Book - A Laboratory Manual and Text-book of Embryology 5

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Prentiss CW. and Arey LB. A laboratory manual and text-book of embryology. (1918) W.B. Saunders Company, Philadelphia and London.

Human Embryology 1918: The Germ Cells | Germ Layers | Chick Embryos | Fetal Membranes | Pig Embryos | Dissecting Pig Embryos | Entodermal Canal | Urogenital System | Vascular System | Histogenesis | Skeleton and Muscles | Central Nervous System | Peripheral Nervous System | Embryology History
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Pages where the terms "Historic Textbook" and "Historic Embryology" appear on this site, and sections within pages where this disclaimer appears, indicate that the content and scientific understanding are specific to the time of publication. This means that while some scientific descriptions are still accurate, the terminology and interpretation of the developmental mechanisms reflect the understanding at the time of original publication and those of the preceding periods, these terms and interpretations may not reflect our current scientific understanding.     (More? Embryology History | Historic Embryology Papers)

Chapter V. The Study of Six and Ten Millimeter Pig Embryos

A. The Anatomy of a Six mm Pig Embryo

Very young pig embryos of the primitive streak and neural fold stages have been seen already {Fig. 26). In its early stages the pig embryo is flattened out on the surface of the yolk sac like a chick embryo (Fig. 92), but as the head and tail folds elongate the body becomes flexed and twisted spirally, making it difficult to study. In embryos 5 to 7 mm. long the twist of the body begins to disappear and its structure may be seen to better advantage.


External Form of 6 mm. Embryo

When compared with the form of the 4 mm. human embryo, the marked difference in the 6 mm. pig is the convex dorsal flexure which brings the head and tail regions close together (Fig. 93). The cephalk flexure at the mesencephalon forms an acute angle and there is a marked neck or cervical flexure. As a result, the head is somewhat triangular in form. The body is bent dorsad in an even convex curve and the tail is flexed sharply dorsad. Lateral to the dorsal line may be seen the segments, which become larger and more differentiated from tail to head. At the tip of the head a shallow depression marks the anlage of the oljactory pit. The lens vesicle of the eye is open to the exterior. Caudal to the eyes at the sides of the head are four branchial arches separated by three grooves, the branchial clefts. The fourth arch is partly concealed in a triangular depression, the cervical sinus, formed by the more rapid growth of the first and second arches (cf. Fig. 97). The first, or mandibular arch, forks ventrally into two processes, a smaller maxillary and a larger mandibular process, and the latter with its fellow forms the mandible or lower jaw. The position of the mouth is indicated by the cleft between these processes. The groove between the eye and the mouth is the lacrimal groove.


Fig. 92.— Pig embryos (A) of seven and (B) of ten primitive segments, in doc^al view wiili amnion away (Keibel, Normentafel). X 20,


The second or hyoid arch is separated from the mandibular arch by a hyomandibular cleft which persists as the external auditory meatus. About the dorsal end of the deft develops Oie external ear.


Fig. 93. — Pig embryo of 6 mm., viewed from the left side. The a: edge is shown in the figure. X 12.


The heart is large and through the transparent body wall may be seen the dorsal atrium and ventral ventricle. Caudal to the heart a convexity indicates the position of the liver. Dorsal to the liver is the bud of the anterior extremity, now larger than in the 4 mm. human embryo. Extending caudal to the anlage of the upper extremity a curved convexity indicates the position of the left mesonephros. At its caudal end is the bud of the lower limb. The amnion has been dissected away along the line of its attachment ventral to the mesonephros. There is as yet no distinct umbilical cord and a portion of the body stalk is attached to the embryo.


Due to a shorter term of development, a young pig embryo is somewhat precodously developed in comparison with a human embryo ol the same size (Fig. 94). In a human embryo 7 mm. long the head is larger, the tail shorter. The cervical flexure is more marked, the olfactory pits larger and deeper. The liver is more prominent than in the 6 mm. pig, the mcsonephros and segments less so.


Lumbar segment o Fig. 94. — A humitn embryo 7 mm. long, viewed from Ihe right side (Mall in KoUmaiin). X 14. , Branchial arches 1, 2, and .1; (, heart; L, liver; L', otic vesicle; K, olfactory placode; Tt, semilunar ganglion of trigeminal ner\'e.


Dissections of the Viscera

To understand the sectional anatomy of an embryo, a study of dissections and reconstructions is essential. For methods of dissection see p. 137, Chapter VI. Before studying sections, the student should become as well acquainted as possible with the anatomy of the embryo and compare each section with the figures of reconstructions and dissections.


Nervous System. — Fig. 95 shows the central nervous system and viscera exposed on the right side of a 5.5 mm. embryo. The ventro-lateral wall of the bead has been left intact with the lens cavity, olfactory pit, and portions of the maxillary and mandibular processes, second and third branchial arches, and cervical sinus (cf. Fig. 93). The brain is differentiated into the five regions, telencepluxlon, diencephalon, mesencephalon, melencephalon, and myelaicepiialon. The spinal cord is cylindrical and gradually tapers off to the tail. The aniages of the cerebral and spinal ganglia and the main nerve trunks are shown. The oculomotor nerve begins to appear from the ventral wall of the mesencephalon.


Fig., 95.— Disseclkin ot a 5.5 ir . pig tmbrj'o, showinR ihe n 5ide. X 18.


Ventro-lateral to the metencephalon and myelencephalon occur in order the semilunar ganglion and three branches of the trigeminal nerve: the geniculate ganglion and nerve trunk of the n. facialis: the ganglionic aniage of the n. acusHcus and the otocysl. It will be observed that the nerve trunks are arranged with reference to the branchial arches and clefts. Caudal to the otocyst a continuous chain of cells extends lateral to the neural tube Into the tail region. Cellular enlargements along this neural crest represent developing cerebral and spinal ganglia. They are in order the superior or root ganglion of the glossopharyngeal nerve with its distal petrosal ganglion; the ganglion jugulare and distal ganglion nodosum of the vagus nerve; the ganglionic crest and the proximal portion of the spinal accessory neroe; and the aniage of Froriep's ganglion, an enlargement on the neural crest just cranial to the first cervical ganglion. Between the vagus and Froriep's ganglion may be seen the numerous root fascicles of the hypoglossal nerve, which take their origin along the ventro-lateral wall of the myelencephalon and unite to form a single trunk. The posterior roots of the spinal ganglia are very short; their anterior or ventral roots are not shown.


The position of the heart with its ventricle, atrium, and sinus venosus are shown. The liver is divided into a small dorsal and a large ventral lobe. The fore-gut emerges from between the liver lobes and curves ventrad to the yolk stalk and sac. The hind-gut is partly hidden by the fore-gut; it make a Ushaped bend from the yolk stalk to the caudal region. The gut is attached to the dorsal body wall by a double layer of splanchnic mesoderm which forms the mesefttery. The long, slender mesonephros lies ventral to the spinal cord and curves caudad from a p)oint opposite the eighth cervical ganglion to the tail region. The cranial third of the mesonephros is widest and its size diminishes tailwards. Between the yolk sac and the tail the allantois is seen, its stalk curving around from the ventral side of the tail region.

Digestive Canal

The arrangement of the viscera may be seen in median sagittal and ventral dissections (Figs. 96 and 97), also in the reconstruction shown in Fig. 105. The mouth lies between the mandible, the median frontonasal process of the head, and the maxillary processes at the sides. The diverticulum of the hypophysis (Rathke's pocket), flattened cephalo-caudad and expanded laterad, extends along the ventral wall of the fore-brain (Fig. 105). Near its distal end, the wall of the brain is thickened and later the posterior lobe of the hj'pophysis will develop from the brain wall at this point.

The pharynx is flattened dorso-ventrally and is widest near the mouth. Its lateral dimension narrows caudad, and opposite the third branchial arch it makes an abrupt bend, a bend which corresponds to the cervical flexure of the embryo's body (Figs. 104 and 105). In the roof of the pharynx, just caudal to Rathke's pocket, is the somewhat cone-shaped pouch known as SeesseVs pocket, which may be interpreted as the blind cephalic end of the fore-gut. The lateral and ventral walls of the pharynx and oral cavity are shown in Fig. 98. Of the four arches the mandibular is the largest and a groove partly separates the processes of the two sides. Posterior to this groove and extending in the median line to the hyoid arch is a triangular rounded elevation, the tuberculum impar, which later forms a small part of the tongue. At an earlier stage the median thyreoid anlage grows out from the mid-ventral wall of the pharynx just caudal to the tuberculum impar. The ventral ends of the second arch fuse in the mid-ventral line and form a prominence, the copula. This coimects the tuberculum impar with a rounded tubercle derived from the third and fourth pairs of arches, the anlage of the epiglottis. Its cephalic portion forms the root of the tongue (Fig. 156). Caudal to the epiglottis are the arylenoid ridges, and a slit between them, the glottis, leads into the trachea.


Fig. 96.McdiBTi sagittal dissection of


The branchial arches converge caudad and the pharynx narrows rapidly before it is differentiated into the trachea and esophagus (Figs. 104 and 105).


Laterally and ventrally between the arches are the four paired outpocketings of the pharyngeal pouches. The pouches have each a dorsal and ventral divertkuium (Fox, Thyng). The dorsal diverticula are large and wing-like (Fig. 104); they meet the ectoderm of the gill clefts and fuse with it to form the closing plates. Between the ventral diverticula of the third pair of pouches lies the median thyreoid aaiage. The fourth pouch is smaller than the others. Its dorsal diverticulum just meets the ectoderm; its ventral portion is small, tubular in form, and is directed parallel to the esophagus (Fig. 104).


Fig. 97.— Ventral dissection of a 6 t Dnr'nl nnrta iinit iimhilkal nrlery m. pig embryo. X 14. The head has been bent doisally.


The groove on the floor of the pharynx caudal to the epiglottis is continuous with the tracheal groove. More caudally, opposite the atrium of the heart, the trachea has separated from the esophagus (Fig. 96). The trachea at once bifurcates to form the primary bronchi and the aniages of the lungs (Fig. 97). The lungs consist merely of the dilated ends of the bronchi surrounded by a layer of ^lanchnic mesoderm. They bud out laterally on each side of the esophagus near the cardiac end of the stomach, and project into the pleural ccdom. The esophagus is short and widens dorso-ventrally to form the stomach. The long axis of the stomach is nearly straight, but its entodermal walls are flattened together and it has revolved on its long axis so that its dorsal border lies to the left, its ventral border to the right, as seen in transverse section (Fig. 111).


Caudal to the pyloric end of the stomach, and to its right, is given off from the duodenum the hepatic diverticulum. Its opening into the gut is seen in the ventral dissection (Fig. 97). The hepatic diverticulum is a sac of elongated oval form from which the liver and part of the pancreas take origin, and which later gives rise to the gall bladder, cystic duct, and common bile duct. It is connected by several cords of cells with the trabeculae of the liver.


The liver is divided incompletely into four lobes, a small dorsal and a large ventral lobe on each side (Figs. 95 and 112). The lobarion does not show in a median sagittal section. The pancreas is represented by two outgrowths. The ventral pancreas originates from the hepatic diverticulum near its attachment to the duodenum (Fig. 96). It grows to the right of the duodenum and ventral to the portal vein. The dorsal pancreas takes origin from the dorsal side of the duodenum caudal to the hepatic diverticulum and grows dorsally into the substance of the gastric mesentery (Figs. 105 and 113). It is larger than the ventral pancreas, and its posterior lobules grow to the right and dorsal to the portal vein and in later stages anastomose with the lobules of the ventral pancreas.


Fig. 98.— Dissection of the tongue and branchial arches of a 7 mm. pig embryo, seen in dorea! view.


The intestine of both fore-gut and hind-gut has elongated and curves ventrally into the short umbilical cord where the yolk stalk has narrowed at its point of attachment to the gut (Fig. 96). As the intestinal tube grows ventrad, the layers of splanchnic mesoderm which attach it to the dorsal body wall grow at an equal rate and persist as the mesentery.


The cloaca, a dorso-ventrally expanded portion of the hind-gut, gives ofi cephalad and ventrad the allantoic stalk. This is at first a narrow tube, but soon expands into a vesicle of large size, a portion of which is seen in Fig. 95. Dorsolaterad the cloaca receives the primary excretory (mesonephric) ducts. The hindgut is continued into the tail as the tail gut (postanal gut) which dilates at its extremity as in the 7.8 mm. pig described by Thyng. The mid-ventral wall of the cloaca is fused to the adjacent ectoderm to form the cloacal membrane. In this region later the anus arises {Fig. 105). The postanal gut soon disappears.


Urogenital System

This consists of the mcsmephroi, the mesonepkric {Wolffian) duels, the anlages of the melanephroi, the cloaca, and the allantois. The form of the mesonephroi is seen in Figs. 95 and 97. Each consists of large vascular glomeruli associated with coiled tubules lined with cuboidal epithelium and opening into the mesonephric duct (Figs. 114 and 208). The Wolffian ducts, beginning at the anterior end of the mesonephros, curve at first along its ventral, then along its lateral surface. At its caudal end ^ "' each duct bends ventrad and to the midline where it opens into a lateral e.\pansion ,, ^ of the cloaca (Fig. 96). Before this junction takes place, an evagination into the mesenchyme from the dorsal wall of each of the metanephroi, or permanent kidneys. A slight thickening of the mesothelium along the median and ventral surface of each mesonephros forms a light-colored area, the getiilal fold (Fig. 96). This area is pointed at either end and confined to the middle third of the kidney. It is the anlage of the genital gland from which either testis or ovary is developed. Blood Vascular System.— The heart lies in the pericardial cavity, as seen in Fig. 96. The atrial region (Fig. 99), as in the 4.2 mm. human embryo, has piven rise to two lateral sacs, the right and left atria. The bulbo- ventricular loop has become differentiated into right and left vetttriclts much thicker walled than the atria. The right ventricle is the smaller and from it the bulbus passes between the atria and is continued as the ventral aorta. Viewed from the caudal and dorsal aspect (Fig. 100) the sinus venosus is seen dorsal to the atria. It opens into the right atrium and receives from the right side the rigid common cardinal vein, from the left side the left common cardinal. These veins drain the blood from the body of the embryo. Caudally the sinus venosus receives the two vitelline veins. Of these, the left is small in the liver and later disappears. The right vitelline vein, now the common hepatic, carries most of the blood to the heart from the umbilical veins, and from the liver sinusoids, gut, and yolk sac.


Fig. 99, — Ventral and cranial surface of the mesonephric duct gives rise to the anlages heart from a 6 mm. pig embryo. x 14.


Fig. 100.— Dorsal and caudal \-icw o( the heart from a 6 mm. pig embryo. X 21.

Transverse sections of the embryo through the four chambers of the heart show the atria in communication with the ventricles through the alrio-ventricular foramina, and the sinus venosus opening into the right atrium {Fig. 109). This opening is guarded by the right and left valves of the sinus venosus. Septa incompletely separate the two atria and the two ventricles. In Fig. 109 the atrial septum (septum primum) appears complete due to the plane of the section. In Fig. 101, from a slightly smaller embryo, it is seen that the septum primum grows from the dorsal atrial wall of the heart and does not yet meet the endocardial cushions between the atrio-ventricular canals. This opening between the atria is known as the interatrial foramen. Before it doses, another opening appears in the septum, dorsal in position. This is the foramen ovale and persists during fetal life. In Fig. 101 these two openings may be seen, as may also the dorsal and ventral endocardial cushions which bound the atrio-ventricular foramina. The outer mesothelial layer of the ventricles has become much thicker than that of the atria. It forms the epicardium and the myocardium, the sponge-like meshes of which are now being developed.


Fig. 101.— Dissection of a 5.5 mm. pig's heart from the left side, showing the septum primum and the interatrial and oval foramina. X 14.


The Arteries

These begin with the ventral aorta, which takes origin from the bulbus cordis. From the ventral aorta are given off five pairs of aortic arches. These run dorsad in the five branchial arches (Figs. 104 and 105) and join the paired dorsal or descending aortcB, The first and second pairs of aortic arches are very small and take origin from the small common trunks formed by the bifurcation of the ventral aorta just caudal to the median thyreoid gland. The fourth aortic arch is the largest. From the fifth arch small pulmonary arteries are developing. There is evidence that this pulmonary arch is really the sixth in the series, the fifth having been suppressed in development (cf. Fig. 272 B). Cranial to the first pair of aortic arches, the descending aortae are continued forward into the maxillary processes as the internal carotids. Caudal to the aortic arches the descending aorta? converge, unite opposite the cardiac end of the stomach, and form the median dorsal aorta. From this vessel and from the descending aorts paired dorsal intersegmental arteries arise. From the seventh pair of these arteries (the first pair to arise from the medial dorsal aorta) there are developed a pair of lateral branches to the upper limb buds. These vessels are the subclavian arteries. From the dorsal aorta there are also given off ventro4ateral arteries to the glomeruli of the mesonephros, and median ventral arteries. Of the latter the coeliac artery arises opposite the origin of the hepatic diverticulum. The vitelline artery takes origin by two or three trunks caudal to the dorsal pancreas. Of these trunks the posterior is the larger and persists as the superior mesenteric artery. Thyng (Anat. Record, vol. 5, 1911) has figured three trunks of origin in the 7.8 mm. pig. These unite and the single vitelline artery branches in the wall of the yolk sac.


Opposite the lower limb buds the dorsal aorta is divided for a short distance. From each division there arises laterad three short trunks which unite to form the single umbilical artery on each side. The middle vessel is the largest and apparently becomes the common iliac artery. A pair of short caudal arteries, much smaller in size, continue the descending aortae into the tail region.

The Veins

The vitelline veins, originally paired throughout, arc now represented distally by a single vessel, which, arising in the wall of the yolk sac, enters the embryo and courses cephalad of the intestinal loop (Fig. 102). Crossing to the left side of the intestine and ventral to it, it is joined by the stiPerior mesenteric vein which has developed in the mesentery of the intestinal loop. The trunk formed by the union of these two vessels becomes the portcU vein. It passes along the left side of the gut in the mesentery. Opposite the origin of the dorsal pancreas it gives off a small branch, a rudimentary continuation of the left vitelline vein, which courses cephalad and in earlier stages connects with tlie sinusoids of the liver. The portal vein then bends sharply to the right, dorsal to the duodenum, and, in the course of the right vitelline vein, passing between the dorsal and ventral pancreas to the right of the duodenum, it soon enters the Uver and connects with the liver sinusoids. The portal trunk is thus formed by persisting portions of both vitelline veins, and receives a new vessel, the superior mesenteric vein. The middle portions of the primitive vitelline veins are connected with the network of liver sinusoids. Their proximal vitelline trunks drain the blood from the liver and open into the sinus venosus of the heart. The right member of this pair is much the larger (Fig. 100) and persists as the proximal portion of the inferior vena cava. For the development of the portal vein see Chapter IX.


Fig. 102. — ReconstnicttOD in ventral view of a 6 mm. pig embryo to show the vitelline and umbilical veins, the latter opened (original drawing by Dr. K. L. Vehe). X 22. In the small orientation figure {cf . Fig. 105) the various planes are indicated by broken lines — • *.


The umbilical veins, taking their origin in the walls of the chorion and allantoic vesicle, lie caudal and lateral to the allantoic stalk and anastomose (Figs. 102 and 105). Before the allantoic stalk enters the body, the umbilical veins separate and nin lateral to the umbilical arteries. The left vein is much the larger. Both, after receiving branches from the posterior limb buds and from the body wall, pass cephalad in the somatopleure at each side (Fig. 72). Their course is first cephalad, then dorsad, until they enter the liver. The left vein enters a wide channel, the ductus venosus, which carries its blood through the liver, thence to the heart by way of the right vitelline trunk. The right vein joins a large sinuswdal continuation of the portal vein in the liver. This common trunk drains into the ductus venosus.


Fig. 103. — Reconstructioa of the cardinal and subcardmal ^ eins of a 6 mm. pig embryo showing the ttrly deveiopment of the inferior vena caia (k L \ehr) X 22 In the small orientation figure (d. . Fig. 105) the various planes are indicated bj broken lines — .


The anterior cardinal veins (Figs. 103 and 104) are formed to drain the plexus of veins on each side of the head. These vessels extend caudad and lie lateral to the ventral portion of the myelencephalon. Each anterior cardinal vein receives branches from the sides of the myelencephalon, then curves ventrad, is joined by the linguo-facial vein from the branchial arches and at once unites with the posterior cardinal of the same side to form the common cardinal vein. This, as we have seen, opens into the sinus venosus.


The posterior cardinal veins develop on each side in the mesonephric ridge, dorso-lateral to the mesonephros (Figs. 103. 104 and 112). Running cephalad, they join the anterior cardinal veins. When the mesonephroi become prominent, as at this stage, the middle third of each fwsterior cardinal is broken up into sinusoids (Minot). Sinusoids extend from the posterior cardinal vein ventrally around both the lateral and medial surfaces of the mesonephros. The median sinusoids anastomose longitudinally and form the subcardinal veins, right and left. The subcardinals he along the median surfaces of the mesonephroi, more ventrad than the posterior cardinals with which they are connected at either end. There is a transverse capiUary anastomosis between them, cranial and caudal to the permanent trunk of the vitelline artery (Fig. 103). The right subcardinal is connected with the liver sinusoids through a small vein which develops in the mesenchyme of the plica vena; cava: (caval mesentery) located to the right of the mesentery (Fig. 112). This vein now carries blood direct to the heart from the right posterior cardinal and right subcardinal, by way of the liver sinusoids and the right vitelline trunk (common hepatic vein). Eventually the unpaired injertor vena cava forms in the course of these four vesseb. (For the development of the inferior vena cava see Chapter IX.)


Fig. 104.— Reconstruction of 7.8 i (after Thyng). 1. pig embryo sliowing veins and aortic arches from the left ade X 15. Pk. P. I, 2, J, 4, Pharyngeal pouches.

Fig. 105. — Rcconstnictioii of a 6 mm. pig embr>'o in the median sagitlal |Jane, i-icH-ed trom the right side. The numbered heai-y lines indicate the levels of the transvenc lectionK dKiwrt in V'iKf. 106-117. The broken lines indicate the outline of the left mcsonephros and the ciiunc of the left umbilical artery and vein. The biter may be traced from the umbilical cord to the liver where it is tcctioDcd longitudinally. (Original drawing and reconstruction by l>r. K. L. Vche;. X 10.5,


Transverse Sections of a Six Mm. Pig Embryo Having acquainted himself with the anatomy of the embryo from the study 01 dissections and reconstructions, the student should examine serial sections cut in the plane indicated by guide lines on Fig, 105. Refer back to the external structure of the embryo (Fig. 93), to the lateral dissection of the organs (Fig. 95), and having determined the exact plane of each section, interpret the structures seen by comparing with Fig. 105. The various structures may be recognized by referring to the figures of sections in the text, and they should be traced section by section through the series as carefully as time will allow. Remember that the secUons of pig embryos figured here are drawn from the cephahc surface, so that the right side of the section is the left side of the embryo.


Transverse Section through the Mjelencephalon and Otocysts of c 6 mm. Embryo (Fig. 106). — As the head is bent nearly at right angles to the body, this section passes lengthwise through the myelencephalon. The diencepkalon is cut transversely. The cellular walls of the myelencephalon show a series of six pairs of constrictions, the tuuromera. Lateral to the fourth pair of neuromeres are the otocysU, which show a median outpocketing at the point of entrance of the enddymph duct. The ganglia of the nn. trigeminus, facialis, acusticus, and the superior ganglion of the glossopharyngeal nerve occur in order on each side. Sections of the anterior cardinal vein and its branches show on the left side. Ventral to the diencephalon are sections of the internal carotid arteries.


Fig. 106.-— Transverse section through the myelcncephaloc X 26.5. Gang, acuit. n. S, acoustic ganglion of acoustic n



Psflsing along down the series into the pharynx region, observe the first, second, and third pharyngeal pouches. Their dorsal diverticula come into contact with the ectoderm of the bianchiai clefts and form the closing plates.

TMnsvem Section through the Branchial Arches and the Eyes (Fig. 107). — The section passes lengthwise through the four branchial arches, the fourth sunken in the cervical sinus. Dorsad is the spinal cord with the first pair of cervical ganglia. The pharynx is cut across between the third and fourth branchial pouches. In its floor is a prominence, the anlage of the epiglottis. Ventral to the pharynx the ventral aorta gives off two pairs of vessels. The larger pair are the fourth aortic archfs which curve dorsad around the pharyrx to enter the descending aorta. The smaller third aortic arches enter the third branchial arches OD each aide. A few sections higher up in the series the ventral aorta bifurcates and the right and left trunks thus formed give off the first and second pair of aortic arches. Cranially in the angle between (heir common trunks lies the median thyreoid anlage. The anterior cardinal veins are located lateral and dorsal to the descending aorta;. The end of the head is cut through the diencephalon and the optic vesicles. On the left side of the figure the lens reside may be seen still connected with the ectoderm. The optic vesicle now shows a thick inner, and a thin outer layer; these form the nerfous and pigment layers of the retina respectively.


Fig. 107,— Transverse I through the branchial arches and ej-es of i X 26.S. X. aortic aich 4.


Transverse Section through the Tracheal Groove, Bulbus Cordis and Olfactory Pita (FiR. 108).— The ventral portion of the figure shows a section through the tip of the head. The telencephalon is not prominent. The cctmlcrm is thickened and slightly invaginated ventro-laterad to form the anlages of the olfactory pits. These deepen in later stages and become the nasal cavities. In the dorsal portion of the section may be seen the cervical portion of the spinal cord, the nolockord just ventral to it, the descending aorta, and ventrolateral to them the anterior cardinal veins. The nasopharynx now b small with a vertical groove in its floor. This is the tracheal groove and more caudad it will become the cavity of the trachea. The buibus cordis lies in the large pericardial cavity. On either side the section cuts through the cephalic portions of the atria. These will become larger as we go caudad in


Fig. 109. — Transverse section through the fuur ihiinihtTs of a 6 mm. pig embrjo. X 2l).S,


Transverse Section through the Heart (Fig. 109).— Lateral to the descending aortx are the common cardinal veins. The right common cardinal opens into the sinus venosus which in turn empties into the right atrium, its opening being guarded by the two valves of Ike sinus venosus. The entrance of the left common cardinal into the sinus venosus is somewhat more caudad in ihe series. The trachea has now separated from the esophagus and lies ventral to it. Both trachea and esophagus are surrounded by a condensation of mesenchyme. The myocardium of the ventricles has formed a spongy layer much thicker than that of the atrial wall. An incomplete intervenlricular septum leaves the ventricles in dorsad. The»^rum^rtmum is complete in this section, but higher up in the series there is an interatrial foramen (cf. Fig. 101). The foramen ovale is hardly formed.


Fig. 110. — Transverse section through the right lung bud and septum transversun embryo. X 26,5.


Transverse Section through the Lung Buds and Septum Transversum (Fig. 1 10). — The section passes through the bases of the upper Hmh buds. The lips of the ventricles lying in the pericardial cavity still show in this section. Dorsally the pericardial cavity has given place to the pleuroperitoncal cavity. Projecting vcntrad into this cavity are the mrsonephric (Wolffian) folds in which the posterior cardinal veins partly lie. Into the floor of the pleuro-peritoneal cavities bulge the dorsal lobes of the liver, embedded in mesenchyma. This mesenchyma is continuous with that of the somalopleurc. and forms a complete transverse septum ventrally between the liver and heart. This is the septum transversum which takes part in forming the ligaments of the liver and is the anlage of a portion of the diaphragm. The two proximal trunks of the vitelline veins pass through the septum. Projecting laterally into the pleuro-peritoneal cavities are ridges of meseiicyhma covered by splanchnic mesoderm in which the lungs develop as lateral buds from the caudal end of the trachea. The rigkl lung bud is shown in the figure. Between the esophagus and the lung is a crescent-shaped cavity, the cranial end of the letser peritoneal sac.

Transverse Section through the Stoouch (Fig. 111).— The section passes through the upper limb buds and )ust caudal to the point at which the descending aortse unite to form the median dorsal aorta. As the liver develops in early stages, it comes into relation with the plica vena cava along the dorsal body wall at the right side of the dorsal mesogastrium. The space between the liver and plica to the right, and the stomach and its omenta to the left, is a caudal continuation of the lesser peritoneal sac. The dorsal wall of the stomach b rotated to the left, its ventral wall to the right. The liver shows a pair of dorsal lobes and contains large blood spaces and networks of sinusoids lined with endotheUum. Ventral to the liver, the tips of the ventricles are seen.


Fig. 111.— Transverse through the stomach of a 6 mm. pig embryo. X 26.5.


Transverse Section fhrotigh the Hepatic Diverticulum (Fig. 112).— The upper hmb buds are prominent in this section. The mesonephric folds show the tubules and glomeruli of the mesonephroi and the posterior cardinal veins are connected with the mesonephric sinusoids. From the dorsal attachment of the liver there is continued down into this section a ridge on the dorsal body wall just to the right (left in figure) of the mesentery. In this ridge lies a small vein which connects cranially with the liver sinusoids, caudally with the right subcardinal vein. As it later forms a portion of the inferior vena cava, the ridge in which it Ues is termed the plica vena: cava or caval mesentery. The right dorsal lobe of the liver contains a large blood space into which the portal vein opens. The duodenum is ventral to the position occupied by the stomach in the previous section. There is given off from it ventrad and to the right the hepatic diverticulum. In the sections higher up small ducts from the liver trabeculx may be traced into connection with it. In the left ventral lobe of the liver, a large blood space indicates the position of the left umbilical vein on its way to the ductus venosus.


Transverse Section through the Dorsal Pancreas (Fig. 113).'-At this level the upper limb buds still show; the mesonephroi are larger and marked by their large glomeruli.


The right posterior cardinal van is broken up into mesonephric sinusoids. The vein in tl plies vetiK CA\'x will, a few sections lower, connect with the right subcardinal vein. The a lage of the dorsal pancreas is seen extending from the duodenum dorsad into the mesenchyme J

pig embrjo. X 26,5.

pig embryo. X 26.S

To the right of the ventral pancreas lies the portal vein (at this level a portion of the right vileUine). To the left of the dorsal pancreas is seen the remains of the left vildline vein. Tbe ventral lobes of the liver are just disappearing at this level. In the mesenchyme which connects the liver with the ventral body wall lie on each side the umbilical veins, the left being the larger. Between the veins is the extremity of the hepatic diverticulum. The body wall is continued ventrad to form a short umbUicai cord.

Transverse Section at the Level of Origin of the Vitelline Arter; and Umbilical Arteries (Fig. 114). — As the posterior half of the embryo is curved in the form of a half circle, sections caudal to the liver, like this one, pass through the lower end of the body at the level of the posterior limb buds. Two sections of the embryo are thus seen in one,

liatetlitirdR. post, cardinal

MesodirnuU segment

-Transverse section ot a 6 ram. pig embryo at the level of the ori|^a of the vitelline arlery. The lower end of the section passes through the posterior limb buds. X 26.S.

thfir ventral aspects facing each other and connected by the lateral body wall. In the dorsal part of the section the mesonephroi are prominent with large posterior cardinal veins lying dorsal to them. The trunk of the vitelline artery takes origin ventrally from the aorta. It may be traced into the mesentery, and through it into the wall of the yolk sac. On either side of the vitelline artery arc the subcardinal veins, the right being the larger. In the mesentery may be seen two sections of the intestinal loop (the smalt intestine being cut lengthwise, the large intestine transversely), and also sections of the vitelline artery and veins. In the lateral


Tht left vtia U large and body walls ventral to the mesonephros occur ibt umbilitil u cat IcBStbwise. The right vein is cut obliquely twice.


In the ventral portion of the lection. the Itnifr Hmh hudi are prominent laterally. A lu|e pur of arteries, the common iliatf. are given nQ hum the aurta and may be traced into ccNmcction with the umbilkai arUties. The l<tr%t inUilint i\i\^i'incA by a sbon mttentery lies in the ctrlom near the midline. r>n each side are the metotupkric foUi. here small and each showing a section of the mtsompkrU duel and a Eingle vehicular anlage of the mtsontpkrU tmhtdes. The mesonephric ducts are sectioned a<> they cun'c around from their position in ibe dorsal portion of the section.


Section thnni|^ the Primitive Segments and Spinal Cord fFig. 115). — This section is near the end of the series and as the bf^ly Is here curved it b really a frontal section. At the left side of the spinal cord the oval cellular ma^iie^ are the iptnal %an^ia cut across. The ectoderm, arching over the segments, indicates their position. Each segment shows an outer dense layer, the CMlir plitt. lying just l^ntalh the etloderm. This plate curies lateral to the spindle-iihaped mutcU plulr which pves rise to the voluntary mustle. .Next comes a difiuse mass of meseochyma. the scUrotome. which eventually, with its fellow of the


Fig. 116. — Transverse section through the UDiUUcai vc&scIk, allant<usand cloaca of a 6 mm. pig embryo. X 45.


imiHHns Bide, icirvunds the spinal cord and forms the anlage of a vertebra. A pair of spinal nes'vta imi tgi^al ganglia are developed opposite each somite, and pairs of small vcsscIh arc mta UHTveai ^be segments. Thi-sc arc Jcrjii/ inlfrstRmenlid arlrries.


? Tra in teonch the Umbilical Vessels, AUsntois and Cloaca (Fig. 116). Mhv1B( vii* sru^i sections ai various levels to near the end of the series we shall iii-xl fXiiTniiic smiuu -jr",iL^ :he caudal repon nnd study ihc anlage^; of the urogenital iirgiiiiH. Owing u iw "TiTTLT-.Tt oi the emhrvo, it is iicii's.iiiry to go ceph.ilad in our series. The tirsl M-ttion imt*i: .mn^ :he base« of the limb buds m the level where the allaiihir slislk, curving in «nrn tnm -.be ■umbili.,-nl cord, o|<«'ns inio the Jiw.:. At t-ilher side of the iilbnioic stalk ■nil- IK lexz. vbl-quc sections nf ilic ikii/i/:. ,:/ ,:tlfrHs aiiil bleral to these the large led iiiiil Miul ~fpiT. pin u diu'lf mvupy the mesonephric ridges whicli ■imitrr nr: i^-.all .-.uid.il |>t.<l.>iiK.iti.>ii'. oi the nelom. Miiiway lH.-twifn the ilucls lies ihc MMB- he tail is in section at the left of the figure.


Section through the AnlageB of the Metanephroi, Cloaca and Hind^pit (Fig. 117). — The metanephroi are seen as dorsal evaginations from the mesonephric (Wolffian) ducts just before their entrance into the cloaca. Each consists of an epithelial layer surrounded by a condensation of mesenchyme. Traced a few sections cephalad the mesonephric duels open into the lateral diverticula of the cloaca, which, irregular in outline because it is sectioned obliquely, lies ventral to them and receives dorsad the hind-gut. Caudal to the cloaca in this embryo the tail bends abruptly cephalad and to the right. The blind prolongation of the hind-gut may be traced out into this portion of the tail until it ends in a sac-like dilatation.


Fig. 117.— Transverse section through the anlages of the metanephroi of a 6 mm. pig embryo. X 45.


The study of embryos at this stage is important as they possess the anlages of most of the organs. The anatomy of a 12 mm. pig embryo has been carefully studied and described by Lewis {Atner. Jour. Anat., vol. 2, 1903).


External Form (Fig. 118). — The head is now relatively large on account of the increased size of the brain. The third branchial arch is still visible in the embryo, but the fourth arch has sunken in the cervical sinus; usually both have disappeared at a slightly later stage. The olfactory pits form elongated grooves on the under surface of the head and the lens of the eye lies beneath the ectoderm surrounded by the optic cup. The ntaxtUary and mandibular processes of the first branchial arch are larger and the former shows signs of fusing with the median nasal process to form the upper jaw. Small tubercles, the anlages of the external ear, have developed about the first branchial cleft which itself becomes the external auditory meatus.


At the cervical bend the head is flexed at right angles with the body, brining the ventral surface of the head close to that of the trunk, and it is probably owing to this flexure that the third and fourth branchial arches buckle inward to form the cervical sinus. Dorsad, the trunk forms a long curve more marked opposite the posterior extremities. The reduction in the tnink flexures is due to the increased size of the heart, Uver, and mesonephroi. These organs may be seen through the translucent body wall and the position of the septum tronsversummay be noted between the heart and the diaphragm, as in Fig. 120. The limb buds are larger and the umbilical cord is prominent ventrad. Dorsally the mesodermal segments may be seen, and extending in a curve between the bases of the limb buds is the milk line, a thickened ridge of ectoderm which forms the anlages of the mammary glands. The taU is long and tapering. Between its base and the umbilical cord is the genital eminence (Fig. 120).


Fig. 118.— Exterior of a 10 mm. pig embryo vieit-ed from the right side. X 7.


Flo. 119.— Exterior of a hum&n embryo of 12 mm., viewed from the right side, ihowing attachmeDt of amnion (cut away) and yolk stallc and sac. X 4.


Human embryos of this stage or slightly older vary considerably in size (Fig. 119). They differ from pig embryos in the greater size of the head, the shorter tail, the much smaller mesonephric region, the longer umbilical cord, and the less prominent segments. The yolk sac is pear-shaped and the yolk stalk is long and slender.


Fig. 120. — Lateral dissection of a 10 nun. ing embryo, showing the viscera and nervous system from the right side. The eye has been removed and the otic vesicle is represented by a broken line. The ventral roots of the spinal nerves are not indicated. X lO.S. «., Nerve; r., ramus.


Central Nervous System and Viscera

Dissections show well the form and relations of the organs (Figs. 120, 121 and 122). Directions for preparing dissections are given in Chapter V1.


JJfoifi.— Five distinct regions may be distinguished (Figs. 120 and 122): (1) The telencephalon with its rounded lateral outgrowths, the cerebral hemispheres. Their cavities, the lateral venirkUs, communicate by the interventricular foramina with the third ventricle. (2) The diencephaion shows a laterally flattened cavity, the third ventricle. Ventro-laterally from the diencephaion pass off the optic stalks and an evagination of the mid-ventral wall is the anlage of the posterior hypophyseal lobe. (3) The mesencephalon is undivided, but its cavity becomes the cerebral aqueduct leading caudally into the fourth ventricle. (4) The metencephalon is separated from the mesencephalon by a constriction, the isthmus. Dorso-Iaterally it becomes the cerebellum, ventrally the pons. (5) The elongated myelencephalon is roofed over by a thin non-nervous ependymal layer. Its ventro-lateral wall is thickened and still gives internal indication of the neuromeres. The cavity of the metencephalon and myclencephalon is the /our/// ventricle.

Fig. 121.— Dissection of the head of a 15 mm vagus ganglia with peripheral ra , pig embryo from the right side to show the accessory ;s passing to the hypoglossal nerve. X 25.


Cerebral Nerves

Of the twelve cerebral nerves ali but the first (olfactory) and sixth (abducens) are represented in Fig. 120. For a detailed description of these nerves see Chapter VIII. (2) The optic nerve is represented by the optic stalk cut through in Fig. 1 20. (3) The oculomotor, a motor nerve to four of the eye muscles, takes origin from the ventro-lateral wall of the mesencephalon. (4) The trochlear nerve fibers, motor, to the superior oblique muscle of the eye, arise from the ventral wall of the mesencephalon, turn dorsad and cross at the isthmus, thus emerging on the opposite side. From the myelencephalon arise in order (5) the trigeminal nerve, mixed, with its semilunar ganglion and three branches, the ophthalmic, maxillary, and mandibular; (6) the w. abducens, motor, from the ventral wall to the external rectus muscle of the eye; (7) the n. facialis, mixed, with its geniculate ganglion and its chorda tympani, facial, and superficial petrosal branches in the order named; (8) the n, acusticus, sensory, arising cranial to the otocyst, with its acoustic ganglion and sensory fibers to the internal ear; (9) caudal to the otocyst the w. glossopharyngeus, mixed, with its superior and petrosal ganglia; (10) the vagus, sensory, with its jugular and nodose ganglia; (11) accompanying the vagus the motor fibers of the spinal accessory which take origin between the jugular and sixth cervical ganglia from the lateral wall of the spinal cord and myelencephalon; the internal branch of the n. accessorius accompanies the vagus; the external branch leaves it between the jugular and nodose ganglia and supplies the sternocleidomastoid and trapezius muscles; (12) the n. hypoglossuSy motor, arising by five or six fascicles from the ventral wall of the myelencephalon; its trunk passes lateral to the nodose ganglion and supplies the muscles of the tongue.

A nodular chain of ganglion cells extends caudad from the jugiilar ganglion of the vagus. These have been interpreted as accessory vagus ganglia. They may, however, be continuous with Froriep's ganglion which sends sensory fibers to the n. hypoglossus. In pig embryos of 15 to 16 mm. this chain is frequently divided into four or five ganglionic masses, of which occasionally two or three (including Froriep*s ganglion) may send fibers to the root fascicles of the hypoglossal nerve. Such a condition is shown in Fig. 121.

Spinal Nerves

These have each their spinal ganglion, from which the dorsal root fibers are developed (Figs. 120 and 136). The motor fibers take origin from the ventral cells of the neural tube and form the ventral roots which join the dorsal roots in the nerve trunk.

In Fig. 120 the heart with its right atrium and ventricle, the dorsal and ventral lobes of the liver, and the large mesonephros are prominent. Dorsal and somewhat caudal to the atrium is the anlage of the righl lung. The septum transversum extends between the heart and the liver.

Pharynx and its Derivatives

Dorsally the anterior lobe of the hypophysis is long and forks at its end (Figs. 122 and 123). In the floor of the pharynx are the anlages of the tongue and epiglottis (Fig. 156-4). From each mandibular arch arises an elongated thickening which extends caudal to the second arch. Between, and fused to these thickenings, is the triangular iuberculum impar. The opening of the thyreoglossal duct between the tuberculum impar and the second arch is early obliterated. A median ridge, or copula, between the second arches connects the tuberculum impar with the epiglottis, which seems to develop from the bases of the third and fourth branchial arches. On either side of the slit-like glotiis are the arytenoid folds of the larynx. (For the development of the tongue, see p. 149.) The pharyngeal pouches are now larger than in the 6 mm. pig (Fig. 123). The first pouch persists as the auditory tube and middle ear cavity, the closing plate between it and the first branchial cleft forming the tympanic membrane. The second pouch later largely disappears; about it, develops the palatine tonsil. The third pouch is tubular, directed at right angles to the pharynx, and meets the ectoderm to form a "closing plate." Median to the plate, the ventral diverticulum of the third pouch is the anlage of the thymus gland. Its dorsal diverticulum forms an epithelial body, or paraihyreoid. The fourth pouch is smaller and its dorsal diverticulum gives rise to a second parathyreoid body. Its ventral diverticulum is a rudimentary thymus anli^. A tubular outgrowth, caudal to the fourth pouch, is regarded as a fifth pharyngeal pouch in human embryos and forms the ultimobranckial body on each side (see p. 164). The thyreoid gland, composed of branched cellular cords, is located in the midline between the second and third branchial arches (Fig. 123).


Fig. 122. — Mediin sagittal disMCtion of a 10 mm. pin embr^'o. showing the brain, spinal cord and viscera from the right side. X 10.,S.


Fig. 123.— Reconstruction of a !0 mm. pig to show the position of the various organs from the right side. The veins are not indicated. Broken lines indicate the outline of the left metonephrae and tbe positions of the limb buds. X 10.


Trachea and Lungs

Caudal to the fourth pharyngeal pouches the esophagus'and trachea separate and form entodermal tubes (Figs. 122 and 123). Cephalad of the point where the trachea bifurcates to form the primary brottcki there appears on its right side the tracheal bud of the upper lobe of the right lung (Fig. 124). This bronchial bud is developed only on the right side and appears in embryos of 8 to 9 mm. Two secondary broncMai buds arise from the primary bronchus of each lung, and form the anlages of the symmetrical lobes of each lung.


Fig. 124— Ventral dissection of a 9 mm. pig embr>o. The head is bent doi^d. X 9.

Esophagus and Stomach

The esophagus extends as a narrow tube caudal to the lungs, where it dilates into the stomach. The stomach is wide from its greater to its lesser curvature and shows a cardiac diverticulum (Lewis). The pyloric end of the stomach has rotated more to the right, where it opens into the duodenum, from which division of the intestine the liver and pancreas develop.


The liver, with its four lobes, fills in the space between the heart, stomach, and duodenum (Fig. 122). Extending from the right side of the duodenum along the dorsal and caudal surface of the liver is the hepatic diverticulum. It lies to the right of the midline and its extremity is saccular. This saccular portion becomes the gall bladder. Several ducts connect the diverticulum with the liver cords. One of these persists as the hepatic duct which joins the cystic duct of the gall bladder. The portion of the diverticulum proximal to this union becomes the common bile duct, or ductus choledochus. The ventral pancreas arises from the common bile duct near its point of origin (Fig. 123). It is directed dorsad and caudad to the right of the duodenum. The dorsal pancreas arises more caudally from the dorsal wall of the duodenum and its larger, lobulated body grows dorsally and cranially (Figs. 123, 127 and 140). Between the pancreatic anlages courses the portal vein. In the pig, the duct of the dorsal pancreas persists as the functional duct.

Intestine

Caudal to the duodenum, the intestinal loop extends well into the umbilical cord (Figs. 122 and 123). At the bend of the intestinal loop is the slender yolk stalk. The cephalic limb of the intestine lies to the right, owing to the rotation of the loop. The small intestine extends as far as a slight enlargement of the caudal limb of the loop, the anlage of the ccecutn, or blind gut. This anlage marks the beginning of the large intestine (colon and rectum). The intestinal loop is supported by the mesentery which is cut away in Fig. 122. The cloaca is now nearly separated into the rectum and urogenital sinus. The cavity of the rectum is almost occluded by epithelial cells (Lewis).

Urogenital System

The mesonephros is much larger and more highly differentiated than in the 6 mm. embryo (Figs. 120 and 124). Along the middle of its ventro-median surface the genital fold is now more prominent (Fig. 122). In a ventral dissection (Fig. 124) the course of the mesonephric ducts may be traced. They open into the urogenital sinus, which also receives the allantoic stalk (Fig. 122).

The metanephros, or permanent kidney anlage, lies just mesial to the umbilical arteries where they leave the aorta (Fig. 123). Its epithelial portion, derived from the mesonephric duct, is differentiated into a proximal, slender duct, the ureter, and into a distal, dilated pelvis. From this grow out later the calyces and collecting tubules of the kidney. Surrounding the pelvis is a layer of condensed mesenchyma, or nephrogenic tissue, which is the anlage of the remainder of the kidney.

Blood Vascular System

The Heart

In Fig. 125 the cardiac chambers of the right side are opened. The septum primum between the atria is perforated dorsad and cephalad by \he foramen ovale. The inferior vena cava is seen opening into the sinus venosus, which in turn communicates with the right atrium through a sagittal slit guarded by the right and left valves of the sinus venosus. The right valve is the higher and its dorsal half is cut away. The valves were united cephalad as the septum spurium. Between the left valve and the septum primum the sickle-like fold of the septum secundum is forming; the fusion of these three components gives rise later to the adult atrial septum. The aortic bulb is divided distally into the aorta and the pulmonary artery, the latter coimecting with the fifth pair of aortic arches. Proximally the bulb is undivided. The interventricular septum is complete except for the itUervenlricular foramen which leads from the left ventricle into the aortic side of the bulb. Of the bulbar sweUings which divide the bulb into aorta and pulmonary trunk, the left joins the interventricular septum, while the rigfU extends to the endocardial cushion. These folds eventually fuse and the partition of the ventricular portion of the heart is completed. The endocardium at the atrio-ventricular foramina is already undermined to lorm the aniages of the tricuspid and bicuspid valves. From the caudal vail of the left atrium there is given off a single pulmonary vein.


Fig. 125. — Heart of 12 mm. embryo dissected from the right ride.


The Arteries

As seen in Fig. 123, the first two aortic arches have disappeared. Cranial to the third arch, the ventral aortx become the external carotids. The third aortic arches and the cephalic portions of the descending aortae constitute the inlemat carotid arteries. The ventral aortic between the third and fourth aortic arches per^t as the common carotid arteries. The descending aorta; in the same region are slender and eventually atrophy. The fourth aortic arch is largest and on the left side will form the aortic arch of the adult. From the right fourth arch caudad. the right descending aorta is smaller than the left. Ojip'^sitf (be 'ighth segment, the two aorta* unite and continue caudally as the median dorsal aorta. The fifth (sixth ?) aortic arches (cf. p. 99) are connected with the puhnonary trunk, and from them arise small pulmonary arteries to the lungs. Dorsal intersegmental arteries arise, six pairs from the descending aorta, others from the dorsal aorta. From the seventh pair, which arise just where the descending aorta fuse, the subclavian arteries pass off to the upper limb buds and the vertebral arteries to the head. The latter are formed by a longitudinal anastomosis between the first seven pairs of intersegmental arteries on each side, after which the stems of the first rfx pairs atrophy.


Fig. 126 A.— RecoMtraction of a 12 mm. pig embryo to show the veins and heart from the left ade. For names of parts see Fig, 126 B on oi^iosite page (F. T. Lewis). X 9.


Venlro-lateral arteries from the dorsal aorta supply the mesonephros and genital ridge (Fig. 123). Venird arteries form the ccsliac artery to the stomach region, the vitelline or superior meserUeric artery to the small intestine, and the inferior mesenteric artery to the large intestine.

The umbilical arteries now arise laterally from secondary trunks which persist as the common iliac arteries.

The Veins

The cardinal veins have been reconstracted by Lewis in a. 12 mm. pig (Fig. 126). The veins of the head drain into the anterior cardinal vein, which becomes the internal jugular vein of the adult. After receiving the external jugular veins and the subclavian veins from the upper limb buds the anterior cardinals open into the common cardinal veins (duct of Cuvier). '


Fig. 126 B. — Reconstruction of a 12 mm. pig einbr>'c> to show tbc veins (rom tlie left side fl^wis). X 9. A,, Umbiliral artery; At>., aorta; Au., right auricle (atrium); card.', card.", superior and inferior sections of posterior cardinal vein; d, left common cardioal vein; D.C., right common cardinal vein; D.V., ductus venosus; Jug.', Jif.", jugular or ant. cardinal vein; /... liver; t-.s.. antaxe of lateral sinus: mjr, transverse vein; P., pulmonary arter>-; Sc, subcardinal vein: Sd.. subclavian vein; sh., aniage of sup, longitudinal sinus; Um4., right umbilical vein; Vcn., right \-entricle; V.H.C., common hepatic vein; V.op., ophthalmic vein; V.P., portal vein; X, anastomosis between the right and left subcardinal vrini.


The posterior cardinal veins arise in the caudal region, course dorsal to the mesonephroi, and drain the mesonephric sinusoids. The subcardinal veins anastomose just caudal to the origin of the superior mesenteric artery and the posterior cardinals are interrupted at this level. The proximal portions of the posterior cardinals open into the common cardinal veins as in the 6 mm. embryo. Of the two subcardinal wins, the right has become very large through its connection with the right posterior cardinal vein and the common hepatic vein, and now forms the middle portion of the inferior vena cava. For the development of this vein, see Chapter IX.

Fig. 127, — Reconstrucbon of a 10 mm pig embiyo to show the umbilical and vitelline veins horn the ventral side, x indicates sinusoidal connection between left umbilical vein and portal vein. X 15. In the small orientation figure (d. Fig. 133) the varioiu planes are indicated by broken lines —

The umbilical veins (Figs. 126 and 127) anastomose in the umbilical cord, separate on entering the embryo, and course cephalad in the ventro-Iateral body wall of each side to the ventral lobe of the liver. The left vein is much the larger, and, after entering the liver, its course is to the right and dorsad. After connecting with the portal vein, it continues as the ductus venosus and joins the proximal end of the inferior vena cava. The smaller right umbilical vein after entering the liver breaks up into sinusoids. It soon atrophies, while the left vein persists' until after birth.

The Vitelline Veins

Of these, a distal portion of the left and a proximal portion of the right are persistent. The left vitelline vein, fused with the right, a)urses from the yolk sac cephalad of the intestinal loop. Near a dorsal anastomosis between the right and left vitelline veins, just caudal to the duct of the dorsal pancreas, the left receives the superior mesenteric vein, a new vessel arising in the mesentery of the intestinal loop. Cranial to its junction with the superior mesenteric vein, the left vitelline, with the dorsal anastomosis and the proximal portion of the right vitelline vein, fonn the portal vein, which gives off branches to the hepatic sinusoids and connects with the left umbilical vein. For the development of the portal vein, see Chapter IX.


Figures are shown of sections passing through the more important regions and should be used for the idendfication of the organs. The level and plane of each section is indicated by guide Unes on Fig. 128. The student should compare this with Figs. 118 and 12.^, and orient each soilion with reference to the embryo as a whole. To avoid re|)etiti(in most of the levels illustrated in the transverse sections of the 6 mm. pig are not represented in the U)mm. series. For this reason the former series will be found very instructive in supplementing the following descriptions.


Fig. 128.— Rtconstniction of a 10 r numbered lines indicati- iln' kwh »l tm For the names of the ^'a^ious structuris n. pi): cmbTj'o. showing llic chief organs of the IcFt side. The svtrso sections shown in the corresponilinc tiifurcs |12y-14,'(. .ttelliTeiiset Kif!. 12.!. X 8.


Transverse Section through the Eye and Otocysts (Fig. i29) - The brain is sectioned twice, lengthwise through the myelencephalon, transversely through the fore-brain. The brain wall shows differentiation into three layers; (1) an inner ependymat layer densely cellular; (2) a middle mantle layer of nerve cells and fibers; (3) an outer marginal layer chiefly fibrous. These same three layers are developed in the spinal cord. A thin vascular layer differentiated from the mesenchyma surrounds the brain wall and is the anlage of the pia maler. The myelencephalon shows three neuromerea in this section. The telencephalon is represented by the paired cerebral hemUpheres, their cavities, the lateral ventricles, connecting through the inlerventricular foramina with the third ventricle of the diencepbalon. Close to the ventral wall of the diencephalon is a section of the anterior lobe of the hypophysis (Rathke's pocket) near which are the internal carotid and basilar arteries. Lateral to the diencephalon is the npiic cup and lens vesicle of the eye, which are sectioned caudal to the optic stalk. The outer layer of the optic cup forms the thin pigment layer; the inner thicker layer, is the nervous layer of the retina. The lens is now a closed vesicle distinct from the overlying corneal ectoderm.


Fig. 129.— Transverse section passing through the eyesandotocystsofalO mm. pig embryo. X 22.5.


The large vascular spaces are the cavernous sinuses, which drain by way o£ the w. capitis latcrales into the internal jugular veins. Transverse sections may be seen of the maxillary and mandibular branches of the n. trigeminus; the n. abducens is sectioned longitudinally. The snuD nn. oadomctorius and IrocUearis should be identified in sections more cephalad in ibe series. Ventral to the otocyst are seen the geniculate and acoustic ganglia of the nn. facialis ■□d acMstietu. The wall of the otocyst forms 3 sharply defined epithelial layer. More cephalad in the scries the endolymph duct lies median to the otocyst and connects with it. Dorsal to the otocyst the n. glossopharyngcus and the jugular ganglion of the vagus arc cut transversely ■ while the trunk of the n. accessorius is cut lengthwise.

Section through the First and Second Phairngeal Pouches (Fig. 130).— The end of the head, with sections of the telencephalon and of the ends of the olfactory pits, is now distinct from the rest of the section. The pharynx shows portions of ibe first and second pharyngeal pouches, 0|)positc the first pouth externally is the first branchial cleft. A section of the tuberculum impar of the tongue shows near the midline in the pharyngeal cavity. The neural tube is sectioned dorsally at the level of Froricp's ganglion. Ilelwccn the neural lube and the pharynx may Ik seen i)n each side the .several root fascltles of the n. hypogfossus, the fibers of the nn. vagus and accrssorius. and the pelrosiil ganglion of the m. glossopharyngeus. Mesial to the ganglia arc the descending aorta and lateral to the vagus is the internal jugular


Fig. 130.— Transverse section passing Ihroufih the first anil second phar>-ngea1 pouches of a 10 it embryo. X 22.5,


Section through the Third PbuTiigeal Pouches (Fig. 131).— The tip of the head is now small and shows on either side the deep olfactory pits lined with tfaickeoed olfactory epiIhelium. The first, second, and third branchial arches show on either side of the section, the third being slightly sunken in the cervical sinus. The dorsal diverticula of the third pharyngeal pouches extend toward the ectodenn of the third branchial cleft. The ventral diverticula or thymic aniages may be traced caudad in the series. The floor of the pbacynz is sectioned through the epiglottis. Ventral to the pharynx are sections of the third aortic arches and the solid cords of the thyreoid gland. Dorsally the section passes through the spinal cord and first pair of cervical ganglia. Between the cord and pharynx, named in order, are the internal jugular veins, the hypoglossal nerve, and the nodose ganglion of the vagus. Lateral to the ganglion is the external branch of the n. accessorius, and mesial to the ganglia are the small descending aorta.


Fig. 131. — ^Transverse section through the third pharyngeal pouches of a 10 mm. [Mg embryo.

< 22.5.


Section tbrou^ the Fourth Phairngeal Pouches (Fig. 132).— This region is marked by the disappearance of the head and the appearance of the heart in the pericardial cavity. The lips of the atria are sectioned as they project on either side of the bulbus cordis. The bulbus is divided into the aorta and pulmonary artery, the latter connected with the right ventricle, which has spongy muscular walls. The pharynx is crescentic and continued laterally as the smaW fourth pharyngeal pouches. Into the mid-ventral wall of the pharynx opens the vertical slit of the trachea. A section of the vagus complex is located between the descending aorta and the internal jugular vein. At this level the jugular vein receives the linguo-facial vein. The left descending aorta is larger than the right. The ventral aorta may be traced cranially in the series to the fourth aortic arches. The pulmonary artery, if followed caudad, connects with the fifth aortic arches as in Fig. 133.

R. descending aorln Esophagus

R. cealride Fic. 132.— Tnmsverse sectioii through the fourth pharyngeal pouches of a 10 mm. pig enibr>-o. X 22.5.

Fig. 133. — Traiuvenc Mctioa through the fifth pair of aortic arches and bulbus cordis of & 10 mm. [ng embiyo. X 22.5.


Section through the Fifth Aortic Arches (Fig. 133),— The Jiflh aortic arch is complete on ihe left side. From these pulmonary arches small pulmonary arteries may be traced caudad in the scries to the lung anlages. The esophagus, now separate from the trachea, forms a curved horizontal s!it. AH four chambers of the heart are represented, but the aorta and pulmonary artery are incompletely separated by the right and left bulbar swellings or folds.

Section through the Sinus Venosus and the Heart (Fig. 134). — The section is marked by the symmetrically placed atria and ventricles of the heart and by the presence of the upper limb buds. Dorsal to the atria are ihe common cardinal veins, the right vein forming part of the sinus venosus. The sinus venosus drains into the right atrium through a slit-like opyening in the dorsal and caudal atrial wall. The opening is guarded by the right and left tallies of the sinus venosus. which project into the atrium. The septum primum completely divides the right and left atria at this level, which is caudal to the foramen ovale and the atrio-ventricular openings. The septum joins the fused endocardial cushions. Note that the esophagus and trachea are now tubular and that the left descending aorta is much larger than the right. .Ground the epitheUum of both trachea and esophagus are condensations of mesenchyma, from which their outer layers are diilerentiated.


Section through the Foramen Ovale of the Heart (Fig. 135).— The level of this section is cranial to that of the previous figure and shows the septum primum interrupted dorsalty to form the foramen ovale. Each atrium communicates with the ventricle of the same side through the alrio-vtntricular foranun. Between these openings is the endocardial cushion, which in part forms the aniages of the tricuspid and bicuspid valves. The atria are marked oS externally from the ventricles by the coronary sulcus. Between the two venticles is the inlerventricular septum. The ventricular walls are thick and spongy, forming a network of muscular cords or trabecula surrounded by blood spaces or sinusoids. The trabeculse are composed of muscle cells, which later become striated and constitute the myocardium. They are surrounded by an endothelial layer, (he endocardium. The mammalian heart receives all its nourishment from the blood circulating in the sinusoids until later, when the coronary vessels of the heart wall are developed. The heart is surrounded by a layer of mesothclium, the epicardium, which is continuous with the pericardial mesothelium lining the body wall.

Section through the Liver and Upper Limb Buds (Fig. 136). — The section is marked by the presence of the upper limb buds, the liver, and the bifurcation of the trachea to form the primary bronchi of the lungs. The limb buds are composed of dense undiSereatiated mesenchyme surrounded by ectoderm which is thickened at their tips. The seventh pair of cervical ganglia and nerves are cut lengthwise showing the spindle-shaped ganglia with the dorsal root fibers taking origin from their cells. The ventral root fibers arise from the ventral cells of the mantle layer and Join the dorsal root to form the nerve trunk. On ihe right side a short dorsal ramus supplies the anlagc of the dorsal muscle mass. The much larger venlral ramus unites with those of other nerves to form the brachial plexus.

Septum Endocardial cushion

Fig. 135.— Transverse L. alrio-veniricular fori 1 10 imn. pig embryo.


The descending aortae have now fused and the seventh pair of dorsal inlersegmental arteries arise from the dorsal aorta. From these intersegmental arteries the subclavian arteries are given off two sections caudad in the scries. Lateral to the aorla are the posterior cardinal veins. The esophagus, ventral to the aorta, shows a very smalt lumen, while (hat of the trachea b large and continued into the bronchi on either side. Adjacent to the esophagus are the cut vagus nerves. The lung aniages project laterally into the crescenlic pleural cavities, of which the left is separated from the peritoneal cavily by Ihe septum transversum. The liver, with its fine network of trabecule and sinusoids, is large and nearly (ills the peritoneal or abdominal cavity. The /tMrcivib are composed of liver cells surrounded by the endothelium of the sinusoids. Red blood cells are developed in Ihe liver at this stage. The large vein penetrating the septum transversum from the liver lo the heart is the proximal portion of the inferior vena cava, originally the right vitelline vein. Ventral to the bronchi may be seen sections of the pulmonary veins.


Fig. 137.— Dorsal half of a Me.ioncpkric liibidr ■Ferilaneal cavity ■Dorsal lobe of liver

Sinusaids of liver clion through the lung buds cranial lo tht atomach in a 10 m pig embryo. X 22,5.

Section through Lung Buds Cranial to Stomach (Fig. 137). — The lungs are sectioned through theit caudal ends and ihc esophagus is just beginning to dilate into the Stomach. On either side of the circular dorsal aorta are the mesouepkroi, while dorso-laterally are sympathttic ganglia. The pleural cavities now communicate freely on both sides with the peritoneal cavity. A section of the lesser ptritoneul sac appears as a crescent -shaped slit at the right of the esophagus. In the right dorsal lobe of the liver is located the inferior tena cava. Near the median line ventral to the lesser sac is the large ductus venosus.

Section through the Stomach and Liver (Fig. 138).— Prominent in (he body cavity are the mesonephrot and liver lobes. The mesonephroi show sections of coiled tubules lined with cuboidal epithelium. The glomeruli., or renal corpuscles, are median in position and develop as knots of small arteries which grow into the ends of the tubules. The thickened cpithcUum along the median and ventral surface of the mesonephros is the anlage of lYie genital gland. The body wall is thin and Imed with mcsothclium continuous with that which covers the mesenteries and organs. The mesothelial layer becomes the epithelium of the adult periloneum, mesenteries, and serous layer of the viscera. The stomach lies on the left side and is attached dorsally by the greater omentum, ventrally to the liver by the lesser omentum. The right dorsal lobe of the liver is attached dorsally to the right of ihe great omentum. In the liver, ventral to this attachment, courses the inferior vena cava and the attachment forms the fiica ventr cava. Between the attachments of Lhe stomach and liver, and (o the right of the stomach, is the lesser peritoneal sac. In (he liver to the left of the midline is the ductus venosus, sectioned just at the point where it receives the left umbilical vein and a branch from the portal vein. The ventral attachment of the liver later becomes the falciform ligament.


Fig. 138.— Transverse section through the stomach and liver of a 10 mm. pig embryo. X 22.5,


Section throucb the Hepatic Diverticnliim (Fig. 139). — The section passes through the pyloric end of the stomach and duodenum near the attachment of the hepatic diverticulum. The great omentuin of the stomach is larger than in the previous section and to its right, in the plica ven» cave, lies the inferior vena cava. Ventral to the inferior vena fava is a section of the portal vein. The ventral and dorsal lobes of the liver are now separate and in the right ventral lobe is embedded the saccular end of the hepalic diverticulum which forms the gall Madder. To the right of the stomach, the divertitulum is sectioned again just as it enters the duodenum. Ventrally the Uft umbilical vein is entering the left ventral lobe of the liver. It is much larger than ihe right vein, which still courses in the body wail. On the left side of the embryo the spiniil tierve shows in addition to its dorsal and ventral rami a sympathetic ramus, the fibers of which pass to a cluster of ganglion cells located dorso-laleral to the aorta. These cells form one of a pair of sympaihtik ganglia and are derived from a spinal ganglion.

Fig. 139.^Transverse ibryo. X 22.5.

Section through the Pancreatic Anlages {Fig. 140). — The lesser peritoneal sac just above the level of this section has opened into the peritoneal cavity through the epiploic foramen (of Winslow). The mesonephric duels are now prominent ventrally in the mesonephroi. The duct of the dorsal panrrens is sectioned tangentially at the point where it takes origin from the duodenum. From the duct tbe lobulaied gland may be traced dorsad in the mesentery. To the right of ihe dorsal pancreatic duct is a section of the ventral pancreas, which may be traced cephalad in the series to its origin from the hepatic diverticulum. Dorsal to the ventral pancreas is a section of the porlai vein. The inferior vena cava appears as a vertical slit in the clorsal mesentery.

Section through the TJiogenital Sinus and the Lower Limb BudE (Fig. 141).— The figure shows only the caudal end of a section, in the dorsal portion of which the mesonephroi were sectioned at the level of the subcardinal anastomosis. A portion of the mesenUry is shown with a section of the colon. In ihc body wall are veins which drain into the umbilical veins, and on each side are the umbilical arteries, just entering the body from the umbilical cord. Between them, in sections cranial to this, the allantoic stalk is located. Here it has opened into the crescentic urogenital sinus. Dorsal to the urogenital sinus (dorsal now being at the bottom of the figure owing to the curvature of the caudal region) is a seclion of the rectum, separated from the sinus by a curved prolongation of the calom. From the ends of the urogenital sinus, as we trace cephalad in the embryo {doVHWard in the series), are given off the mesonepkric duels.


Ventral fon. Fig. 140.— Portion of a transverse


Fic. 141. — Tmia\-eneKCtion through the urogenital pig embryo. X 22. .S.

Section throufh die Hetonepbric Ducts at the Opening of the Ureter (Fig. 142). — The section cuts through both lower limb buds near their middle. Mesial to their besas are the umbilical arteries, which lie lateral to the mesonephric duels. From the dorsal wall of the left mesonephric duct is given off the ureter or duct of the metunepkros. Tracing the sections down in the series, both ureters appear as minute tubes in transverse section. They soon dilate to form the pelvis of the kidney at the level of Fig. 143. Note the undifierentiated mesenchyme of the lower limb buds and their thickened ectodermal tips.


Fig. 142. — Transverse section of a 10 mm. embrjo passing through the lower limb buds at the level of the openings of the ureters into the mesonephric ducts. X 22.5.

Section through the Metanephroi and Umbilical Arteries (Fig. 143).— The section passes caudal to the mesonephric ducts which curve along the ventral surfaces of the mesonephroi (Fig, 124). The umbilical arteries course lateral to the metanephroi which consist merely of the thickened epithelium of the pelvis surrounded by a layer of condensed mesenchyma, the nephrogenic tissue.


Fig. 143. — Transverse section through the aniages of the metanephroi in a 10 mm. pig embryo. X 22.5.




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Prentiss CW. and Arey LB. A laboratory manual and text-book of embryology. (1918) W.B. Saunders Company, Philadelphia and London.

Human Embryology 1918: The Germ Cells | Germ Layers | Chick Embryos | Fetal Membranes | Pig Embryos | Dissecting Pig Embryos | Entodermal Canal | Urogenital System | Vascular System | Histogenesis | Skeleton and Muscles | Central Nervous System | Peripheral Nervous System | Embryology History
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