and sixth aortic
 
arches; the first and second have degenerated, while the fifth
 
  
 
Spinal cord .
 
Spinal cord .
  
 
 
   
 
 
 
 
  
Ganglion
 
Notochord
 
  
Anterior
 
limb bud
 
  
Common Dorsal aorta
+
 
cardinal vein
+
 
Eparterial Oesophagus
+
Ganglion Notochord
bronchus Trachea
+
 
Valves of
+
Anterior limb bud
sinus Left atrium
+
 
Right
+
Common Dorsal aorta cardinal vein Eparterial Oesophagus bronchus Trachea Valves of sinus Left atrium Right atrium Left Right \ ventricle ventricle
atrium
 
Left
 
Right \ ventricle
 
ventricle
 
  
 
Fia. 231. — 10 mm. pigembryo. Transverse section through sinus venosus. 16}.
 
Fia. 231. — 10 mm. pigembryo. Transverse section through sinus venosus. 16}.
  
seldom appears as a separate structure. The pulmonary arteries
+
seldom appears as a separate structure. The pulmonary arteries are growing back from the sixth aortic arches.
are growing back from the sixth aortic arches.
 
  
As prolongations of the original paired ventral and dorsal aortae,
+
As prolongations of the original paired ventral and dorsal aortae, the external and internal carotid arteries, respectively, run forward into the head. The internal carotid arteries are united at the level of the isthmus between the mesencephalon and the metencephalon with the basilar artery, which serves to unite them with the vertebral arteries, arising from the anastomosis of intersegmental arteries in the cervical region. At the 10 mm. stage the vertebral arteries have lost their intersegmental connections with the aorta except at the posterior end, where the MESODERMAL DERIVATIVES 323
the external and internal carotid arteries, respectively, run forward into the head. The internal carotid arteries are united at
 
the level of the isthmus between the mesencephalon and the
 
metencephalon with the basilar artery, which serves to unite
 
them with the vertebral arteries, arising from the anastomosis of
 
intersegmental arteries in the cervical region. At the 10 mm.
 
stage the vertebral arteries have lost their intersegmental connections with the aorta except at the posterior end, where the
 
MESODERMAL DERIVATIVES 323
 
  
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
  
Anterior
+
 
limb bud:
+
 
 +
 
 +
 
 +
 
 +
 
 +
 
 +
 
 +
 
 +
 
 +
Anterior limb bud:
  
 
Coelom
 
Coelom
  
Posterior
+
Posterior vena cava
vena cava
 
  
 
Right
 
Right
  
atrium {
+
atrium { i Left Right i ventricle ventricle
i Left
 
Right i
 
ventricle ventricle
 
  
 
Notochord
 
Notochord
Line 2,097: Line 1,157:
 
Glomerulus
 
Glomerulus
  
ketee . g ‘
+
ketee . g ‘ Posterior Vea me AN Stomach vena cava i
Posterior Vea me AN Stomach
+
 
vena cava i
+
by \ v
 +
 
 +
Septum 4 Pericardial
 +
ransversum — cavity a
  
by \
+
Fig. 233. — 10 mm. pig embryo. Transverse section through stomach. X16}. 324 THE ANATOMY OF THE 10 MM. PIG EMBRYO
v
+
 
: Septum 4 Pericardial
+
seventh cervical intersegmental artery persists and grows out into the pectoral limb bud to form the subclavian artery. Near the point of origin of the subclavian, the dorsal aortae are fused and run back as a single median aorta into the tail. Dorsally, branches are given off from the aorta as intersegmental arteries of the trunk. Laterally, many small branches supply the glomeruli of the mesonephros. Ventrally, the dorsal aorta gives off the coeliac artery and anterior mesenteric arteries to the gut.
ransversum — cavity
 
a
 
  
Fig. 233. — 10 mm. pig embryo. Transverse section through stomach. X16}.
 
324 THE ANATOMY OF THE 10 MM. PIG EMBRYO
 
  
seventh cervical intersegmental artery persists and grows out into
 
the pectoral limb bud to form the subclavian artery. Near the
 
point of origin of the subclavian, the dorsal aortae are fused and
 
run back as a single median aorta into the tail. Dorsally,
 
branches are given off from the aorta as intersegmental arteries
 
of the trunk. Laterally, many small branches supply the glomeruli of the mesonephros. Ventrally, the dorsal aorta gives
 
off the coeliac artery and anterior mesenteric arteries to the gut.
 
  
   
 
 
  
Ganglion
+
Ganglion Notochord
Notochord
 
  
Left umbilical
+
Left umbilical vein
vein
 
  
 
Fig. 234. — 10 mm. pig embryo. Transverse section through gall bladder. 163.
 
Fig. 234. — 10 mm. pig embryo. Transverse section through gall bladder. 163.
  
Two large umbilical (allantoic) arteries run from the dorsal aorta
+
Two large umbilical (allantoic) arteries run from the dorsal aorta into the umbilical cord. The aorta continues into the tail as a relatively slender vessel, the caudal artery.
into the umbilical cord. The aorta continues into the tail as a
 
relatively slender vessel, the caudal artery.
 
  
The vitelline veins are much smaller than in the chick of
+
The vitelline veins are much smaller than in the chick of seventy-two hours, for the yolk sac from which they drew their blood is nearly degenerated. In the pig at this stage they drain the gut area and cross into the liver where they become the portal vein. Within the liver they are broken up into capillaries which emerge as the hepatic veins to the sinus venosus. Of the somatic MESODERMAL DERIVATIVES 325
seventy-two hours, for the yolk sac from which they drew their
 
blood is nearly degenerated. In the pig at this stage they drain
 
the gut area and cross into the liver where they become the portal
 
vein. Within the liver they are broken up into capillaries which
 
emerge as the hepatic veins to the sinus venosus. Of the somatic
 
MESODERMAL DERIVATIVES 325
 
  
veins, the anterior cardinals are still prominent and are joined
+
veins, the anterior cardinals are still prominent and are joined by an extensive series of head veins. In the cervical region the anterior cardinals receive the dorsal intersegmental veins as well as the external jugular from the mandible. As the anterior cardinals enter the common cardinal veins, they are joined by the posterior cardinals, which have already lost part of their drainage
by an extensive series of head veins. In the cervical region the
 
anterior cardinals receive the dorsal intersegmental veins as well
 
as the external jugular from the mandible. As the anterior
 
cardinals enter the common cardinal veins, they are joined by the
 
posterior cardinals, which have already lost part of their drainage
 
  
 
Spinal cord oO .
 
Spinal cord oO .
  
 
 
 
 
 
 
 
 
 
 
 
 
 
  
Notochord
 
Posterior
 
cardinal
 
vein
 
Posterior
 
vena
 
  
Left
+
 
vitelline
+
 
(portal)
+
 
vein
+
 
Left
+
 
mbilical
+
 
vein
+
 
Left
+
 
vitelline
+
 
artery
+
Notochord Posterior cardinal vein Posterior vena
 +
 
 +
Left vitelline (portal) vein Left mbilical vein Left vitelline artery
  
 
duct
 
duct
  
umbilical
+
umbilical artery
artery
 
  
Fused
+
Fused umbilical veins
umbilical
+
 
veins
+
Fig. 235. 10 mm. pig embryo. Transverse section through umbilical stalk in region of intestinal loop. X16}.
 +
 
 +
area to the subcardinal veins passing through the ventral portions of the mesonephroi. Numerous small venous channels serve to connect the subcardinals and postcardinals during this period. The posterior caval vein has already made its appearance as a direct connection from the subcardinals to the liver. The umbilical (allantoic) veins proceeding from the allantois toward the heart are fused together in the umbilical cord. In the body they 326 THE ANATOMY OF THE 10 MM. PIG EMBRYO
 +
 
 +
pass through the liver, within which they are, like the vitelline veins, broken up into capillaries. The left umbilical maintains a broad channel through the liver. This vessel, now known as the ductus venosus, connects the umbilical with the posterior caval vein.
  
Fig. 235. 10 mm. pig embryo. Transverse section through umbilical stalk in
 
region of intestinal loop. X16}.
 
  
area to the subcardinal veins passing through the ventral portions
 
of the mesonephroi. Numerous small venous channels serve to
 
connect the subcardinals and postcardinals during this period.
 
The posterior caval vein has already made its appearance as a
 
direct connection from the subcardinals to the liver. The umbilical (allantoic) veins proceeding from the allantois toward the
 
heart are fused together in the umbilical cord. In the body they
 
326 THE ANATOMY OF THE 10 MM. PIG EMBRYO
 
  
pass through the liver, within which they are, like the vitelline
 
veins, broken up into capillaries. The left umbilical maintains a
 
broad channel through the liver. This vessel, now known as the
 
ductus venosus, connects the umbilical with the posterior caval
 
vein.
 
  
 
 
   
 
 
 
  
 
Posterior Mesonephric
 
Posterior Mesonephric
  
limb bud duct
+
limb bud duct Umbilical artery Metanephric duct
Umbilical
+
 
artery Metanephric
+
Caudal artery
duct
+
 
 +
Notochord Spinal cord
 +
 
 +
Fig. 236. — 10 mm. pig embryo. Transverse section through metanephric duct and posterior limb buds. X16}.
 +
 
 +
Ectodermal derivatives. — The epidermal derivatives of the ectoderm have already been enumerated in the description of external form. There remain for consideration the nervous system and sense organs. ‘The five definitive vesicles of the brain are well marked. From the telencephalon arise the two lateral cerebral vesicles. This division of the brain is separated from the diencephalon by two points of reference, the optic recess in the floor, and the velum transversum in the roof. From the diencephalon spring the optic stalks, leading to the optic cups, and the infundibulum, now in contact with the hypophysis as mentioned above. The posterior boundary of the diencephalon is indicated by the tuberculum posterius arising from the brain floor. The epiphysis seldom appears at this stage. The mesencephalon, with the third cranial nerve arising from its floor, is ECTODERMAL DERIVATIVES 327
 +
 
 +
demarcated at its posterior end by the deep constriction of the isthmus. The metencephalon is distinguished from the myelencephalon by its thicker roof. From the isthmus the fourth cranial nerve runs forward laterally over the sides of the brain to the mass of mesoderm surrounding the eyeball, from which the
 +
 
 +
 
 +
 
 +
 
 +
 
 +
 
 +
 
 +
 
 +
 
  
Caudal
 
artery
 
  
Notochord
 
Spinal
 
cord
 
  
Fig. 236. — 10 mm. pig embryo. Transverse section through metanephric duct and
 
posterior limb buds. X16}.
 
  
Ectodermal derivatives. — The epidermal derivatives of the
 
ectoderm have already been enumerated in the description of
 
external form. There remain for consideration the nervous system and sense organs. ‘The five definitive vesicles of the brain
 
are well marked. From the telencephalon arise the two lateral
 
cerebral vesicles. This division of the brain is separated from
 
the diencephalon by two points of reference, the optic recess in
 
the floor, and the velum transversum in the roof. From the
 
diencephalon spring the optic stalks, leading to the optic cups,
 
and the infundibulum, now in contact with the hypophysis as
 
mentioned above. The posterior boundary of the diencephalon
 
is indicated by the tuberculum posterius arising from the brain
 
floor. The epiphysis seldom appears at this stage. The mesencephalon, with the third cranial nerve arising from its floor, is
 
ECTODERMAL DERIVATIVES 327
 
  
demarcated at its posterior end by the deep constriction of the
 
isthmus. The metencephalon is distinguished from the myelencephalon by its thicker roof. From the isthmus the fourth
 
cranial nerve runs forward laterally over the sides of the brain to
 
the mass of mesoderm surrounding the eyeball, from which the
 
  
 
 
 
 
   
 
   
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
  
Basilar
+
Basilar artery
artery
 
  
Anterior
+
Anterior cardinal
cardinal
 
  
vein Internal
+
vein Internal carotid artery Thymus Olfactory pit | Visceral 8rd Aortic arch
carotid artery
 
Thymus Olfactory pit |
 
Visceral 8rd Aortic arch
 
  
4th Aortic arch
+
4th Aortic arch 6th Aortic arch. Sinus
6th Aortic arch.
 
Sinus
 
  
 
venosus
 
venosus
  
Right
+
Right atrium
atrium
 
  
 
pouch
 
pouch
  
Ductus
+
Ductus venosus
venosus
+
 
  
 
  
 
[J Subcardinal
 
[J Subcardinal
#/J anastomosis,
 
  
 +
/J anastomosis,
 
Notochord
 
Notochord
  
Fig. 237. — 10 mm. pig embryo. Frontal section through aortic arches and ductus
+
Fig. 237. — 10 mm. pig embryo. Frontal section through aortic arches and ductus venosus. X16}.
venosus. X16}.
 
  
eyeball muscles will be formed. Conspicuous at the anterior
+
eyeball muscles will be formed. Conspicuous at the anterior ventro-lateral margin of the metencephalon is the large semilunar ganglion of the fifth cranial nerve. From the floor of the myelencephalon, the sixth cranial nerve emerges to run forward toward the eye. Immediately following this, the geniculate ganglion of the seventh and the acoustic ganglion of the eighth are in close 328 THE ANATOMY OF THE 10 MM. PIG EMBRYO
ventro-lateral margin of the metencephalon is the large semilunar
 
ganglion of the fifth cranial nerve. From the floor of the myelencephalon, the sixth cranial nerve emerges to run forward toward
 
the eye. Immediately following this, the geniculate ganglion of
 
the seventh and the acoustic ganglion of the eighth are in close
 
328 THE ANATOMY OF THE 10 MM. PIG EMBRYO
 
  
connection. The ninth cranial nerve has two ganglia, the dorsal
+
connection. The ninth cranial nerve has two ganglia, the dorsal superior ganglion and ventral petrosal, while the tenth similarly possesses a dorsal jugular and ventral nodose ganglion. The eleventh cranial nerve possesses at this stage a small ganglion (of Froriep) which disappears in the adult. The last of the cranial nerves, the twelfth, arises from the floor of the myelencephalon by a number of small roots and without a ganglion. In the region of the spinal cord the segmental nerves arise from the cord by two roots, of which the dorsal is associated with a spinal ganglion. The trunk is very short and soon divides into three main branches. The dorsal and ventral rami run to these respective regions of the body wall, while the third, or communicating ramus, unites the spinal nerve with a ganglion of the sympathetic chain. The sympathetic ganglia may be recognized as small masses of cells dorsal to the aorta.
superior ganglion and ventral petrosal, while the tenth similarly
 
possesses a dorsal jugular and ventral nodose ganglion. The
 
eleventh cranial nerve possesses at this stage a small ganglion
 
(of Froriep) which disappears in the adult. The last of the cranial
 
nerves, the twelfth, arises from the floor of the myelencephalon
 
by a number of small roots and without a ganglion. In the region
 
of the spinal cord the segmental nerves arise from the cord by two
 
roots, of which the dorsal is associated with a spinal ganglion.
 
The trunk is very short and soon divides into three main branches.
 
The dorsal and ventral rami run to these respective regions of
 
the body wall, while the third, or communicating ramus, unites
 
the spinal nerve with a ganglion of the sympathetic chain. The
 
sympathetic ganglia may be recognized as small masses of cells
 
dorsal to the aorta.
 
  
The nose is represented by the olfactory pits. The eye is in
+
The nose is represented by the olfactory pits. The eye is in the optic cup stage with a well-marked choroid fissure and groove, while the lens is completely separated from the outer ectoderm and is in the vesicle stage. Of the various regions of the ear, all the primordia are now established. The otic vesicle with its endolymphatic duct, representing the inner ear, is in close juxtaposition to the first visceral pouch (hyomandibular) which will give rise to the auditory tube and chamber of the middle ear; the external auditory meatus, or outer ear, will arise from the first or hyomandibular groove.
the optic cup stage with a well-marked choroid fissure and
 
groove, while the lens is completely separated from the outer ectoderm and is in the vesicle stage. Of the various regions of the
 
ear, all the primordia are now established. The otic vesicle with
 
its endolymphatic duct, representing the inner ear, is in close
 
juxtaposition to the first visceral pouch (hyomandibular) which
 
will give rise to the auditory tube and chamber of the middle ear;
 
the external auditory meatus, or outer ear, will arise from the
 
first or hyomandibular groove.
 
  
REFERENCES
+
===References===
  
 
Arey, L. B. 1934. Developmental Anatomy, 3rd Ed., Chap. 19.
 
Arey, L. B. 1934. Developmental Anatomy, 3rd Ed., Chap. 19.
  
“Boyden, E. A. 1933. A Laboratory Atlas of the Pig Embryo.
+
Boyden, E. A. 1933. A Laboratory Atlas of the Pig Embryo.
  
Keibel, I’. 1897. Normaltafeln, I, des Schweines (Sus scrofa domesticus).
+
Keibel, F. 1897. Normaltafeln, I, des Schweines (Sus scrofa domesticus).
  
Lewis, F. T. 1902. The gross anatomy of a 12 mm. pig, Am. Jour. Anat., Vol. 2,
+
Lewis, F. T. 1902. The gross anatomy of a 12 mm. pig, Am. Jour. Anat., Vol. 2, pp. 211-226.
pp. 211-226.
 
  
‘Minot, C.S. 1911. A Laboratory Textbook of Embryology, 2nd Ed.
+
Minot, C.S. 1911. A Laboratory Textbook of Embryology, 2nd Ed.
  
 
Patten, B. M. 1931. The Embryology of the Pig, 2nd Ed.
 
Patten, B. M. 1931. The Embryology of the Pig, 2nd Ed.
  
Wallin, E. 1917. A teaching model of a 10 mm. pig embryo, Anat. Rec., Vol. 5,
+
Wallin, E. 1917. A teaching model of a 10 mm. pig embryo, Anat. Rec., Vol. 5, pp. 17-45.
pp. 17-45.
+
 
 +
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Shumway W. Introduction to Vertebrate Embryology. (1935) John Wiley & Sons, New York

Shumway (1935): Preface - Contents | Part I. Introduction | Part II. Early Embryology | Part III. Organogeny | Part IV. Anatomy of Vertebrate Embryos | Part V. Embryological Technique
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Introduction to Vertebrate Embryology (1935)

Part IV Anatomy Of Vertebrate Embryos

Chapter XI The Anatomy Of Frog Embryos

In earlier chapters we have discussed the fertilization of the frog’s egg (page 57), its cleavage (pages 97, 103), and germ-layer formation (pages 109, 118), and have observed that while the germ layers are being laid down the process is complicated by the early localization of some of the organ systems, notably the sensorynervous complex (page 129). In this account of later organogeny, three stages of development seem especially significant: first, an early embryo of about 3 mm. body length in which the visceral grooves are apparent, a stage attained in Rana pipiens about the second day after the eggs are laid; second, the newly hatched larva of about 6 mm. with external gills developing, about two weeks old; third, a young “ tadpole ” stage of about 11 mm. with the opercula covering the internal gills, about the age of one month.

These stages are easily identified even though the lengths and ages can be given only approximately, for the rate of development is greatly influenced by the prevailing temperature, and the size of the tadpole is determined largely by external factors, such as the amount of food available.

The student must bear in mind that the sections illustrated in this and the two chapters following are for the sole purpose of giving him starting points from which he 1s expected to study all the sections in the series furnished him. He will probably never encounter sections exactly like those selected for these illustrations, but he will discover sections very like them from which he can commence his own observations.

A. THE EARLY EMBRYO (3 MM.)

External form. — This stage corresponds approximately to the embryo of 33 mm. described by Marshall. The head region, through its more rapid growth, has become easily distinguishable from the trunk, which bulges ventrally on account of the large

amount of contained yolk, and a well-marked tail bud is present. 275 276 THE ANATOMY OF FROG EMBRYOS

The neural folds have fused throughout their length, and enclosed the blastopore. In the head the stomodeum appears as an antero-posterior slit on the anterior ventral surface, and is enclosed by ridges identifiable as the maxillary processes and mandibular arches. On either side and slightly ventral to the stomodeum, are the primordia of the sucker or oral gland. At the dorso-lateral margins the olfactory placodes have begun to evaginate. Lateral bulges on either side of the head are due to the developing optic vesicles. The ear is now in the otic vesicle stage. The gill region shows five visceral grooves. Immediately behind the last arch, a swelling is caused by the developing pronephros. Dorsally, slight furrows indicate the boundaries of thirteen soEpiphysie mites. Beneath the tail Optic vesicle bud, the proctodeum Prosencephalon has united with the

Oral gland hind-gut to form the Visceral pouch eloacal aperture.

Fore gut Endodermal derivaay Liver tives. - The anterior portion of the gastrocoel is now a large fore-gut with a thin-walled lining. From this, on a . either side, the begin eurenteric . : canal nings of three visceral Fig. 182. —3 mm. frog embryo, viewed from right pouches can be seen.

side as a transparent object. X15.

From the fore-gut a narrow evagination grows backward into the floor of the mid-gut as the primordium of the liver. The mid-gut is distinguishable by its relatively narrow lumen and thick yolk-laden floor. The small but thin-walled hind-gut opens above into the neurenteric canal by which it is connected with the neurocoel, and opens ventrally to the exterior by way of the proctodeum. An axial rod, the hypochord, is found beneath the notochord. It originates from the roof of the gastrocoel and disappears soon after hatching.

Mesodermal derivatives. — The notochord is large and vacuolated and enclosed by two sheaths. The somites have now attained their maximum number (13) in the trunk, but are not



Mesencephalon

Otic vesicle Rhombencephalon

Somite I Notochord + THE EARLY EMBRYO 277

yet distinguishable in the tail region. The intermediate mesoderm, after a temporary division into nephrotomes, is now reunited into a nephrotomal band in which spaces have appeared opposite the second, third, and fourth somites, indicative of the pronephric tubules which are to develop. A thickening along the



Mesencephalon PD, Prosencephalon


Neurenteric canal

Fia. 1838. —3 mm. frog embryo. Sagittal section: 50.

nephrotomal band immediately below the ventro-lateral margins of the somites is the primordium of the pronephric duct. Immediately below the floor of the fore-gut, the lateral mesoderm has separated into dorsal splanchnic and ventral somatic layers, while the contained space is the beginning of the pericardial cavity, the only region of the coelom yet apparent.

Ectodermal derivatives. — The epidermis at this stage is ciliated. The neurocoel, as has been remarked above, is con278 , THE ANATOMY OF FROG EMBRYOS

nected with the hind-gut by the neurenteric canal. At the anterior end, the brain is distinguishable by its relatively larger lumen and by the cranial flexure over the anterior end of the notochord. The divisions between the three primary vesicles are not marked by the constrictions characteristic of many vertebrates, but are distinguished by the following points of reference: the prosencephalon extends to a Optic line projected from a thickvesicle ening on the floor known as the tuberculum posterius to a point just in front of a similar thickening on the Hypophysis — dorsal wall; the mesencephOral gland = aon, from the boundary of the prosencephalon to a line Fia. 184. —3 mm. frog embryo. Transverse connecting the tuberculum section through optic vesicle. 50. anda point just behind the dorsal thickening; the rhombencephalon merges imperceptibly into the spinal cord. From the prosencephalon, the optic vesicles extend on either side and cause the external bulges already noted. From the ventral side of the prosencephalon, a depression, the infundibulum, extends towards the hypophysis, which in the frog grows inward as a solid wedge of ectodermal cells anterior to the stomodeum. Dorsally, the epiphysis appears as a median evagination.



Fore-gut

B. THE LARVA AT HATCHING (6 MM.)

External form. — Although the larva, if it may be so called, has emerged from the protecting membranes of egg jelly, the mouth has not yet opened and for several days the yolk is still the sole source of food. The head region is still easily distinguishable from the trunk, while the tail has increased greatly in length and has become bilaterally compressed. In the head, the stomodeal pit has deepened at the anterior end, and the maxillary processes and mandibular arches are more sharply sculptured. The invagination of the nasal (olfactory) placodes has THE LARVA AT HATCHING 279


Fig. 185.—3 mm. frog embryo. Transverse section through otic (auditory) vesicle. 50.


Fig. 186. —8 mm. frog embryo. Transverse section through mid-gut and liver. X50. 280 THE ANATOMY OF FROG EMBRYOS


Fig. 187. — 3 mm. frog embryo. Frontal section through optic stalks, liver, and hind-gut. 50. THE LARVA AT HATCHING 281

continued to the point where they may be called pits, connected to the anterior margins of the stomodeal pit by oro-nasal grooves. The bulge of the eye is still prominent. The primordia of the oral glands have fused to form a U-shaped sucker ventral and posterior to the stomodeum. The visceral grooves are still separated from the visceral pouches by closing membranes, while on the third and fourth arches external gills have appeared. Behind them the pronephric elevation is well marked, and continues backward as a slight ridge marking the pronephric duct. Intersomitic grooves are still apparent. On the ventral side at the base of the tail is the cloacal aperture.

Prosencephalon




Optic cup

Mesencephalon é

Otic vesicle a

Heart Rhombencephalon S External gills Pronephros i a— Liver


Yolk

Myotomes

A Fig. 188. — 6 mm. frog larva (just hatched). Transparent preparation, viewed from right side. X15.

Endodermal derivatives. — On either side of the fore-gut are to be seen five visceral pouches, although they would hardly be recognized as such since they are so compressed. A groove on the ventral side of the pharyngeal cavity is the primordium of the thyroid gland. At this stage, also, the dorsal epithelial 282 THE ANATOMY OF FROG EMBRYOS

bodies of the first two visceral pouches (hyomandibular and first branchial) may be distinguished. The liver diverticulum has increased in length. The hind-gut has lost its connection with the neurocoel through the occlusion of the neurenteric canal, but now receives the posterior ends of the pronephric ducts.

Mesodermal derivatives. — The notochord has grown back into the tail. The somites have now become differentiated into the myotomes, dermatomes, and sclerotomes, while from the myotomes muscle cells have been formed. The pronephros is now established. There are three pronephric tubules, each opening into the coelom by means of a ciliated nephrostome. Opposite these, a mass of capillaries, connected with the dorsal aorta, forms the so-called glomus, equivalent to the separate glomeruli of other vertebrates. The pronephric tubules grow backward into the pronephric ducts, which have acquired lumina. At the time of hatching, the primordia of the heart have fused to form a tube, twisted slightly and almost S-shaped, suspended in the pericardial cavity by a dorsal mesocardium. ‘Two regions may be distinguished, the posterior atrium and anterior ventricle. From the ventricle leads the bulbus, arising from the fusion of paired primordia. This connects with the dorsal aorta, also the result of fusion, by means of aortic arches in the third and fourth visceral arches (vestiges of the first and second aortic arches have already appeared and disappeared). At a slightly later stage, loops from these arches will grow out into the external gills to form a branchial circulation. The anterior ends of the dorsal aortae are prolonged to form the internal carotids, while the posterior ends unite directly above the heart, and just after uniting give off the glomi on either side. Both the somatic and splanchnic venous systems are represented at this stage. Two vitelline veins unite to enter the heart at the sinus venosus. The cardinal veins at this time are represented by irregular lacunar spaces in the head and near the pronephros.

Ectodermal derivatives.— The epidermis is still ciliated. From the prosencephalon the thin-walled cerebral vesicle has appeared. The epiphysis is well marked, and the infundibulum is in contact with the hypophysis. At this time the primordia of cerebrospinal nerves may be distinguished. In the spinal nerves, dorsal roots arise from the ganglia produced by the segTHE LARVA AT HATCHING


283 Infundibulum Epiphysi Mesencephalan. piphysis Prosencephalon Rhombencephalon Fore gut Oral gland

Heart

Notochord

Liver

Sittegretee SEN Bengawere ee

ee

pear = a 2

os

a ‘5-3 eo Re Od SA eGo

a Hoe Raa pide Pina SRL

eC PRES ee

BJ

eS Fs “

i) ce ‘ ret OKs Ve on

a

A)

a

Yolk

Ps ae oe

Fig. 189. — 6 mm. frog larva.

Sagittal section, anterior portion. 50. 284 THE ANATOMY OF FROG EMBRYOS



Prosencephalon

Optic cup Lens Optic . ZH stalk FF 5 —_ Notochord

Fore-gut

Stomodeum

Fig. 190. — 6 mm. frog larva. Transverse section through optic cup. 50.

Otic vesicle

cavity

Fig. 191. — 6 mm. frog larva. Transverse section through otic vesicle. 50. THE LARVA AT HATCHING 285

mentation of the neural crest while the ventral roots arise from neuroblasts in the spinal cord. In the head, four ganglia arise and with each is associated a placode of nervous ectoderm. From the first ganglion and placode, the trigeminal (V) nerve arises. The second combination gives rise to the facial (VII) and acoustic (VIII) cranial nerves, while the remainder of this placode invaginates to form the otic vesicle. The third ganglion and placode produce the glossopharyngeal (IX) cranial nerve, and the

Pronephric tubules


Fig. 192. — 6 mm. frog larva. Transverse section through pronephros. 50.

fourth gives rise to the vagus (X). The fourth placode grows back as far as the tail, giving off as it goes small groups of cells which later become the lateral line organs of the trunk. Those of the head arise from the second and third placodes. At this time, also, ganglion cells are migrating toward the dorsal aorta to aggregate as the ganglia of the autonomic nervous system. The eye is well advanced in development, as the optic vesicles have invaginated to form the optic cup and the lens placode has separated from the epidermis and acquired a cavity. The ear is in the otic vesicle stage with an endolymphatic duct. The nose is still represented by the nasal pits. From the prolongation of the fourth placode referred to above, the lateral line system is in process of formation. 286 THE ANATOMY OF FROG EMBRYOS




Visceral 1 pouch

I Visceral arch

Pronephric tubules

Segmental muscles

Fig. 193. —6 mm. frog larva. Frontal section through nasal pit and visceral

pouches. 450. THE YOUNG TADPOLE 287

C. THE YOUNG TADPOLE (11 MM.)

External form. — The head and trunk are now fused into a common ovoid mass, sharply distinguished from the long bilaterally compressed tail. The mouth is open and equipped with horny raspers, while the oral gland is reduced to two vestiges on the ventral side of the head. On the dorsal surface, the large eyes, now functional, protrude slightly. Anterior to these are the external openings of the nasal tubes (external nares). The external gills, which were developing at hatching, have now degenerated and been replaced by internal gills concealed from view by the opercula. On the left side, the opercular aperture serves as a means of egress for the water from which the gills obtain their oxygen. The tail, now two-thirds the length of the tadpole, has a dorsal and a ventral fin. Close to the margin of the latter, at the base of the tail, is the cloacal opening.

Endodermal derivatives. — The mouth has been formed by the breaking through of the oral membrane. From the pharynx, all the visceral pouches except the hyomandibular and the vestigial sixth pouch open to the exterior as visceral clefts; and demibranchs have arisen on the anterior and posterior margins of the third, fourth, and fifth visceral arches and on the anterior margin of the sixth. These are the internal gills which hang down into the opercular cavity. The epithelial bodies from the hyomandibular pouch have disappeared. Those from the second pouch form the thymus gland, while similar buds arise from the third and fourth but presently disappear. The ventral epithelial bodies of the second pouch are said to give rise to the carotid gland, and those of the third and fourth to “ parathyroids.” The fifth pouch never gains communication with the exterior but gives rise to the ultimobranchial bodies. The thyroid is now separated from the pharynx. In the tadpole the pulmonary organs consist of a pair of thin-walled sacs, the lungs, arising from a laryngeal cavity connected with the pharynx by a narrow opening, the glottis. Posterior to the pharynx comes the esophagus, which was occluded just before the opening of the mouth but now possesses a narrow lumen opening into the stomach, which is not greatly dilated. The vesicle, which formerly represented the liver, persists as the gall bladder and common bile duct, rela288 THE ANATOMY OF FROG EMBRYOS


Internal gills

Intestine

Fig. 194. — 11 mm. frog larva.1_ Transparent preparation viewed from right side. X15.

1 Figs. 194-198 inclusive are from preparations loaned me by Dr. A. R. Cahn. In earlier editions they were labelled 9 mm., as measured after preservation. THE YOUNG TADPOLE 289


Infundibulum

Stomach Notochord Intestine Dorsal aorta Yolk Muscles of tail

ie Fig. 195. — 11 mm. frog larva. Sagittal section, anterior part. 40. 290 THE ANATOMY OF FROG EMBRYOS

tively small in comparison with the great glandular mass of the liver. Although the pancreas arose from paired primordia of the duodenum, these have now shifted their position so that their ducts open into the common bile duct. The intestine is extremely long and coiled into a double spiral. It terminates in a slightly dilated rectum, opening into the cloacal cavity which also receives the pronephric ducts and opens to the exterior by the cloacal aperture.

Mesodermal derivatives. — The notochord has elongated toward the posterior end, accompanying the growth of the tail. The two most anterior somites have disappeared, leaving eleven in the trunk region and a much larger and variable number in


Fig. 196. — 11 mm. frog larva. Transverse section, through eye. X40.

the tail. In the tail the myotomes have given rise to the dorsal and ventral musculature. The pronephros has become larger and more complicated through the branching of the pronephric tubules. The coelom consists of a pericardial cavity containing the heart, whose myocardia have disappeared, and an abdominal cavity in which the gut is suspended by the dorsal mesentery. These cavities are still continuous up to the time of metamorphosis. In the heart the sinus venosus is now a large transverse sac; the atrium is partially divided by the interatrial septum; the ventricle has thick muscular walls; and the short bulbus opens into the ventral aorta which is divided into proximal and distal portions by a pair of valves. The ventral aorta is divided into THE YOUNG TADPOLE 291

four afferent branchial arteries, the ventral portions of aortic arches III-VI. From these the blood passes through the internal gills by means of capillaries and is conveyed to four efferent branchial arteries, the dorsal portions of the aortic arches referred to above, which in turn lead to the dorsal aortae. The carotid arteries are connected in front of and behind the infundibulum by commissural vessels, and continue forward as the anterior cerebral arteries. From the anterior commissure the basilars run backward and the anterior palatines forward. The pharyngeal

Myelencephalon


Otic vesicle


Horizontal

Fig. 197. — 11 mm. frog larva. Transverse section through ear. X40.

artery, running forward from the dorsal aorta, at a point just posterior to the anterior commissure, represents the dorsal portion of the mandibular arch; the lingual artery arises independently and unites with the first efferent branchial. From the efferent branchial arteries of the sixth arch, the pulmonary arteries grow backward to the lungs. The vitelline veins have been broken up, by their inclusion in the developing liver, into hepatic veins, opening into the sinus venosus, and hepatic-portal veins from the intestine. The anterior cardinal veins are formed by the union of the superior jugular and facial veins and empty into the common cardinals. From the ventral side of the head the inferior jugulars drain into the common cardinals. The posterior somatic veins are the posterior cartlinals, which return the blood from the 292 THE ANATOMY OF FROG EMBRYOS

region of the pronephros into the common cardinals. The lymphatic vessels of the tadpole have arisen from the confluence of numerous, small intercellular spaces in the mesenchyme. Ectodermal derivatives. — The epidermis is no longer ciliated. The cerebral vesicle is now subdivided into right and left portions, while immediately behind this is the choroid plexus of the diencephalon. The pineal gland is connected with the diencephalon by:a small stalk; the pituitary gland has lost all connection with the exterior. In the mesencephalon the optic

Neural tube



Notochord



Intestine

Fig. 198. — 11 mm. frog larva. Transverse section through pronephros. X40.

lobes are just apparent. The metencephalon is distinguishable by the thickness of its walls as compared with the choroid plexus of the myelencephalon. All cranial nerves and spinal nerves are now established. The eye now contains all elements necessary for functioning; rods and cones of the sensory layer connect with the neurons of the optic nerve; pigment is deposited in the pigment layer; the choroid and sclerotic layers have been formed from mesenchyme; the lens is transparent, as is the cornea formed from the ectoderm. The otocyst is partially divided by a dorsal partition into an outer saccule and inner utricle. The nasal pits have grown backward as solid rods which by now have acquired lumina and will soon open into the


Fig. 199. — 11 mm. frog larva. Trontal section through nose, eye, and ear. 40.

References

Huxley, J. S., and de Beer, G. R. 1934. The Elements of Experimental Embryology, Chap. 2.

Jenkinson, J. W. 1913. Vertebrate Embryology, Chap. 7.

Kellicott, W. E. 1913. Chordate Development.

Marshall, A. M. 1898. Vertebrate Embryology, Chap. 3.

McEwen, R.S. 1931. Vertebrate Embryology, 2nd Ed., Part 2.

Morgan, T. H. 1897. The Development of the Frog’s Egg.

Zeigler, H. E. 1902. Lehrbuch der vergleichenden Entwickelungsgeschichte der niederen Wirbeltiere.

Chapter XII The Anatomy Of Chick Embryos

The traditional stages in the development of the chick Gallus domesticus) for laboratory practice are those at the end of each of the first three days of incubation. So many important changes take place during the second day, however, that it is advisable to study an additional stage intermediate between twenty-four and forty-eight hours in age. The chick of thirty-three hours is selected because the form of the embryo is not yet affected by torsion or flexure, and the headfold of the amnion has not yet slipped over the head of the chick.

As it is a well-known fact that, in these first few days of incubation, embryos of the same age have attained varying degrees of development, the length of the embryo has been proposed as a mark of identification. The flexures of the body, however, make this standard impracticable, and the remaining alternative is to select the specific development of some particular structure as a basis of arrangement. For this purpose the number of somites, suggested by Lillie, is admirable. Still, it must be remembered that on account of the effect of temperature upon the rate of development, the number of somites is not correlated exactly with the number of hours of incubation, as may be seen from the following table.



TABLE 12 Duval Keibel Lillie Patten About 24 hours Fig. 76 Vig. 9, 9A Vig. 59 Fig. 36

(24 hrs. 6S) (24 hrs. 7-88) (25 hrs. 7S) (27 hrs. 8S)


About 33 hours Fig. 93 Fig. 10, 10A Fig. 68 Fig. 39 (33 hrs. 168) (32 hrs. 9 S) (33 hrs. 128) (33 hrs. 128)


About 48 hours Fig. 109 Fig. 16, 16A Fig. 93 Fig. (48 hrs. 27-28S)| (52 hrs. 278) a8 hrs. 278) (55 hrs. 2 S)


About 72 hours Fig. 115 Fig. 18, 18A g. 117 g. 63 (68 hrs. 37S) {(67 hrs. 35-37 S) (ak 18s, 35S) ah ee 368)






° 294 TWENTY-FOUR HOURS 295

A. THE TWENTY-FOUR HOUR STAGE

At the end of the first day of incubation, the chick embryo has completed the period of cleavage (pages 98, 105) and germ-layer formation (pages 111, 121), and is in the early stages of organogeny.


Fig. 200. — 24 hour chick embryo. Cleared preparation from dorsal side. X25.

External form. — The embryo, 3.3 mm. in length, lies along the axial line of the slipper-shaped area pellucida which in turn is surrounded by the crescent-shaped area vasculosa, whose anterior horns, separated by the proamnion, reach about to the level of 296 THE ANATOMY OF CHICK EMBRYOS

tip of the head. At the anterior end, the head fold of the embryo is lifted above the proamnion from which it is separated by the subcephalic pocket. In the head fold is contained the fore-gut,

0.59 mm. in length, which opens at its

posterior end into the yolk cavity by

means of the anterior intestinal portal.

On either margin of the portal the pri mordia of the vitelline veins are to be

recognized in thick bands of splanchnic mesoderm. The neural plate has already given rise to the neural folds which extend back as far as the first somite. They have united just posterior to the region where the optic vesicles are _ to appear and thus have given rise to a neural tube 0.3 mm. in length, which is widely open in front and behind as the anterior and posterior neuropores, respectively. Behind the head fold the axial mesoderm is segmented into six somites. Between the neural folds the notochord can be recognized as a faint line which joins, at its posterior end, the

primitive streak, now reduced to 0.83

mm. in length.

Endodermal derivatives. — The only differentiation which has taken place in the endoderm consists of the establishment of the fore-gut by means of the folding off of the head from the proamnion. As this process continues the fore gut will be lengthened at the expense of

Fia. 201. — 24 hour chick em- the widely open mid-gut, and the an ean Sagittal section. terior intestinal portal: will progress steadily backward.

Mesodermal derivatives. — The mesoderm proper does not extend into the head, but a loose aggregate of mesenchyme derived from it is present. Posterior to the head the axial mesoderm is divided into six somites. Transverse sections show that TWENTY-FOUR HOURS 297

Epidermis , , Brain




Mesenchyme

Splanchnic mesoderm

Proamnion


Somatic mesoderm Fore-gut


Ectoderm Endoderm.

Fig. 202. — 24 hour chick embryo. Transverse section through brain region. The neural folds have met but are not yet fused together. X50.


Axial mesoderm Notochord ge ie See SO ete Ry oe ayer Vitelline vein Amnio-cardiac Splanchnopleure vesicle

Fig. 203. — 24 hour chick embryo. Transverse section through region of intestinal portal. X50.

Neural groove

| Somite IY Ectoderm

Exocoel





PO Blood island nom i ram) Canes.

Notochord So Endoderm ~

Fig. 204. — 24 hour chick embryo. Transverse section through fourth somite. X50.

Intermediate mesoderm

Primitive groove

Endoderm

HH, BY Fig. 205. — 24 hour chick embryo. Transverse section through primitive streak. x50. 298 THE ANATOMY OF CHICK EMBRYOS

each has a minute cavity, or myocoel. The intermediate mesoderm does not divide into nephrotomes as in the frog. The lateral mesoderm is divided into the somatic and splanchnic layers. In the latter, numerous blood islands appear and give the characteristic mottled appearance to the area vasculosa. The coelom of the embryo is continuous with that of the extra-embryonic regions, or exocoel. In the region on either side of the head, between the proamnion and the intestinal portal, the coelom is distended into an amniocardiac vesicle, so called because the somatopleure will contribute to the head fold of the amnion, while the splanchnic mesoderm will give rise to the primordia of the heart, and the cavities of the vesicles will unite to form the pericardial cavity. The notochord, from its point of origin, the primitive streak, extends forward into the head.

Ectodermal derivatives. — The ectoderm at this stage consists of the elongate neural plate, with its groove and folds which are already in process of fusion, and the epidermis or non-nervous ectoderm.

B. THE THIRTY-THREE HOUR STAGE

External form. — In the chick embryo, after thirty-three hours’ incubation, the length has increased to 4.3 mm. There is a slight bending of the head downward over the end of the notochord, foreshadowing the cranial flexure. The area vasculosa, in which the blood islands are being converted into capillaries, now has grown in toward the embryo, so that the area pellucida persists only around the head and tail regions. The anterior horns of the area vasculosa have met in front, completely inclosing the proamnion. The head has increased in length not only by actual forward growth but also by the backward extension of the lateral margins of the head fold, so that the enclosed foregut is now 1 mm. long. The vitelline veins are prominent at the margins of the intestinal portal and continue on the ventral side of the fore-gut to meet at the posterior end of the heart, which is now a single tube, slightly bent toward the right. The neural folds are fused as far back as the eleventh somite, where the posterior neuropore is now known as the rhomboidal sinus. The anterior neuropore is about to close, and in the head the neural tube shows three regions of dilation which represent the THIRTY-THREE HOURS 299

Head fold |. of amnion



4 neuropore Prosencephalon

Optic 7 vesicle Mesencephalon Foregut Rhombencephalon & Heart

- Vitelline vein

Somite 6

Sinus rhomboidialig

Primitive streak

Fig. 206. — 33 hour chick embryo. Cleared preparation from dorsal view. X25. 300 THE ANATOMY OF CHICK EMBRYOS

fore-brain, mid-brain, and hind-brain, respectively. The sides of the fore-brain are evaginating to produce the optic vesicles.

Head fold . of amnion

Prosencephalon— Subcephalic F pocket Mesencephalon Fore-gut Pericardial cavity


Rhombencephalon# 6.4. /"

Notochord

Fig. 207. — 33 hour chick embryo.

Sagittal section.

are ~ Anterior

intestinal portal

i Primitive cm, streak

aN

X25.





In the hind-brain, five neuromeres can be identified. Twelve somites may be counted. The notochord extends forward to the fore-brain from the primitive streak which is now reduced to 0.3 mm.

Endodermal derivatives. — The anterior end of the fore-gut is in contact ventrally with the stomodeum separated only by the oral plate, composed of ectoderm and endoderm. At the sides, the walls of the fore-gut are fused to the ectoderm at points where the first visceral pouches (hyomandibular) will be located.

Mesodermal derivatives. — The somites now number twelve, and myocoels are still apparent. The mesomere is still unsegmented, but pronephric tubules have appeared in the region corresponding to somites 5-12. The four posterior tubules are growing back to form the pronephric duct. In the splanchnic mesoderm the blood islands are being converted into capillaries. The vitelline veins are prominent and continue forward into the heart, of which the endo -cardium and myocardium are dis tinct. The heart is supported by the dorsal mesocardium, the ventral mesocardium having disappeared. The primordial tubes, from the

fusion of which the heart arose, continue forward as the ventral aortae which bend around the pharynx (first aortic arches) and THIRTY-THREE HOURS 301

continue backward along the dorsal surface of the pharynx as the dorsal aortae. At the level of the primitive streak they are lost in a capillary nexus which foreshadows the vitelline arteries. From a point immediately in front of the optic vesicle, the anterior cardinals course backward on either side of the neural tube, bending down ventrally to enter the heart with the vitelline veins. The notochord is slightly bent at the anterior end.

Ectodermal derivatives. The ncural folds now extend to the eleventh somite and have fused throughout the length of the head. The anterior neuropore is almost closed. The three

Prosencephalon

Epidermis Mesenchyme

Optic vesicle

=» _

ts wa P Exocoel é OOF


Splanchnopleure Sub-cephalic pocket

Fia. 208. — 33 hour chick embryo. Transverse section through optic vesicles. X50.

dilations which represent the prosencephalon, mesencephalon, and rhombencephalon are distinct. From the prosencephalon the two optic vesicles extend to the ectoderm of the sides of the head. Five neuromeres may be identified in the rhombencephalon. It has been asserted that in earlier stages three neuromeres may be identified in the prosencephalon and two in the mesencephalon, while the first of the five noted above has resulted from the fusion of two original neuromeres destined to give rise to the metencephalon. At about this time a shallow depression in the floor of the prosencephalon, just in front of the tip of the notochord, marks the appearance of the infundibulum. The auditory placodes may sometimes be seen in sections as thickenings at the level of the constriction separating the last two neuromeres on either side. 302 THE ANATOMY OF CHICK EMBRYOS

Rhombencephalon






Notochord Fore-gut Otic ( auditory) placode Somatopleure TET Dorsal aorta Dorsal [Bi soy 7S uateral sulcus mesocardium B Feria

5 Way, Oe ee ae ox ed fhe & wou ee se Os a lamest ~ Endocardium Splanchnopleure Fig. 209. — 33 hour chick embryo. Transverse section through otic placodes.

x50.

Spinal cord




Dorsal aorta Somite

Intermediate mesoderm Exocoel

Se

Vitelline vein

Fie. 210. — 33 hour chick embryo. Transverse section through vitelline veins. x50.

Spinal cord

Neural crest Somite



Intermediate mesoderm Somatic layer

NAPRY

Fig. 211. — 33 hour chick embryo. Transverse section through sixth somite. 50. FORTY-EIGHT HOURS . 803

Cc. THE FORTY-EIGHT HOUR STAGE — External form. — The chick at the end of the second day of incubation has usually attained a length of 7 mm., but the form

of the body has been altered profoundly. As the head has been lifted away from the blastoderm, it has increased greatly in size,









Ww ir ot y h h, Mesencephalon ‘Rhombencephalon

Otic vesicle Optic cup

Lens vesicle Visceral cleft I Prosencephalon

0 i

Sinus venosus— Vitelline vein—

Atrium Bulbus arteriosus Ventricle

Amniotic fold Somite XIV

Vitelline artery

Neural tube

Tail fold

aed cent Fia. 212.—48 hour chick embryo. Transparent preparation from dorsal view (head from right side). X15.

and the cranial flexure, which was just appearing in the thirtythree hour chick, has become so pronounced that the anterior end of the head is directed backwards. With this growth and flexure the head is twisted normally to the right, until it lies on one side, a phenomenon known as torsion. At forty-eight hours, this torsion involves the chick as far back as the seventeenth somite. The posterior end of the chick lies in its original position, and at the extreme caudal end a tail fold is being formed. In the Fig. 213. — 48 hour chick embryo. (304)

frontal section due to torsion.

x50.

Head in sagittal section, somite region in

Stomodaeal plate Telencephalon.


Diencephalon

Myelencephalon


Metencephalon




Mesencephalon FORTY-EIGHT HOURS 305

area vasculosa the capillaries have formed attachments with the vitelline arteries and veins, and at the border of this area is a circular vessel, the sinus terminalis. The fore-gut is now 1.4 mm. in length, and the first of the three visceral pouches now communicates to the exterior following the rupture of the closing plate which separated it from the corresponding visceral groove. The second and third visceral grooves are apparent, but their closing plates are still unperforated. In the visceral arches the first three aortic arches are apparent, arising from the ventral aorta. The heart is now twisted so that the ventricular loop is upper Anterior cardinal vein Dorsal aorta

Otic pit





eB 5

Chorion



Yolk sac tent Notochord eet Blood island Visceral groove I a Pigment layer

Visceral pouch I Hypophysis Sensory layer


Fia. 214. — 48 hour chick embryo. Transverse section through otic pit and optic cup. 650.

most. The vitelline veins are large and conspicuous, as are the vitelline arteries which leave the body at the level of the twentysecond somites. The neural tube is completely closed. In the head the five definitive regions of the brain are outlined, the prosencephalon having given rise to the telencephalon and diencephalon, and the rhombencephalon to the metencephalon and myelencephalon. The eye is now in the optic cup stage, and the invagination of the optic vesicle continues down the stalk to form the choroid fissure. The lens is in the form of a pit which has almost attained the vesicle stage. The ear is represented by an otic pit which, owing to the cervical flexure, is about on a level with the eye. There are twenty-seven somites at this stage. The primitive streak is found only in the tail fold. At this time 306 THE ANATOMY OF CHICK EMBRYOS

the head fold of the amnion has grown back over the chick as far as the sixteenth somite.

Endodermal derivatives. — The stomodeum, an ectodermal invagination from the ventral surface of the head fold, has formed the oral membrane by contact with the fore-gut a little back of its most anterior point. Hence there is a blind pocket in front of the oral plate, known as the preoral gut. Three visceral pouches are present, the first of which opens into the corresponding visceral furrow following the rupture of its closing membrane. The primordium of the thyroid is represented by a ventral depression in the floor of the pharynx at the level of the second visceral pouches. The primordia of the lungs (sometimes difficult to distinguish) extend to the level of the sinus venosus. The liver arises at the level of the anterior intestinal portal from two evaginations of the endoderm, one below and one above the meatus venosus. ‘The mid-gut now has two shifting boundaries, the anterior intestinal portal and the posterior intestinal portal. The latter is barely apparent as the opening of a shallow endodermal pocket or hind-gut in the tail fold.

Mesodermal derivatives.—-The somites, twenty-seven in number, show a varying degree of specialization, with the most advanced at the anterior end. In these two regions can be distinguished: a loose aggregate of cells at the median ventral angle (the sclerotome); and a cap of epithelial cells at the lateral dorsal angle. The cells of this cap nearest the epidermis will form the dermatome, while those nearest the neural tube will form the myotome.

The pronephric tubules in the more anterior somites have disappeared and mesonephric tubules are appearing in the mesomere posterior to the thirteenth somite. The pronephric (now the mesonephric) duct has acquired a lumen but has not yet attained its complete backward growth.

The heart is still tubular, but the ventricular limb of the cardiac loop has grown back and over the atrial limb so that the ventricular region is now caudal and dorsal with relation to the atrial region. Three aortic arches are present as a rule, but infrequently the third has not developed. From the first aortic arch a network of capillaries extends into the head. From these the carotid arteries will be formed. The dorsal aortae have fused FORTY-FIGHT HOURS 307

from a point back of the sixth somite as far as the level of the fifteenth somite. The vitelline arteries leave the dorsal aortae at the level of the twenty-second somite but the aortae continue

Common cardinal Bulbus vein arteriosus Chorion

I.- poem 2s Cra, / Dorsal aorta Fore-gut er , bAY

Notochord




Spinal cord

,

as

Epidermis 5 J

Amnion

“ Somiter Coelom

‘Dorsal mesocardium Fia. 215. — 48 hour chick embryo. ‘Transverse section through heart. X50.

backward as the caudal arteries to the last somite. The vitelline veins are fused at their point of entrance into the heart as the sinus venosus. The anterior cardinals are prominent and extend from a capillary plexus in the head back toward the heart, where they

Somite

Notochord | Dorsal aorta





Vitelline vein,

Choria

- Armnion— Gi N Coelom —

Posterior cardinal vein

Ventricle


Meatus Mid-gut: *

venosus

Fig. 216. — 48 hour chick embryo. Transverse section through liver. 50.

are joined by the posterior cardinals and proceed as the common cardinals to: enter the heart in the angles between the sinus venosus and the vitelline veins. The posterior cardinals may be traced back to the last somite. The heart of the chick commenced 308 THE ANATOMY OF CHICK EMBRYOS

beating at the forty-fourth hour of incubation, so that the course of the blood is through the ventral aorta to the aortic arches and thence to the dorsal aorta. From the first aortic arch a network of capillaries supplies the head with blood (which is returned by way of the anterior cardinals). The main current of the stream passes down the dorsal aortae to the point where these fuse to form the median dorsal aorta. From the dorsal aorta, the somites are supplied by capillaries, which will later become the intersegmental arteries. This blood is returned through the posterior cardinals. Leaving the dorsal aorta by way of the vitelline arteries, the blood passes through the capillaries of the area vasculosa to the sinus terminalis, and thence to the capillary drainage of the vitclline veins which return it to the heart.

The notochord is bent, not only at its tip (cranial flexure) but also at the point where the myelencephalon merges with the spinal cord (cervical flexure).

Ectodermal derivatives. — The brain now has acquired its five definitive vesicles. The telencephalon is enlarged but shows

Amniotic raphe




— Posterior cardinal vein

Mesonephric tubules

Beast

Lateral sulcus Fig. 217. —48 hour chick embryo. Transverse section through mesonephros. 50.

no particular differentiation. From the diencephalon project the constricted optic stalks which bear the optic cups with their inner sensory layer and outer pigmented layer. (The pigment will not arise until later.) The invagination by which the cups were formed continues down the stalk as the choroid groove. On the ventral surface of the diencephalon the infundibulum has deepened. Growing in toward it from the stomodeum is an ectodermal invagination, the hypophysis, which will fuse with the infundibulum to form the pituitary gland. The lens of the eye SEVENTY-TWO HOURS 309

is in the pit stage, resulting from the invagination of a sensory placode. When the process is complete, the lens will be a vesicle completely withdrawn beneath the surface of the ectoderm, as will the otic vesicle, the primordium of the inner ear. Along the rhombencephalon and cord, the neural crest is to be seen as a narrow band of cells on each dorso-lateral angle.

Myelencephalon


Otic vesicle %, Metencephalon

he 4 te

Visceral cleft I * q Mesencephalon wy Choroid fissure Optic cup Atrium and lens

Diencephalon Nasal pit - Epiphysis Telencephalon Ventricle Anterior limb bud 3 Somite 26 Vitelline 2 artery Vitelline ; . . : Posterior vem limb bud

Fig. 218. — 72 hour chick embryo. Transparent preparation from dorsal view, head seen from right side. X15.

D. THE SEVENTY-TWO HOUR STAGE External form. — At the end of the third day of incubation, the total length of the embryo is 9.5 mm., but the curvature of the body is so great, on account of the cranial and cervical flexures in addition to the newly developed caudal flexure, that the greatest length, from neck to tail, is 7 mm. Torsion involves the 310 THE ANATOMY OF CHICK EMBRYOS

body as far back as the vitelline arteries and will become complete during the fourth day. Anterior and posterior limb buds are now apparent at the levels of somites 17-19 and 26-32 respectively. The tail is curved forward. The fore-gut is still 1.4 mm. in length but has undergone further differentiation, indicated externally by the fact that the first three visceral clefts are open while the fourth is still interrupted by its closing plate. In the branchial arches four aortic arches may be seen. The telencephalon has given rise to the primordia of the cerebral hemispheres, and from the roof of the diencephalon, a small evagination represents the epiphysis or primordium of the pineal gland. The eye and ear, which were formerly in the same transverse section, are now nearly in an antero-posterior relationship. The olfactory pits have made their appearance in the head. The semilunar (fifth cranial nerve), geniculo-acoustic (seventh and eighth), and petrosal (ninth) ganglia may be seen. There are approximately thirty-five somites. The primitive streak has disappeared. The amnion is completed by the fusion of head and tail folds. The allantois, a small sac-like evagination, protrudes ventrally between the posterior limb buds.

Endodermal derivatives. — At the end of the third day the oral aperture has been formed by the rupture of the oral membrane separating the stomodeum and the fore-gut. Immediately anterior to this opening the preoral gut persists. The fore-gut is still the same length as in the chick of forty-cight hours, but is more complex in structure. The thyroid gland, which appeared during the second day, has now become differentiated into the distal dilation which will give rise to the gland proper and the thyroglossal duct. The first three visceral pouches are open to the exterior, but the epithelial buds destined to give rise to the thymus and parathyroids are not yet apparent. The fourth visceral pouch is still separated from the corresponding groove by the closing plate. The laryngeo-tracheal groove has developed in the floor of the pharynx just posterior to the fourth visceral pouches. At its posterior end the dorsal margins of this groove have closed together to form the primordium of the trachea which is thus set free from the esophagus above. The trachea is bifurcated at the posterior end, thus giving rise to the two bronchial buds which are the primordia of the lungs. SEVENTY-TWO HOURS 311

The esophagus, which is relatively narrow, is followed by a dilation which is to become the stomach. Posterior to this, the primordium of the liver may be seen as an evagination from the








Aortic arches

Dorsal.

aorta Myelencephalon

Metencephalon . Roy fandibatam

Sinus

venosus Isthmus

Atrium Spinal cord , Notochord ++—%

>Amnion Mesencephalon

Diencephalon

Telencephalon Epiphysis Mesonephros

Notochord

Spinal cord—

Fiq. 219. — 72 hour chick embryo. Sagittal section. X25.

ventral floor of the duodenal region of the gut. The dorsal pancreas arises from the duodenal region just dorsal to the liver at the end of the third day. The ventral primordia will not appear for another day. 312 THE ANATOMY OF CHICK EMBRYOS



halon

fe


Fig. 220. — 72 hour chick embryo. ‘Transverse section through otic vesicle. X25.

L esicle Dorsal aorta Optic ome

Aortic arches

d cup Sensory layer ii


Pharynx yyy

Visceral arches Fia. 221. — 72 hour chick embryo. Transverse section through optic cup. X25.









/ Esophagus Primary ‘Common cardinal Bulbus arteriosus “ . : i Chorion Amnion bronchus yon polite eee! oa 3 A ang ots + ~ Somite AfSead- m,

\ rst; Epidermis ip

3

‘Yolk sac



Pleural groove Sinus Atrium Nasal pit “venosus Pericardial cavity Fig. 222. — 72 hour chick embryo. Transverse section through heart and lung. X25.

‘ SEVENTY-TWO HOURS 313

The mid-gut region is gradually lessened by the advancing sulci which are cutting off the body of the embryo from the yolk. This region opens into the yolk stalk which is still quite wide.

The hind-gut contained in the tail fold has not yet acquired its cloacal aperture nor has the proctodeum appeared. The floor of the hind-gut between the tail bud and the posterior intestinal portal evaginates to give rise to the allantoic primordium.

Mesodermal derivatives. — The somites, typically thirty-five in number, still show a varying degree of differentiation which is carried to its furthest point in the more anterior somites. The dermatome is now a thin sheet of cells along the dorso-lateral

Posterior Dorsal cardinal} Dorsal Li _Amnion aorta vein mesentery iver






ao

Spinal cord

Notochord

Coelom

Allantoic vein


Ventral mesentery

Meatus venosus Fig. 223. — 72 hour chick embryo. Transverse section through liver. X25.

angle of the embryo, with the myotome parallel and internal; the sclerotome in these anterior segments is a large and loose aggregate of cells investing the neural tube, notochord, and aortae.

The pronephric tubules have degenerated to a considerable extent, but the nephrostomes opening into the coelom may persist. The mesonephric tubules are now in process of development, with those in the more anterior segments most highly differentiated. The tubules between the thirteenth and _ thirtieth somites have progressed from the vesicle stage characteristic of those behind the twentieth somite, and some have acquired a lumen and joined the pronephric duct which henceforward is known as the mesonephric duct. A few of the more anterior tubules develop nephrostomes, but these soon disappear. 314 THE ANATOMY OF CHICK EMBRYOS

Behind the twentieth somite, as far back as the thirtieth, only vesicles are formed. The mesonephric ducts have grown back and united with the cloaca.

The heart now shows a constriction between the atrial and ventricular region. Four aortic arches are developed, of which

Amniotic raphe

Dermatome, Sclerotome \ Spinal cord


Mesonephric Vitelline tubule artery

Lateral sulcus Dorsal aorta

Fig. 224. — 72 hour chick embryo. Transverse section through vitelline arteries leaving body. X25.

the first is becoming smaller, and somctimes has disappeared at this stage. The internal carotid arteries are now well developed, growing forward into the head from the point of union between the first arches and the dorsal aortae. From the ventral end of the first aortic arch the external carotid takes its origin. The

Chorion Mesonephric . Amnion duct Somite Dorsal ao


Fig. 225. 72 hour chick embryo. Transverse section through allantois. 25.

pulmonary is sometimes apparent as a posterior prolongation of the ventral aorta at the point where the fifth arches will appear during the next twenty-four hours. The intersegmental arteries are now apparent as dorsal diverticula from the aorta between each pair of somites. The vitelline veins have fused for a short distance behind the sinus, thus giving rise to the meatus venosus.

The anterior cardinal vein now possesses many branches from the head, among which are three intersegmental veins. The posterior cardinal has continued its backward growth dorsal to the mesonephric duct as far as the thirty-third somite. It receives the intersegmental veins of this region. Where the posterior cardinals unite with the common cardinals, a capillary network indicates the beginnings of the allantoic veins.

Ectodermal derivatives. —-'The brain at the end of the third day has its five definitive vesicles even more sharply demarcated. From the telencephalon two lateral vesicles have evaginated to form the primordia of the cerebral hemispheres. In the diencephalon the epiphysis has appeared as a dorsal evagination. On the floor of this vesicle the infundibulum is almost in contact with the hypophysis. The mesencephalon is separated from the metencephalon by a deep constriction known as the isthmus. Along the sides of the myelencephalon may be distinguished the following cerebral ganglia: the semilunar of the fifth cranial nerve; the acoustico-facialis which will later separate into the geniculate ganglion of the seventh and the acoustic of the eighth; and the petrosal ganglion of the ninth. The eye has increased in size, and the lens is now free from the epidermal ectoderm. The ear, too, is in the vesicle stage and possesses a short endolymphatic duct, which has lost its connection with the epidermis. On the third day the primordium of the nose is represented by two olfactory pits anterior to the mouth.

References

Arey, L. B. 1934. Developmental Anatomy, 3rd Ed., Chap. 18.

Duval, M. 1889. Atlas d’embryologie.

Keibel and Abraham. 1900. Normaltafeln II, des Huhnes (Gallus domesticus). Lillie, F. R. 1919. The Development of the Chick, 2nd Ed.

McEwen, R. 8S. 1931. Vertebrate Embryology, 2nd Ed., Part 4.

Patten, B. M. 1929. The Early Embryology of the Chick, 3rd Ed. CHAPTER XIII THE ANATOMY OF THE 10 MM. PIG EMBRYO

Pig embryos of 10 to 12 mm. body length are particularly instructive for laboratory work in mammalian embryology as they

Myelencephalon Metencephalon

Ear,



Mesencephalon

Trachea +

Anterior __| a —Body stalk

limb bud Roots of spinal nerves Posterior

Fig. 226. — 10 mm. pig embryo. Transparent preparation from right side. X11.

are sufficiently large for the study of external structures and yet small enough to afford serial sections for a detailed study of the anatomy. The primordia of practically all the organ systems, excepting the skeleton and musculature, are present. In comparing the accounts given by different authors of this particular stage, it should be remembered that a large amount of shrinkage

takes place during the preparation of fresh sections, so that, as 316 ENDODERMAL DERIVATIVES 317

pointed out by Patten, an embryo of 12 mm. will not measure more than 93} mm. when prepared for sectioning. The account which follows corresponds in general to the pig (Sus scrofa) of 10 mm. described by Keibel, of 12 mm. (Minot), 10 mm. (Prentiss) and 9.4 mm. (Patten), but is not so advanced as the 13.5 mm. pig (Boyden).

External form. — The pig embryo at this stage is relatively ‘more advanced than the chick of seventy-two hours. The body is sharply flexed, owing to the presence of the cranial, cervical, dorsal, and caudal flexures. In the head region the olfactory pits are well developed and are connected by the naso-lachrymal groove to a depression which surrounds the bulging eyeball. The five divisions of the brain are apparent through the relatively thin overlying epidermis. Four visceral grooves can be seen, the first of which, or hyomandibular, is the primordium of the external auditory meatus. The third and fourth grooves are compressed by the cervical flexure into a deeper depression known as the cervical sinus. A frontal view of the head shows the oral cavity bounded above by the frontal process in the middle, the maxillary processes at the side, while the lower jaw is represented by the mandibular arch.

In the trunk region, the buds of the pectoral and pelvic appendages are large but show no further differentiation. The contours of the somites, now forty-four in number, are apparent along the back, and ventral to these can be seen the outlines of the heart, liver, and mesonephros. In some specimens there appears between the limb buds a thickened ridge from which the mammary glands develop and which is therefore known as the milk line. | The umbilical cord projects from the ventral side of the embryo. Between this and the base of the slender tail is a small protuberance, the genital tubercle, or primordium of the external genitalia.

Endodermal derivatives. — The preoral gut still persists anterior to the oral aperture. Ventral to this, and seen best in sagittal section, is the long and slender hypophysis, now in contact with the infundibulum of the diencephalon. Both the hypophysis and infundibulum, it should be remembered, are of ectodermal origin. The pharynx is dorso-ventrally compressed, and from its floor the tongue is arising. Four visceral pouches 318 THE ANATOMY OF THE 10 MM. PIG EMBRYO

are present, corresponding to the visceral grooves already noted. These do not unite to become visceral clefts but remain separated by their closing membranes. Between the second and third

Metencephalon Myelencephalon



Pharynx


Posterior vena cava





//_ . Ductus venosus

i Liver Duct of ventral pancreas

Spinal { nw ; artery fDuodenum___Vitelline vein on—__ Body Dorsal stalk pancreas i ses mbilical SSS ) rte Vitelline 4 7 SN d Af ty (ant. mesenteric)’ oN i R artery cl Notochord oaca Metanephros Aorta

Mesonephros

Fia. 227.— 10 mm. pig embryo. Sagittal section. 164.

pouches the thyroid gland appears. From the level of the fourth pouch a short laryngeal groove is prolonged into the trachea which has given rise to the bronchial buds, three in number. Two of these, the primary bronchi, have arisen by the bifurcation of ENDODERMAL DERIVATIVES 319

the trachea; the third or apical bud, which will give rise to the eparterial bronchus, develops anterior to the right primary bronchus. The esophagus is relatively long and narrow and, just posterior to the level of the lung buds, passes into the stomach which is dilated and shows a slight dorsal curvature. Posterior to the stomach the duodenal glands, liver, and pancreas are well developed. The liver, now a large glandular mass traversed by




W- Nerve XI

Nerve X and jugular F Ganglion IX ganglion (superior)

Otic vesicle

~Ganglion VOI Myelencephalon ¥} (acoustic) Ganglion Y~}(semilunar) P Pog IT a1 Basilar Fee eof artery Nerve III Internal

carotid artery

Mesencephalon


Fig. 228. — 10 mm. pig embryo. Transverse section through otic vesicles. 163}.

the capillaries of the hepato-portal veins, retains its original connection with the duodenum as the common bile duct from the distal end of which the gall bladder is forming. Both dorsal and ventral primordia of the pancreas are present, the duct of the latter arising from the common bile duct. The long and slender intestine extends into the umbilical cord as the intestinal loop, to which the yolk stalk is still attached. Just posterior to this, a slight enlargement may sometimes be observed which indicates the boundary between the large and small intestine. The hind-gut is dividing into a dorsal rectum and ventral urogenital 320 THE ANATOMY OF THE 10 MM. PIG EMBRYO

sinus, prolonged into the allantoic stalk. The sinus and rectum unite in a common cloaca which has not yet established connection with the proctodeum. Immediately posterior to the cloacal plate, a small blind pocket represents the postcloacal gut.

Spinal cord.


it Dorsal root co Spinal ganglion

re Ventral root Dorsal ramus








Notochord

Anterior cardinal vein Ganglion X Aortic ( nodosum ) arch OT Radix aortae Visceral

Visceral arch

a Hypophysis Anterior cardinal vein

Sensory layer Pigment layer

Diencephalon

Fig. 229.10 mm. pig embryo. Transverse section through optic cup. 164.

Mesodermal derivatives. — The notochord extends from the vicinity of the floor of the mesencephalon into the tail, following the flexures of the body.

The somites have long since become differentiated into the myotome, dermatome, and sclerotome. In the tail region, the sclerotomes are separated into the cranial and caudal arcualia from which the vertebrae will originate.

In the pig of 10 mm., the pronephric stage has been passed; the mesonephros is at the height of its development, forming a great “Wolffian” body with a complicated network of interwoven tubules; while the mesonephric duct (originally the pronephric duct) may be recognized along the ventral margin. Emerging MESODERMAL DERIVATIVES 321

from the mesonephros, each duct enters the urogenital sinus at the same level as the allantoic stalk. From each duct a narrow stalk runs dorsally and forward as the metanephric duct, or ureter, which at its distal end is enlarged to form the pelvis of the metanephros. Around the pelvis the posterior portion of the nephrotomal band will produce the secretory tubules of the definitive kidney at a later stage. On the median ventral margin of each




Notochord

Dorsal aorta

Oesophagus Anterior

ardinal vein

Left atrium

Telencephalon

Fig. 230. — 10 mm. pig embryo. Transverse section through nasal (olfactory) pit. X 163.

mesonephros are slight swellings which will later become the genital ridges, primordia of the gonads. The coelom is partially divided into the pericardial and abdominal cavities by the septum transversum. The mesenteries of the principal viscera are in evidence. The liver is still suspended in the ventral mesentery. A dorsal mesocardium is present.

The heart of the 10 mm. pig has the four main chambers established, although not yet completely separated into right and left halves. The sinus venosus now enters the right atrium through 322 THE ANATOMY OF THE 10 MM. PIG EMBRYO

a slit guarded by the valves of the sinus. The right and left atria are partially separated by the interatrial septum in which can be seen an opening, the foramen ovale. The atrio-ventricular canal leading to the ventricle is partially separated into right and left halves by the endocardial cushion. The ventricle is partially divided by the interventricular septum. From the ventral aorta three aortic arches curve around the pharynx to unite with the dorsal aorta. These are the third, fourth, and sixth aortic arches; the first and second have degenerated, while the fifth

Spinal cord .




Ganglion Notochord

Anterior limb bud

Common Dorsal aorta cardinal vein Eparterial Oesophagus bronchus Trachea Valves of sinus Left atrium Right atrium Left Right \ ventricle ventricle

Fia. 231. — 10 mm. pigembryo. Transverse section through sinus venosus. 16}.

seldom appears as a separate structure. The pulmonary arteries are growing back from the sixth aortic arches.

As prolongations of the original paired ventral and dorsal aortae, the external and internal carotid arteries, respectively, run forward into the head. The internal carotid arteries are united at the level of the isthmus between the mesencephalon and the metencephalon with the basilar artery, which serves to unite them with the vertebral arteries, arising from the anastomosis of intersegmental arteries in the cervical region. At the 10 mm. stage the vertebral arteries have lost their intersegmental connections with the aorta except at the posterior end, where the MESODERMAL DERIVATIVES 323







Anterior limb bud:

Coelom

Posterior vena cava

Right

atrium { i Left Right i ventricle ventricle

Notochord

Dorsal aorta

Glomerulus

ketee . g ‘ Posterior Vea me AN Stomach vena cava i

by \ v

Septum 4 Pericardial ransversum — cavity a

Fig. 233. — 10 mm. pig embryo. Transverse section through stomach. X16}. 324 THE ANATOMY OF THE 10 MM. PIG EMBRYO

seventh cervical intersegmental artery persists and grows out into the pectoral limb bud to form the subclavian artery. Near the point of origin of the subclavian, the dorsal aortae are fused and run back as a single median aorta into the tail. Dorsally, branches are given off from the aorta as intersegmental arteries of the trunk. Laterally, many small branches supply the glomeruli of the mesonephros. Ventrally, the dorsal aorta gives off the coeliac artery and anterior mesenteric arteries to the gut.



Ganglion Notochord

Left umbilical vein

Fig. 234. — 10 mm. pig embryo. Transverse section through gall bladder. 163.

Two large umbilical (allantoic) arteries run from the dorsal aorta into the umbilical cord. The aorta continues into the tail as a relatively slender vessel, the caudal artery.

The vitelline veins are much smaller than in the chick of seventy-two hours, for the yolk sac from which they drew their blood is nearly degenerated. In the pig at this stage they drain the gut area and cross into the liver where they become the portal vein. Within the liver they are broken up into capillaries which emerge as the hepatic veins to the sinus venosus. Of the somatic MESODERMAL DERIVATIVES 325

veins, the anterior cardinals are still prominent and are joined by an extensive series of head veins. In the cervical region the anterior cardinals receive the dorsal intersegmental veins as well as the external jugular from the mandible. As the anterior cardinals enter the common cardinal veins, they are joined by the posterior cardinals, which have already lost part of their drainage

Spinal cord oO .







Notochord Posterior cardinal vein Posterior vena

Left vitelline (portal) vein Left mbilical vein Left vitelline artery

duct

umbilical artery

Fused umbilical veins

Fig. 235. 10 mm. pig embryo. Transverse section through umbilical stalk in region of intestinal loop. X16}.

area to the subcardinal veins passing through the ventral portions of the mesonephroi. Numerous small venous channels serve to connect the subcardinals and postcardinals during this period. The posterior caval vein has already made its appearance as a direct connection from the subcardinals to the liver. The umbilical (allantoic) veins proceeding from the allantois toward the heart are fused together in the umbilical cord. In the body they 326 THE ANATOMY OF THE 10 MM. PIG EMBRYO

pass through the liver, within which they are, like the vitelline veins, broken up into capillaries. The left umbilical maintains a broad channel through the liver. This vessel, now known as the ductus venosus, connects the umbilical with the posterior caval vein.



Posterior Mesonephric

limb bud duct Umbilical artery Metanephric duct

Caudal artery

Notochord Spinal cord

Fig. 236. — 10 mm. pig embryo. Transverse section through metanephric duct and posterior limb buds. X16}.

Ectodermal derivatives. — The epidermal derivatives of the ectoderm have already been enumerated in the description of external form. There remain for consideration the nervous system and sense organs. ‘The five definitive vesicles of the brain are well marked. From the telencephalon arise the two lateral cerebral vesicles. This division of the brain is separated from the diencephalon by two points of reference, the optic recess in the floor, and the velum transversum in the roof. From the diencephalon spring the optic stalks, leading to the optic cups, and the infundibulum, now in contact with the hypophysis as mentioned above. The posterior boundary of the diencephalon is indicated by the tuberculum posterius arising from the brain floor. The epiphysis seldom appears at this stage. The mesencephalon, with the third cranial nerve arising from its floor, is ECTODERMAL DERIVATIVES 327

demarcated at its posterior end by the deep constriction of the isthmus. The metencephalon is distinguished from the myelencephalon by its thicker roof. From the isthmus the fourth cranial nerve runs forward laterally over the sides of the brain to the mass of mesoderm surrounding the eyeball, from which the









Basilar artery

Anterior cardinal

vein Internal carotid artery Thymus Olfactory pit | Visceral 8rd Aortic arch

4th Aortic arch 6th Aortic arch. Sinus

venosus

Right atrium

pouch

Ductus venosus


[J Subcardinal

/J anastomosis, Notochord

Fig. 237. — 10 mm. pig embryo. Frontal section through aortic arches and ductus venosus. X16}.

eyeball muscles will be formed. Conspicuous at the anterior ventro-lateral margin of the metencephalon is the large semilunar ganglion of the fifth cranial nerve. From the floor of the myelencephalon, the sixth cranial nerve emerges to run forward toward the eye. Immediately following this, the geniculate ganglion of the seventh and the acoustic ganglion of the eighth are in close 328 THE ANATOMY OF THE 10 MM. PIG EMBRYO

connection. The ninth cranial nerve has two ganglia, the dorsal superior ganglion and ventral petrosal, while the tenth similarly possesses a dorsal jugular and ventral nodose ganglion. The eleventh cranial nerve possesses at this stage a small ganglion (of Froriep) which disappears in the adult. The last of the cranial nerves, the twelfth, arises from the floor of the myelencephalon by a number of small roots and without a ganglion. In the region of the spinal cord the segmental nerves arise from the cord by two roots, of which the dorsal is associated with a spinal ganglion. The trunk is very short and soon divides into three main branches. The dorsal and ventral rami run to these respective regions of the body wall, while the third, or communicating ramus, unites the spinal nerve with a ganglion of the sympathetic chain. The sympathetic ganglia may be recognized as small masses of cells dorsal to the aorta.

The nose is represented by the olfactory pits. The eye is in the optic cup stage with a well-marked choroid fissure and groove, while the lens is completely separated from the outer ectoderm and is in the vesicle stage. Of the various regions of the ear, all the primordia are now established. The otic vesicle with its endolymphatic duct, representing the inner ear, is in close juxtaposition to the first visceral pouch (hyomandibular) which will give rise to the auditory tube and chamber of the middle ear; the external auditory meatus, or outer ear, will arise from the first or hyomandibular groove.

References

Arey, L. B. 1934. Developmental Anatomy, 3rd Ed., Chap. 19.

Boyden, E. A. 1933. A Laboratory Atlas of the Pig Embryo.

Keibel, F. 1897. Normaltafeln, I, des Schweines (Sus scrofa domesticus).

Lewis, F. T. 1902. The gross anatomy of a 12 mm. pig, Am. Jour. Anat., Vol. 2, pp. 211-226.

Minot, C.S. 1911. A Laboratory Textbook of Embryology, 2nd Ed.

Patten, B. M. 1931. The Embryology of the Pig, 2nd Ed.

Wallin, E. 1917. A teaching model of a 10 mm. pig embryo, Anat. Rec., Vol. 5, pp. 17-45.

Shumway (1935): Preface - Contents | Part I. Introduction | Part II. Early Embryology | Part III. Organogeny | Part IV. Anatomy of Vertebrate Embryos | Part V. Embryological Technique


Cite this page: Hill, M.A. (2021, May 6) Embryology Book - Introduction to Vertebrate Embryology 1935-4. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Book_-_Introduction_to_Vertebrate_Embryology_1935-4

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