Book - Introduction to Vertebrate Embryology 1935-4

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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

pharynx. REFERENCES 293

Diencephalon

Ear

Notochord

Pronephros

Wall of Intestine

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.

Anterior neuropore

Head fold Proamnionrn t hy

Amnio te Anteriorcardiac ntestinal portal vesicle Neural fold

Neural groove

4th Somite

Area & pellucida Primitive knot

Primitive streak

Area 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

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. REFERENCES 315

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.