Book - Introduction to Vertebrate Embryology 1935-3

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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 III Organogeny

Chapter VIII Endodermal Derivatives

The tissues derived directly from the endoderm are for the most part of the epithelial type and form the inner lining of the gastrocoel and the organs that arise therefrom. These organs are grouped into two closely connected organ systems, the digestive system and the respiratory system. The digestive (enteric) tube, however, becomes ensheathed in splanchnic mesoderm which contributes largely to the ultimate structure of the organ systems just mentioned. Furthermore, this tube opens to the exterior at both the anterior and posterior ends by means of two ectodermal pits, the stomodeum and proctodeum, respectively. All three germ layers, therefore, contribute to the organogeny of these systems.

The stomodeum. — There is an ectodermal invagination on the ventral side of the head to form the stomodeum (Fig. 118), which is bounded on the sides by the maxillary ridges and on the rear by the mandibular ridges. The rupture of the oral plate, which separates the stomodeum from the fore-gut, results in the formation of the oral cavity, or mouth. From the stomodeum another invagination, the hypophysis, grows upward in front of: the fore-gut, and eventually fuses with an evagination from the floor of the neural tube, the infundibulum, to form the pituitary gland, an organ of internal secretion. As the stomodeum joins the fore-gut a little posterior to the anterior end of the latter cavity, there is a blind pocket of endoderm, anterior to the mouth, called the preoral gut.

Fig. 118. — Diagram of an early vertebrate embryo, to show endodermal derivatives.

The oral cavity. — The cavity of the mouth is a compound structure, derived in part from the ectodermal stomodeum and in part from the endodermal fore-gut. The boundary line between these is soon lost after the rupture of the oral plate owing to unequal local growth of the different regions of the mouth. The boundaries of the mouth are the upper jaws, formed from the maxillary ridges, and the lower jaws, derived from the mandibular ridges. On these ridges the teeth arise in exactly the same way as the placoid scales of the elasmobranchs (page 230). Two elements are concerned: an ectodermal enamel organ, shaped like an inverted cup; and a mesodermal dental papilla, which fills the cavity of the enamel organ. The enamel organ gives rise to the outer enamel layer of the tooth, while the papilla forms the dentine (Fig. 119). The dentine is in the general form of a hollow cone, the cavity of which is filled with connective tissue, nerves, and blood vessels. The tongue (Fig. 120) is also a compound organ, arising from an endodermal primary tongue which is formed from the floor of the pharynx in the region of the hyoid arches, and from an ectodermal secondary tongue which arises from the floor of the oral cavity in front of the thyroid gland (page 184). Into the tongue a migration of mesoderm takes place, by means of which the musculature is formed. The glands of the mouth (salivary glands, etc.) arise from the ectodermal lining of the mouth. The taste buds, however, are endodermal (Holtfreter, 1933). The connection between the oral cavity and the nasal cavity will be discussed in Chapter X.

Fig. 119. — Diagram to show origin of vertebrate tooth (lower jaw).

Fig. 120. — Diagram showing derivatives of vertebrate fore-gut.

The pharynx. — The region of the fore-gut which follows the oral cavity is the pharynx, particularly important on account of the respiratory organs and other structures which arise from it.

Respiratory organs. — Respiratory exchange may take place in any thin epithelium in which the blood corpuscles are brought into contact with the oxygen-carrying medium. ‘These epithelia may be either ectodermal or endodermal in origin. Thus, we find that among the amphibia, respiration may take place in the skin as a whole (lungless salamandeys) ; ; An specialized outgrowths on the visceral arches, external gills (N. ecturus); or in the so-called “hairs ” of the African frog, Astylosternus. In this group are to be found also examples of endodermal respiratory organs, the internal gills and lungs. Internal gills are otherwise found only among the fish, while the lungs are characteristic respiratory organs also of the amniotes.

The internal gills. -- The internal gills (branchiae) arise in the visceral clefts (Fig. 120) common to all chordates. Among the aquatic vertebrates these are typically six in number (see Table 8, page 131). In the cartilage fish the first cleft (the spiracle) opens on the dorsal side of the head and is otherwise modified. The clefts are separated by the visceral arches, of which the first is known as the mandibular arch and the second is called the hyoid arch. The visceral clefts are formed by the coming together of paired evaginations of the endoderm (visceral pouches) and complementary invaginations of the ectoderm (visceral grooves). The ectoderm and endoderm come into direct connection to form closing plates. Later, these plates rupture and a series of fingerlike projections grow out into the cleft from the anterior and posterior sides of each arch. These filamentous processes usually fuse to form a demibranch (Fig. 197). The demibranchs in some fish are apparently of endodermal origin, while in the amphibia they are derived from the ectoderm. It is interesting to note that in the spiracle of the cartilage fish a gill-like structure, the pseudobranch, develops. In amphibians and the amniotes generally the first visceral pouch does not open to the exterior but gives rise to the tympanic cavity and auditory tube (see Chapter X). In all fish except the elasmobranchs, a projection grows back from the hyoid arch to cover the remaining visceral clefts. This is the operculum. Internal gills do not appear in the development of the amniotes; but the visceral clefts, or at least the visceral pouches and grooves, are of invariable occurrence.

The lungs. — In all the vertebrates except the cyclostomes and cartilage fish, there develops from the pharynx a sac (or a pair of sacs) which becomes the air bladder in pisces and the lungs in tetrapoda. We shall confine our attention here to the development of the lungs (Fig. 120). The first indication of lung formation is the appearance of a longitudinal groove in the floor of the pharynx posterior to the last pair of visceral pouches. This is the tracheal groove. This groove separates from the pharynx, the process commencing at the posterior end, so that the dorsal portion of the tube, or esophagus, is separated from the ventral portion, or trachea, except for a narrow opening, the glottis. The trachea grows backward rapidly and divides into two lobes, the primordia of the lungs. There is some evidence that the trachea is bifurcated from its first appearance, suggesting that the lungs arise from paired primordia. In the birds and mammals the lung primordia subdivide many times to form the bronchi, or branches of the respiratory tree.

The thyroid gland. — This structure arises as a median ventral evagination of the pharyngeal floor between the primary and the secondary tongue primordia or at the level of the hyoid arches. The diverticulum grows downward and expands at its distal end (Fig. 120). Eventually, its connection with the pharyngeal floor, the thyroglossal duct, becomes occluded and disappears, and the gland itself subdivides into a mass of vesicles which migrate backward and assume somewhat different positions in various vertebrates, often ending as a paired organ on either side of the trachea.

The epithelial bodies.— In all the vertebrates there arise, from the upper or lower angles of the visceral pouches, small buds of epithelium which often give rise to endocrine glands of varying — and mostly unknown — function (Figs. 120, 121). The dorsal buds (except among the mammals, where conditions are reversed) contribute in varying number to the formation of a large gland, the thymus, which loses connection with the pharynx and moves backward to its definitive position, which differs according to the form studied. The remainder of the dorsal bodies become lymphoid and degenerate. The ventral buds (absent in fish) detach themselves from the pharyngeal wall and take up varying positions. Among the mammals it is the ventral buds which form the thymus, while the dorsal buds of the third and fourth pouches move to the sides of the thyroid gland where they are known as the parathyroids.

Fig. 121. — Diagrams showing origin of epithelial bodies in A, frog; B, chick; and C, man.

The esophagus. — The digestive canal behind the pharynx becomes specialized into four regions: (1) the esophagus; (2) the stomach; (3) the intestine and its derivatives; and (4) the cloaca. Of these, the esophagus (Fig. 120) remains comparatively unspecialized; it is a narrow tube, short in the anamniotes, elongate in the amniotes. No digestive glands are found in this region.

The stomach. — This portion of the digestive tract is distinguished by its dilation (Fig. 120) into a large sac or series of sacs, and by the development of a thick wall of muscle from the splanchnic mesoderm in which it is enveloped. The stomach is rich in glands which aid in digesting the passing food.

The intestine. — All the regions of the digestive tract mentioned so far are derived from the fore-gut. The intestine is derived in part from the fore-gut, in part from the mid-gut, and in part from the hind-gut. It is impossible to indicate exactly which regions arise from these divisions of the gut, as both the fore-gut and the hind-gut expand at the expense of the mid-gut during the consumption of the yolk. As was said in the discussion of the development of body form, the division of the alimentary canal into these regions is the result of the method by which the head and tail are formed. The intestine becomes subdivided in various ways in the different groups, but we need notice only the most anterior of these, the duodenum, which is that portion of the intestine immediately succeeding the stomach and generally held to be derived from the fore-gut. The intestine is richly glandular throughout its length, but from the duodenum, in particular, we find developed two most important glands, the liver and the pancreas (Fig. 120).

The liver. — This gland arises from the ventral side of the duodenum as an evagination which grows forward, expanding into a vesicle at the distal end and retaining its connection with the duodenum by a narrow hollow stalk, the common bile duct, (Fig. 120). The sac-like distal end becomes subdivided, by the ingrowth of mesenchyme, into many tubules which often anastomose. In this process of growth and subdivision the liver grows about the vitelline veins (Chapter [X) and breaks these up into a system of hepatic capillaries. The cavity of the sac becomes the gall-bladder, to which the bile, formed in the glandular portion of the liver, is carried by means of the hepatic ducts. It releases these secretions into the duodenum via the common bile duct (ductus choledochus).

The pancreas. — This gland arises usually from three diverticula of the duodenum (Fig. 120), but the number of primordia is variable. One appears on the dorsal side of the duodenum just posterior to the stomach; the others arise on the ventral side, usually in connection with the hepatic diverticulum. The primordia increase in size, and break up into masses of secretory THE FROG 187

tubules at the distal end of each. The primordia unite and their proximal ends become the pancreatic ducts, one or more of which may be suppressed in later organogeny. The pancreas, as well as elaborating a digestive pancreatic juice discharged through the pancreatic duct, forms a hormone (insulin), which is carried away by the blood stream. It functions therefore as an endocrine gland in addition to its digestive function. Insulin, as is well known, is important in the treatment of diabetes.

The cloaca. — The intestine behind the duodenum is variously subdivided in the different vertebrate classes, but all are alike in the possession of a terminal region which receives in addition the ends of the nephric ducts and of the genital ducts (see Chapter IX). From the cloaca also arises the urinary bladder and the allantois of the amniotes.

The cloaca, like the pharynx, communicates with the exterior by means of an aperture lined with ectoderm, which arises as a median ventral pit, the proctodeum (Fig. 118), just in front of the tail region. The proctodeum is formed at the point where the blastopore was obliterated and is separated from the hind-gut temporarily by means of the cloacal plate, which is comparable with the oral plate. For a time there is a blind pocket of endoderm posterior to the cloaca, which is known as the postcloacal gut. The region of the cloaca anterior to the entrance of the nephric ducts is known as the rectum; its aperture is called the vent. In mammals the rectum becomes separated from the remainder of the cloaca, which is then known as the urogenital sinus. Each of these cavities has a separate exit, the two openings being the anus and the urogenital aperture, respectively.

THE FROG (SEE ALSO CHAPTER XI).— The mouth of the tadpole does not open until a few days after hatching. It remains round during larval life and is enclosed by the mandibular ridges. Outside these, folds of ectoderm project as the larval lips, on which horny larval teeth develop. These larval structures are lost at metamorphosis, when the definitive jaws and teeth are formed in the usual way. The tongue is compound, arising from a primary tongue and a gland field, relatively late in larval life. The hypophysis is solid (Fig. 181).

Six visceral pouches appear, of which the first never ‘becomes perforated, its closing plate becoming the tympanum of the ear, 188 ENDODERMAL DERIVATIVES

and its cavity persisting as the tubo-tympanic cavity. Of the five remaining pouches, the second and third open to the exterior before the first and fourth, and the fifth remains vestigial. External gills appear on the third, fourth, and fifth arches (that on the fifth arch being rudimentary), but are resorbed later when covered by the operculum. This structure fuses with the body surface on the right side, but on the left it opens to the exterior by an opercular aperture. The internal gills appear as demibranchs commencing on the anterior side of the third arch. The first three gills, therefore, have two demibranchs, while the fourth has but one, formed from the anterior side of the sixth arch. The visceral clefts, gills, and opercular cavity are lost as separate structures by cell proliferation and reorganization just before metamorphosis. The lungs appear early in larval life as solid primordia of the pharynx. These acquire cavities prior to the formation of the tracheal groove which is relatively late in formation. The thyroid arises, just before hatching, as a solid diverticulum of the pharynx; it soon detaches itself and divides into two bodies which later become vesicular. The two thymus glands are formed from epithelial bodies on the dorsal side of the first and second visceral pouches. Epithelial bodies arise from the ventral sides of the second visceral pouches. It has been claimed that those of the third and fourth pouches become the carotid glands. The sixth pharyngeal pouches give rise to the ultimobranchial (suprapericardial) bodies. (Fig. 121A.)

The esophagus is short, and the stomach a simple dilation. The liver arises as a backward ventral diverticulum of the duodenum (Fig. 181). All three pancreatic primordia appear and fuse; the dorsal duct disappears, while the two ventral ducts fuse to become the adult pancreatic duct. The intestine of the tadpole, which is long and coiled (about nine times the body length), becomes resorbed during metamorphosis until it is about one-third of its larval length (Fig. 122).

The postcloacal gut loses its connection with the neural tube (neurenteric canal) during the backward growth of the tail. The urinary bladder does not appear until after metamorphosis.

THE CHICK (SEE ALSO CHAPTER XII). — The mouth opens on the third day of incubation. The teeth are represented only by the tooth ridges which are the first stage in the appearance of the THE CHICK 189

enamel organs. These appear on the sixth day of incubation and disappear shortly after the cornification of the jaws. This results in the formation of the beak and the egg tooth, the latter a horny projection on the upper jaw which is used in breaking through the shell at the time of hatching, and soon after disappears. The primordia of the tongue appear on the fourth day.

Five visceral pouches appear, of which the first three open to the exterior during the third day of incubation (Fig. 218). The


Fig. 122. — Digestive tube in A, tadpole, and B, frog, to show actual shortening of intestine. (After Leuckart wall-charts.)

first cleft closes during the fourth day, and the dorsal part of the pouch becomes the tubo-tympanic cavity. With the extension of the cervical flexure, the remaining pouches are crowded together and disappear. The thyroid appears on the second day, separates from the pharynx on the fourth, and on the seventh divides inte two bodies which migrate backward to the junction of the common carotid and subclavian arteries. The thymus arises from the dorsal epithelial bodies of the third and fourth visceral pouches, while the parathyroid rudiments arise from 190 ENDODERMAL DERIVATIVES

the ventral epithelial bodies. The fifth pouch gives rise to the ultimobranchial bodies. The lung primordia (Fig. 123) appear on the third day and grow back, becoming surrounded by mesenchyme. The primary bronchi subdivide to form a respiratory tree, some branches of which extend among the viscera and even into the hollow bones, as the accessory air sacs.

The esophagus is relatively long; and a dilation, the crop, forms at its posterior end. The stomach is divided into an anterior proventriculus, which contains the gastric glands, and a

Visceral arches

Dorsal pancreas

Yolk stalk

Fig. 123. — Endodermal derivatives in a 72-hour chick.

muscular gizzard at the posterior end. The liver primordium arises at the edge of the anterior intestinal portal on the second day and, therefore, presents the aspect of an anterior ventral and two posterior lateral diverticula for a short time. These fuse, however, by the end of that day, as the backward extension of the fore-gut continues. Three pancreatic diverticula are formed, the dorsal one on the third day, the ventral ones on the fourth. They fuse in later development, and either two or three of the ducts persist. The anterior portion of the mid-gut becomes the small intestine, the large intestine arising from the posterior

region.. MAN 191

The cloaca is first distinguishable on the fourth day, when the proctodeum also is first apparent. The cloaca is ultimately divided into three regions: an anterior portion, the coprodeum, into which the rectum enters; an intermediate part, the urodeum, into which the nephric ducts and gonoducts enter; and the terminal proctodeum.

MAN (SEE ALSO CHAPTER XIII). — The mouth opens in the second or third week, and, like that of all vertebrates, develops lips (fifth week). Ten teeth papillae and enamel caps, the primordia of the milk teeth, appear in each jaw. This is a long-drawn-out process, the germs of the third molar not appearing until the fifth year of infancy. The tongue arises from swellings on the first three arches, the secondary tongue, or gland field, appearing as the tuberculum impar, which does not, however, appear to contribute to the ultimate structure of the tongue.

Five pairs of visceral pouches appear, none of which becomes perforated. The first gives rise to the tubo-tympanic cavity. The ventral portion of the second persists as the fossa in which the tonsil develops. The dorsal epithelial bodies from the third and fourth pair of pouches become the parathyroids. The ventral epithelial bodies of the third pair of pouches unite to form the thymus gland. Similar bodies from the fourth pair may give rise to vestigial thymus-like bodies which remain attached to the parathyroids from the same pouch. The fifth pair become the ultimobranchial bodies. The thyroid gland undergoes an incomplete division into two lobes which remain connected by a narrow isthmus. The lungs (Fig. 124) arise toward the end of the fourth week, from a laryngo-tracheal groove. The cartilages and musculature of the larynx arise from the branchial arches.

The esophagus, at first relatively short, lengthens as the backward movement of the heart and lungs displaces the stomach. The latter organ arises as a dilation of the fore-gut posterior to the esophagus. Continued growth, mainly on the dorsal surface, produces the greater curvature, and a displacement of the whole organ so that the cephalic end is moved to the left and the caudal end to the right. This is followed by a rotation of the stomach on its long axis through 90° to the left. The liver ari ring the third week as a ventral groove in the duodenum. The-pancreas appears slightly later, with either two or three 192 ENDODERMAL DERIVATIVES

primordia according to whether or not one of the ventral primordia is suppressed. The ventral pancreatic duct persists and opens into the common bile duct. The point of division between small and large intestines is marked by the formation of a blind pouch,

Visceral arches

( Hypophysis )



pancreas Intestinal loop

Dorsal pancreas,

Allantoic stalk



( Mesonephric duct ) liverticulum


Fig. 124. — Endodermal derivatives in 10-mm. pig. (From a wax reconstruction by G. W. Hunter and L. T. Brown.)

the cecum. The distal end of the cecum does not grow as rapidly as the proximal region and so remains a finger-like projection known as the vermiform appendix. The small intestine, growing more rapidly than the large, is thrown into a set of six primary coils, each of which develops secondary coils.

The cloaca becomes divided, by a frontal partition, into a SUMMARY 193

dorsal rectum and a ventral urogenital sinus. The cloacal membrane is correspondingly divided into a rectal and a urogenital plate, and the final openings are the anus and the urogenital aperture. The urogenital sinus later is divided into a phallic portion (see page 211) and a vesico-urethral portion. The latter gives rise to the urinary bladder at its distal end, and to. the urethra at its proximal end.


The endoderm gives rise to the epithelial lining of the following structures:

A. Fore-gut I. Oral cavity (also partly from ectoderm of stomodeum)

Teeth (also partly from ectoderm) Tongue II. Pharynx Trachea and lungs Thyroid Visceral pouches Auditory tube and chamber Fossa of palatine tonsil Thymus Parathyroids Ultimobranchial bodies III. Esophagus

IV. Stomach V. Duodenum

Liver Pancreas B. Mid-gut I. Intestine C. Hind-gut I. Cloaca (also partly from ectoderm of proctodeum) Rectum

Urogenital sinus

Urinary bladder

Urethra (also partly from mesoderm, page 204) 194 ENDODERMAL DERIVATIVES


Keey, L. B. 1934. Developmental Anatomy, 3rd Ed., Chaps. 7 and 8.

Brachet, A. 1921. Traité d’embryologie des vertébrés, Part 2, Bk. 1, Chap. 5; Bk. 2, Chap. 4.

Hertwig, O. 1906. Handbuch, Vol. 2, Chaps. 1, 2, and 4.

Keibel and Mall. 1910-1912. Human Embryology, Chap. 17.

Kellicott, W. E. 1913. Chordate Development.

Kerr, J.G. 1919. Textbook of Embryology, Vol. II, Chap. 3.

Kingsley, J.S. 1926. Comparative Anatomy of Vertebrates, 3rd Ed.

Lillie, F. R. 1919. The Development of the Chick, 2nd Ed.

MeMurrich, J. P. 1923. The Development of the Human Body, 7th Ed.

Chapter IX Mesodermal Derivatives

The middle germ layer arises as three different aggregates of cells between the ectoderm and endoderm: the notochord; the mesoderm; and the mesenchyme. The origin of the notochord has already been described, and its later history will be discussed in connection with the skeleton. Organs of mesenchymatous origin will be taken up in connectiqn with the history of the region from which their mesenchyme originates. Of the structures derived from the mesoderm, we shall consider first those arising from the lateral mesoderm, then those whose origin is from the intermediate mesoderm, and finally those derived from the axial mesoderm.


Cavities may appear in all three divisions of the mesoderm; if in the myotomes, they are known as myocoels; if in the nephrotomes, they are called nephrocoels; the cavity of the lateral mesoderm is the coelom (Fig. 76). In some forms the three cavities are confluent. The connection, however, is a temporary one, and the myocoels soon disappear. In other forms they make a transitory appearance and are entirely disconnected with the other cavities, and in many vertebrates myocoels are never formed. The nephrocoels will be considered with the nephric organs. The coelom_ in amphioxus has a metameric origin from the ventral portions of the enterocoels, which become confluent at this point by the disappearance of the intervening anterior and posterior partitions. In vertebrates the coelomic cavity arises from the splitting of the lateral mesoderm into a dorsal somatic and a ventral splanchnic layer. In the amniotes this: split continues out into the extra-embryonic mesoderm, thus giving rise to the exocoel, or cavity of the chorion. The coelom does not extend anterior to the visceral arches. Transitory

cavities have been found in the arches and, indeed, in the head 195 196 MESODERMAL DERIVATIVES

itself, and these have been interpreted as the remains of a cephalic coelom. It will appear later that these are more probably the rudiments of cephalic myotomes. The coelom does not extend into the tail.

Somatopleure and splanchnopleure. — The somatopleure has already been defined as the outer layer of the lateral mesoderm together with the ectoderm with which it becomes associated. Between these two there is an invasion of mesenchymatous cells from the dermatomes and myotomes which give rise to the corium of the skin (see Chapter X) and to its dermal musculature (see page 239). The somatic mesoderm lining the outer wall of the coelom becomes the outer peritoneal lining. The splanchnopleure is the inner layer of the lateral mesoderm plus the endoderm with which it is associated. Between these two occurs a migration of mesenchyme cells which give rise to the splanchnic musculature and blood vessels, while the splanchnic mesoderm itself forms the inner peritoneal lining of the coelom.

The mesenteries (Fig. 125).—JIn all the vertebrates, the coelom is divided for a time into right and left halves by sagittal partitions above and below the alimentary canal, known as the dorsal mesentery and the ventral mesentery, respectively. These are formed by the inward growth of the splanchnic mesoderm above and below the digestive tube and the subsequent fusion of these sheets in the median line. The ventral mesentery disappears posterior to the liver, probably in connection with the coiling of the intestine. The dorsal mesentery (Fig. 125) persists as the support of the alimentary canal, and frequently becomes subdivided into regions which are named from the supported organ, such as the mesogastrium which supports the stomach, the mesoduodenum, etc. In the formation of the ventral mesentery, two organs, the heart and the liver, owing to their ventral position, are caught in between the two advancing sheets of splanchnic mesoderm. In these regions, therefore, the ventral mesentery is divided into an upper and a lower half. The ventral mesentery dorsal to the heart becomes the dorsal mesocardium; that part which is ventral to the heart is the ventral mesocardium (Fig. 126A). Both eventually disappear as the heart increases in size and complexity. In the region of the liver, the dorsal half of the mesentery becomes the dorsal mesohepar, while the ventral porTHE MESENTERIES 197

Pericardial cavity—__f 4 Dorsal mesocar, a dium

Ventricle of heart


Ventral mesocardium +- Septum transversum

Liver Stomach Ventral mesentery st omac (faeiform Hgament) Ventral mesentery

(lesser omentum) Dorsal mesogastrium

Dorsal pancreas

Fia. 125. — Diagram of mesenteries in early human embryo from left side. A, B, and C indicate planes of sections shown in Fig. 126. (From Arey after Prentiss.)

Neural tube Notochord Neural tub Aorta NotochordPostcardinal vein Aorta

Dorsal mesentery,

— Fore-gut Lesser ‘omentum


Peritoneal cavity Foalciform ligament

B Cc

Fra. 126. — Diagrams of mesenteries in early human embryo as seen in transverse sections. Compare lig. 125. (From Arey after Prentiss.) 198 MESODERMAL DERIVATIVES

tion is the ventral mesohepar (Fig. 126B). The primordia of the pancreas lie originally in the dorsal and ventral mesenteries, respectively, but with the rotation of the stomach all are included in the dorsal mesentery. The peritoneal supports of the nephric and genital organs will be considered in the following section. The spleen (see page 224) arises in the mesogastrium, close to the wall of the alimentary canal, and is probably mesodermal in origin.

Later divisions of the coelom. — The coelom becomes divided into an anterior pericardial cavity surrounding the heart, and a posterior abdominal cavity surrounding the viscera, by the septum transversum, a transverse partition which grows out from the bridge of mesoderm surrounding the vitelline veins

( Coe nos


Pericardial cavity


fx-+- Peritoneal cavity

Abdominal “cavity |

Fie. 127. — Diagrams of coelom and its divisions in A, fish, B, amphibia, reptiles and birds, and C, mammals. (After Kingsley.)

where they cross the coelom en route from the body wall to the heart (Fig. 127A). These cavities are connected during a large part of the embryonic period by pericardio-peritoneal canals where the septum has failed to unite with the ventral body wall. In the amniotes, additional septa develop behind the lungs and separate the pleural cavities, which contain the lungs, from the remainder of the abdominal cavity, which is now known as the peritoneal cavity (Fig. 127B). The pleural cavities are separated from each other in the median line by the mediastinum. In the mammals (Fig. 127C) the partition separating the lungs from the viscera receives musculature from the myotomes and becomes the diaphragm.

THE FROG (SEE ALSO CHAPTER XI.) — In the frog, the ventral mesentery disappears as soon as it has been formed, except in the region of the heart and liver. The ventral mesocardium appears THE NEPHRIC ORGANS 199

before the dorsal mesocardium is formed, and disappears soon after, to be followed by the disappearance of the dorsal mesocardium. The ventral mesohepar also has but a short period of existence. The septum transversum receives much of its substance from the mesodermal sheath of the liver. No pleural cavities are formed.

THE CHICK (SEE ALSO CHAPTER xII.) — In the chick, both dorsal and ventral mesenteries are formed. The latter, however, persists only in the region of the fore-gut, and gives rise to the mesocardia, which soon disappear; the dorsal mesohepar, which becomes the gastro-hepatic omentum, and the ventral mesohepar, which becomes the falciform ligament. The septum transversum is not completed until the eighth day of incubation. The pleural cavities are cut off from the pericardial cavities by a pleuro-pericardial septum, and from the peritoneal cavity by the pleuro-peritoneal septum.

MAN (SEE ALSO CHAPTER XIII).— From the first, the pericardial cavity is distinguishable from the abdominal cavity, inasmuch as it never communieates directly with the extraembryonic coelom as does the abdominal cavity. As in the chick, its posterior boundary is coterminous with that of the fore-gut, but it is in communication with the abdominal cavity by means of the parietal recesses, passages which correspond to the peritoneo-pericardial canals of the anamniotes. The recesses are divided frontally by the vitelline veins into dorsal and ventral parietal recesses. With the formation of the septum transversum, the ventral recesses are incorporated into the pericardial cavity. The dorsal recesses become the pleural cavities; and the pleuroperitoneal septum, which divides them from the peritoneal cavity, is formed by the upward growth of the diaphragm. The musculature of this organ arises from the fourth cervical myotome during the backward growth of the diaphragm. The rotation of the stomach results in a rearrangement of the mesenteries, for an account of which the reader is referred to Hertwig or Keibel and Mall.


The nephric or excretory system of vertebrates is essentially a paired series of tubes (nephridia), developed in the intermediate mesoderm, which collect nitrogenous wastes from the blood and 200 MESODERMAL DERIVATIVES

discharge them to the exterior by two longitudinal ducts emptying into the cloaca. The intermediate mesoderm in the anterior part of the body is divided into nephrotomes corresponding to the somites. There are three different types of kidneys among the vertebrates (Fig. 128). The first is the pronephros, which arises from the anterior nephrotomes and is the functional kidney in the larval stages of the fish and the amphibians. The second is the mesonephros, which arises from nephro 9 } Pronephros

Mesonephric duct tomes posterior to the pronephros and is the functional kidney of Metanephros adult anamniotes and embryonic or Mefanephric duct f4¢4] amniotes. The third is the Cloaca metanephros which is the functional

rae, kidney of adult amniotes.

Fig. 128. — Diagram to show rela- The pronephros. — This organ is tionships of vertebrate excretory formed during development by all systems. :

vertebrates, but is best developed

in larval types like the frog, where it arises from nephrocoels (Fig. 129) in the anterior nephrotomes (III, IV, V), the dorsal ends of which grow caudally and unite with each other to form the pronephric duct which grows backward toward the cloaca. The ventral ends of the nephrocoels open into the coelom, and these openings, the nephrostomes, become lined with long cilia. The tubules meantime elongate and become contorted as they project into the surrounding posterior cardinal vein. Median to each nephrostome, the splanchnic mesoderm bulges out and in this projection develops a net of capillaries, or glomerulus, which becomes connected with the dorsal aorta. The pronephros is functional, at most, for a short time; and it disappears as the mesonephros develops to replace it.

The mesonephros. — The mesonephros, like the pronephros, is developed by all vertebrates. It becomes the adult kidney of the anamniotes, but is functional during the embryonic (and fetal) period only of the amniotes. Portions of the mesonephros THE MESONEPHROS 201

become associated with the genital organs of the adult (see next section).

The mesonephros also develops as a series of segmental nephrocoels, but in the nephrotomes posterior to those containing the

Primary tubule

Nephrostomal _

IP \nc_Nephrostome ECS

i A

Fig. 129. — Diagrams showing three stages in the development of the pronephric tubule. (After Felix.)

pronephric ducts with which they unite (Fig. 130). After the degeneration of the pronephros, the tube is known as the mesonephric or Wolffian duct. The ventral ends of the nephrocoels acquire coelomic nephrostomes in the anamniotes. In amniote development, nephrostomes are seldom formed. A glomerulus connected with the dorsal aorta and the cardinal veins arises in connection with each tubule, as in the pronephros. An important difference between the pronephros and the mesonephros lies in the fact that the number of nephric tubules in each nephrotome is greater in the mesonephros (Fig. 131). These arise by the constriction of the posterior median part of each nephrocoel into a small vesicle which gives rise to a secondary tubule; each of these tubules acquires a glomerulus and nephrostome at the 202 MESODERMAL DERIVATIVES

proximal end. The connection of these secondary tubules with the Wolffian duct, however, is attained by an evagination from the duct itself which grows out as the collecting duct to meet the developing secondary tubule. From these secondary tubules, tertiary ones bud off and develop in like manner, acquiring connections with the collecting duct through an evagination of this canal. As many as eight tubules may be formed in a single segment by this process of budding. This complexity is greatest at the posterior end of the mesonephros. In the amBowman’s capsule niotes, the mesonephros

Fig. 130. — Diagrams showing four stages in (except for that portion development of mesonephric tubule. (From associated with the genital Arey after Felix.) organs) disappears after

Mi esonephric duct

Anlage of mesonephric tubule!

the metanephros has been formed. v OT Aorta

The metanephros. “(WS NN — The metanephros, which is found as a separate kidney only


in adult amniotes, iry 3ry 2ry Iry 2ry duct. ry ty . . —_—— ened

probably is equiva- Collecting Nephrostomes Secretory tubules P tubules

lent to the posterior portion of the meso- F1¢. 181. — Diagram to show origin of secondary and

terti . : 7 eph ros of the anam- ( ar nary meson yPhne tubules from primary tubules

niotes, which it resembles greatly in its organogeny. THE METANEPHROS 203

The region in which the metanephros arises is, like that in which the earlier kidneys are found, the intermediate mesoderm. But in the posterior region of the body this mass is never segmented into separate nephrotomes. The first indication of metarephros formation is the appearance of an evagination from the dorsal surface of the mesonephric duct near the point at which the latter enters the cloaca. This evagination grows dorsally and

4 5 Fig. 182. — Diagrams to show origin and development of metanephric tubules. Collecting tubule in center, secretory tubules to right and left, the one on the right relatively more advanced. (From Arey after Huber.)

then turns forward to become the metanephric duct, or ureter, in much the same manner as the collecting ducts of the mesonephros arose. The metanephric duct then sends out into the nephrogenous tissue evaginations which increase in length and branch repeatedly to form the collecting tubules of the metanephros. Around the distal end of each tubule, a small mass of the nephrogenous tissue condenses and acquires a lumen like the nephrocoels of the pronephros and mesonephros (Fig. 132; 1, 2). From these vesicles the secretory nephric tubules arise by a process of elonga204 MESODERMAL DERIVATIVES

tion and later fuse with the collecting tubules just described (Fig. 132; 3,4). In each of the tubules a capsule develops for the reception of a glomerulus which later acquires a connection with a branch of the renal artery (Fig. 1382; 4, 5). Development proceeds from the posterior end toward the anterior, instead of in the opposite direction as in the earlier types of kidneys. The portion of the Wolffian duct nearest to the cloaca is absorbed by it so that the ureter has an opening into the cloaca separate from that of the mesonephric duct. From the region of the cloaca into which the ureters open is formed the urinary bladder and urethra (page 193). In mammals, at least, the enlarging bladder includes part of the ureter.

The later history of the kidneys and their ducts is considered in the next section.

THE FROG (SEE ALSO CHAPTER XI). — Threc pronephric tubules are formed (somites II, III, IV), each with a nephrostome. The region of the coelom into which these open is cut off ventrally by the development of the lungs and becomes the pronephric chamber. The glomeruli soon unite to form a glomus. Before metamorphosis the pronephric tubes, and that portion of the duct into which they open, degenerate.

Mesonephric tubules appear in the nephrotomes (somites VIIXII). These have nephrostomes in early larval life; but at the time the pronephroi degenerate the portion of each mesonephric tubule next to the nephrostome (peritoneal canal) breaks away from the remainder of the tubule and fuses with the posterior cardinal vein. The mesonephros is the functional kidney of the adult, and the Wolffian duct, therefore, functions as the ureter.

THE CHICK (SEE ALSO CHAPTER xi). — About twelve pronephric tubules arise (somites V-XVI) beginning on the second day of incubation. Nephrostomes are formed, but glomeruli do not appear until the third and fourth days of incubation, at which time the pronephros is degenerating. The pronephric duct arises at the ninth somite.

Mesonephric tubules arise from the intermediate mesoderm between somites XII and XXX, the more anterior of which develop nephrostomes. The main part of the mesonephros, however, arises between somites XX and XXX, where the continued growth of the tubules causes the projection of this region THE GENITAL ORGANS 205

into the coelom as the Wolffian body. It is extremely doubtful whether the mesonephros ever functions as a kidney, as it begins to degenerate on the eleventh day.

The metanephros arises on the fourth day of incubation, from two primordia as usual, the intermediate mesoderm in somites XXXI-XXXIII, and an evagination of the mesonephric duct, comparable to the collecting ducts of the mesonephric tubules, which becomes the ureter, pelvis, and collecting ducts.

MAN (SEE ALSO CHAPTER XIII). — Pronephric tubules arise in somites VII—XIII, develop nephrostomes and glomeruli, but degenerate rapidly.

Mesonephric tubules appear in the intermediate mesoderm between the sixth cervical and fourth lumbar segments, but those of the cervical and thoracic segments soon degenerate. Nephrostomes are formed by the more anterior tubules but have only a transitory existence. The mesonephros does not function as a kidney.

The metanephros has a double origin as in the chick.


The genital organs may be grouped into two classes: (1) the primary genital organs, or gonads, in which the germ cells develop; and (2) the accessory genital organs, whose original function is the discharge of the germ cells from the body.

The gonads consist of the germ cells and the subordinate tissues, blood vessels, nerves, connective tissue, ete., which make up a large part of these glands. In an earlier chapter it has been shown that the primordial germ cells may first appear in the endoderm of the gut wall and thence migrate by way of the splanchnic mesoderm, dorsal mesentery, and peritoneum to their definitive position in a thickening of the peritoneum on the mesial side of the nephrotomes. This thickening is called the genital ridge (Fig. 133B). A considerable body of evidence is accumulating to indicate that germ cells may also arise from the cells of the genital ridge itself.

The genital ridge is now invaded by mesenchymal cells, and projects into the coelomic cavity. In some amphibians, a metameric arrangement corresponding to the myotomes has been recorded, but following this the segments unite. The anterior 206 MESODERMAL DERIVATIVES

and posterior ends of the ridge degenerate, and the middle portion enlarges and is separated by a longitudinal groove from the mesonephros so that it hangs in the coelom suspended by a fold of the peritoneum, known as the mesorchium in the male or the mesovarium in the female. The germ cells have by this time become transformed into gonia (Chapter III) and the germ glands are known as gonads.

Within the gonads, the gonia come to lie in nests, close to the germinal epithelium. Tubular outgrowths from the nephric

Glomerulus Wolffian di

Fig. 133. — Diagrams to show early development of the gonads in transverse sections. A, testis. B, genital ridge. C, ovary. (After Corning.)

tubules of the mesonephros approach these nests. The later history of the gonads differs in the two sexes.

Testis. — In the male, the nests of spermatogonia become tubules which connect with the tubules growing in from the mesonephros (Fig. 133A). The testicular parts of these canals are known as the seminiferous tubules, the nephric portions as the efferent ductules. The walls of the seminiferous tubules are composed of spermatogonia and sustentacular cells which act as nurse cells to the developing sperm. Between the tubules lie partitions of mesenchyme which make up the stroma of the testis and contain the interstitial cells, which are supposed to be concerned in the formation of the male hormone. _ It is because of the presence of these cells that the testis is sometimes spoken of as the “ interstitial gland.” It is now well established that the testis secretes a “male”? hormone whose presence in the blood has much to do with the male secondary characters. Eventually, the tubules become separated from. the surrounding germinal OVARY 207

epithelium by the development of a mesenchymatous layer called the tunica albuginea.

Ovary. — In the female, the nests of odgonia become separate follicles (Fig. 133C) which never attain connection with the mesonephric tubules. These tubules consequently degenerate. A follicle consists of a single o6gonium surrounded by follicle cells which may be compared to the sustentacular cells of the male. In the mammalian ovary the primary follicle is greatly enlarged to form a vesicular (Graafian) follicle (Fig. 184), which protrudes

Tunica externa

Tunica interna

Stratum granulosum

Cumulus odphorus Ovum


Fig. 134. —Section of human vesicular (Graafian) follicle. (From Arey after Prentiss.)

from the surface of the ovary. The follicle cells multiply and secrete a follicular fluid which presses the outer wall (stratum granulosum) away from the egg and a layer of follicle cells immediately surrounding it. These form a projection (cumulus odphorus) into the cavity of the follicle. When ovulation takes place the wall of the follicle is ruptured, and the egg,. ‘still surrounded by its investment of follicle cells, now known as the corona r | radiata (page 41), is washed out with the_ follicular fluid. After ovulation the follicle cells enlarge, ‘multiply, and secrete a yellow “substance, the whole forming a corpus luteum. Hisaw has identified hormones from corpus luteum which produce 208 MESODERMAL DERIVATIVES

definite effects on the uterus and other parts of the female body associated with pregnancy and parturition. The existence of female hormones formed in the ovary is now definitely proved. These hormones appear to be formed in the follicles and to be quite distinct from the hormones derived from the corpus luteum (Hisaw). The tunica albuginea of the ovary develops much later than that of the testis but is also of mesenchymal origin.

The genital ducts. — The sperms formed in the seminiferous tubules of the testis are discharged into the mesonephric tubules and thence make their way into the mesonephric duct, which accordingly becomes the male genital duct. The ova, on the other hand, are discharged directly into the cavity of the coelom whence they are received into a new tube, the oviduct, by means of an opening, the ostium tubae (abdominale). The mesonephric duct is often called the Wolffian duct; the oviduct is frequently called the Miillerian duct. Both ducts appear in every embryo (Fig. 135A), but the later histories of the two differ according to the sex.

The Wolffian duct.— In the male (Fig. 135B), the efferent ductules toward the posterior end of the series become occluded, leaving only a few at the anterior end functional. These lose their renal corpuscles and shorten greatly. In the amniotes, where the metanephros acts as the functional kidney, this anterior group becomes the epididymis, while the more posterior, nonfunctional vestige becomes the paradidymis. The mesonephrie duct persists as the deferent duct. At the point where the deferent duct enters the cloaca, there develops a dilation, the seminal vesicle. In the female (Fig. 185C), the anterior portion of the mesonephros persists as the vestigial epodphoron, and the posterior portion becomes the paroédphoron. Traces of the Wolffian duct sometimes persist, as in mammals, where this structure is known as Gartner’s canal.

The Millerian duct. — This canal arises in the elasmobranchs by the constriction of the pronephric duct into two tubes, of which the ventral becomes the Miillerian duct, while the dorsal tube becomes the Wolffian duct. The opening of the Miillerian duct into the coelom, the ostium tubae abdominale, is a persistent nephrostome. In all other vertebrates, this duct arises independently of and after the formation of the Wolffian duct, a ESTROUS CYCLE 209

fact possibly correlated with the delayed functioning of the oviduct as compared with the primary renal function of the Wolffian duct. In these vertebrates the duct arises in the mesoderm lateral to the Wolffian duct and grows both forward and backward until the abdominal and cloacal openings are formed. It is not formed until late in embryogenesis. In the female (Fig. 135C), the posterior ends of the ducts are usually dilated as





deferens Seminal vesicle Utriculus prostaticus

Fia. 135. — Diagrams showing origin and early development of genital ducts. A, early stage showing mesonephros, gonads, (male on left, female on right) and ducts. B, later stage in male, showing in broken lines the structures which degenerate. C, later stage in female. (After Felix.)

storage chambers, and not infrequently fuse to form a uterus. In the male (Fig. 135B), the Miillerian duct degenerates, but vestiges are to be found even in the adult, such as the appendix testis and prostatic utricle of man, which represent the anterior and posterior ends of the female duct, respectively.

Estrous cycle. — Most vertebrates have an annual breeding season. Among the mammals, however, the fact that the young develop for a longer or shorter period (of gestation) in the uterus of the mother is associated with a periodical set of changes in the 210 MESODERMAL DERIVATIVES

activity of the uterus which are known as the estrous cycle. There are three main stages: proestrum, estrus, and anestrum.

During the proestrum the blood vessels of the uterine wall are congested, and in some animals (dog) there is destruction of the uterine wall accompanied by the discharge of blood into the cavity of the uterus.

In estrus the destructive changes of the proestrum are repaired while the cavity itself often contains the secretions of the uterine glands and the materials discharged in the preceding period (“ uterine milk’’). It is in this period that ovulation usually takes place and the wall of the uterus is in the condition most favorable for the implantation of the blastocyst. The estrus receives its name from the fact that this is the time in which the sexual drive is strongest. If implantation (page 140) and pregnancy do not take place, a condition known as pseudopregnancy occurs in some animals (rat, rabbit, etc.). In the closing stages of the estrus, the wall of the uterus returns to its normal condition, accompanied in some animals (dog) by slight hemorrhages. This period of repair is distinguished (Marshall) as the metestrum.

The estrus is succeeded by the anestrum, a name given to the interval lasting until the next proestrum commences. In many mammals estrus occurs but once during the breeding season, but in others it may take place more frequently. The period between each estrus and the next proestrum is sometimes known as a diestrum in these polyestrous species.

There is a considerable difference of opinion among the authorities as to the exact relation between ovulation and menstruation, a term applied to the periodic hemorrhages characteristic of the female primate. It is assumed that the period of ovulation corresponds to the estrus, but the clinical evidence is not clear as to whether the menstrual discharge is comparable to that of the proestrum or that of the closing stages of the estrus itself.

The external genitalia. — The genital organs so far considered are common to all vertebrates and are sometimes spoken of as the internal genitalia. External genitals are found only in those animals in which fertilization is internal. These organs serve the function of transmitting or receiving the sperm at the time of copulation. Internal fertilization is a phenomenon which has THE EXTERNAL GENITALIA 211

been observed in all classes of the vertebrates, but it is characteristic of all amniotes.

Although the external genitalia differ in the sexes, they are embryologically homologous. Two types are recognized, duplex and simplex. In the duplex type, characteristic of the sauropsids, sac-like extensions arise on each side of the cloaca, which in the male become the hemipenes or intromittent organ, while in the female they remain vestigial.

In the simplex type, characteristic of mammals, a single median ectodermal prominence arises anterior Genital tubercle to the cloacal aperture, to become Phallus the phallus (Fig. 136). In the male, the phallus enlarges and encloses the greater part of the urogenital sinus. In this way it becomes the penis, while the enclosed Fie. 136.— Diagram to show the sinus becomes the penile urethra, °"i#in_of the mammalian external genitalia. (After Ielix.) In the female mammal, the phallus becomes the vestigial clitoris, while the sides of the urogenital sinus remain open as the labia minora which guard the opening of the urogenital vestibule. At the base of the phallus is a swelling, the genital tubercle, from which labio-scrotal folds arise on either side of the urogenital opening. In the male they fuse to form the scrotum, an external sac into which the testes descend; in the female they remain separate as the labia majora.

TABLE 9 Homo.ocies oF THE MAMMALIAN GentTraL System


Male Indifferent Female Testis Gonad Ovary Epididymis Mesonephros Epoéphoron Paradidymis Paroéphoron Ductus deferens Mesonephric Gartner’s canal

(Wolffian) duct

Appendix testis Miillerian duct Uterus Prostatic utricle Vagina Penis Phallus Clitoris

Labia minora

Scrotum Labio-scrotal swellings | Labia majora 212 MESODERMAL DERIVATIVES

THE FROG (SEE ALSO CHAPTER XI). — The genital ridges arise soon after hatching. Sex can be distinguished at the time when the embryo is about 30 mm. in body length. The anterior portion of each genital ridge degenerates and becomes a fat body.

The Wolffian duct in the male acquires connection with the testis by means of some of the mesonephric tubules (vasa efferentia), and serves as the deferent duct as well as the ureter. A seminal vesicle is formed. A rudimentary Miillerian duct appears. In the female the Wolffian duct functions solely as a ureter while the Miillerian duct becomes the oviduct.

No external genitalia are developed.

THE CHICK (SEE ALSO CHAPTER XII).— The genital ridge arises with the mesonephros as the urogenital ridge. Of this the anterior region gives rise to the gonad on the mesial side. Sex is not distinguishable until the seventh day of incubation. In the female, the right ovary develops only partially and finally disappears.

The Wolffian duct becomes the deferent duct, connected with the testis by vasa efferentia forming the epididymis. The persisting mesonephric tubules of the posterior region of the mesonephros form a paradidymis. In the female a vestigial epodphoron and parodphoron represent these bodies respectively. The Miillerian ducts degenerate in the male without ever acquiring a cloacal exit. In the female the right Miillerian duct disappears while the left becomes the oviduct. The shell gland appears on the twelfth day of incubation, but the cloacal opening is not formed until the hen is six months old.

No external genitalia are formed, although hemipenes are formed in some other birds.

MAN (SEE ALSO CHAPTER XIII).— The genital ridge arises on the mesial side of the mesonephros. Sex is not distinguishable until after the fifth week.

Each Wolffian duct functions as a deferent duct, and both epididymus and paradidymis are formed, as is a seminal vesicle at the distal end. In the female, epodphoron and paroédphoron are formed, while some portion of the duct itself may persist as Gartner’s canal. The Miillerian ducts become the uterine tubes, which unite at their posterior ends to form the uterus and vagina. The latter is partially closed by a semicircular THE ADRENAL ORGANS 213

fold, the hymen, where it enters the urogenital sinus. In the male, vestiges of the anterior end of each Miillerian duct persist as the appendix testis, while the posterior end is represented by the rudimentary prostatic utricle. The dilation of the bladder results in the inclusion of the ureters (metanephric ducts) in its walls. The genital ducts (Wolffian or Miillerian ducts) empty into the urogenital sinus posterior to the bladder, in a region which constricts to form the urethra. About this develop a number of outgrowths which acquire cavities and form the prostate gland in the male, and the para-urethral glands of the female. The external genitalia are of the mammalian type.


Closely associated with the nephric organs are the mesodermal interrenal glands, which frequently become associated with the suprarenal glands, of ectodermal origin, to form the so-called


Sympathetic @~


Suprarenal @. 2D QO} \ Inter - ——ep renal Genital ridge A

Fig. 137. — Diagrams to show the origin of the suprarenal and interrenal components of the adrenal gland. A, origin as shown in cross section (after Corning). B, condition in amphibia. C, in birds. D, in Tammals. .






adrenal glands. All are endocrine (or ductless) glands. The suprarenal portion of the adrenal forms the powerful hormone epinephrin (adrenalin); the interrenal portion secretes a hormone known as cortin (Swingle), which is employed in the treatment of Addison’s disease. 214 MESODERMAL DERIVATIVES

The interrenals. — These arise as paired thickenings of the splanchnic mesoderm mesial to the nephrocoels. In some of the amphibians there are traces of a segmentation which is soon lost by fusion. There is no direct connection between the interrenal and the mesonephros. These glands may fuse to form an elongate median organ or become associated with the suprarenals.

The suprarenals. — Although these glands are found in the vicinity of the mesonephros, they originate from the sympathetic ganglia (ectodermal) as described in the following chapter. They are separate structures in the fish, but unite with the interrenals in the tetrapods.

The adrenals (Fig. 137). — These compound glands are not found in the fish. In the amphibians the suprarenal portion of the gland is external to the interrenal portion. In the chick they are intermingled. In the amniotes, however, the interrenal substance (cortex) surrounds the suprarenal (medulla).


The vascular system is mesenchymatous in origin. It consists of separate cells, the blood corpuscles, floating in a fluid matrix,

Blood island Ectoderm Somatic mesoderm Splanchnic mesoderm Blood vessel Blood cells

Enloderm fused to yolh Fia. “138. — Diagrams showing three stages in the development of seplary from blood island based on transverse sections of the area vasculosa in a seven somite chick. (From Arey.)

the blood plasma, in a closed system of interconnected tubes, the blood vessels. Some vessels become lined eternally with muscle fibers, and in one locality this muscular development gives rise THE BLOOD CORPUSCLES 215 to a pulsating heart by means of which the blood is kept in circulation.

Origin of the blood-vascular system. — The first indications of the vascular system are found in the splanchnopleure as blood islands (Fig. 138). In the telolecithal vertebrates this is always in the extra-embryonic splanchnopleure. These blood islands originate as local aggregates of mesenchyme. Later, the inner cells separate as corpuscles, while the outer ones form the endothelial lining of a vesicle. These vesicles anastomose with each other to form the extra-embryonic vitelline circulation.

The blood corpuscles. — The first corpuscles formed are the inner cells of the blood islands. Later corpuscles are budded off

. (" 2 @ “SoC

°e@ ef

Fie. 139. — Stages in the development of human red blood corpuscles. A, hemoblasts. B, megaloblasts (anamniote type). C, D, normoblasts (sauropsid type). E, normoblasts in process of becoming F, erythrocytes. (From Arey after Prentiss.)

from the walls of the capillaries into their cavities. Mesenchymal cells in regions where the capillary network is forming may develop into blood corpuscles and enter the blood stream. These first corpuscles are the hemoblasts (Fig. 139).

Hemoblasts become differentiated into the different types of blood corpuscles in the following blood-forming centers: (1) the yolk sac; (2) the embryonic capillaries; (3) the liver, the spleen, and the lymph glands; (4) the bone marrow. In the adult the lymph glands give rise to lymphocytes, and the bone marrow to all types of corpuscles.

The erythrocytes, or red corpuscles, are distinguished by the presence of hemoglobin which gives them their color. In the 216 MESODERMAL DERIVATIVES

anamniotes the erythrocytes have a large vesicular nucleus with granular chromatin and a distinct cell membrane. In the sauropsida, the erythrocytes have a small compact nucleus. The mammalian erythrocyte is distinguished by the absence of the nucleus in the adult. In the development of mammals there is a succession of erythrocytes: first the anamniote type; then the sauropsid type; and finally the mammalian erythrocyte, which is produced by the extrusion of the nuclei from the blood cells of the sauropsid type (Fig. 139).

The leucocytes, or white corpuscles, are of many types, for a discussion of which the reader is referred to the textbooks on histology. The preponderance of evidence indicates that these, like the erythrocytes, are derived from the hemoblasts.

Origin of the intra-embryonic vessels. — The first embryonic blood vessels (Fig. 140) are the vitelline veins which appear at the ventro-lateral margins of the fore-gut. These vessels unite in the region of the anterior intestinal portal to form the heart, then separate as the ventral aortae, which bend up around the pharynx in the mandibular arch as the first aortic arches, and continue backward as the dorsal aortae. These fuse at a very early stage as the dorsal aorta, from which branches are sent to each myotome and to the vitelline circulation. The posterior ends of the vitelline veins fuse in small-yolked forms, such as the frog, to form a subintestinal vein which continues back to the tail. In largeyolked forms like the chick, the vitelline veins are widely separated and brought into connection only by the sinus terminalis which makes a circuit of the area vasculosa. The vitelline veins are the ventral venous channels of the splanchnic circulation. <A dorsal set of vessels soon originates independently to form the somatic venous circulation. The first of these to appear are the anterior cardinal (precardinal) veins of the head. A similar pair, the posterior cardinal (postcardinal) veins, arise in connection with the nephric region. These, however, do not discharge their contents directly into the heart but into the anterior cardinals. The portions of the original anterior cardinals proximal to this juncture with the posterior cardinals are now called the common cardinal veins.

The heart. — Although the heart is primitively a paired organ, we have seen that the two primordia are soon fused into a single 217


Dorsal aorta

Aortic arch Ventral aorta

Vitelline Vitelline vein A area vasculosa

Caudal artery

osterior cardinal vein

Aortic arches Anterior cardinal vein

Internal Common cardinal vein

carotid artery

Ventral aorta

External Vitelline carotid vein artery

Internal carotid External carotid

I y_S c—~\ — (EAI ESXJ ¢ IAN

CS Anterior cardina) Dorsal aorta Common cardinal . | ==— Vitelline vein 4 Posterior cardinal Vitelline artery —= _ C

Fig. 140. — Diagrams to show fundamental plan of embryonic circulation. A, early stage in side view. _B, later stage in side view. C, same from above, aortic

Ventral aorta

roots pulled apart. 218 MESODERMAL DERIVATIVES

median tube connected with the ventral aortae in front, and the vitelline veins (and later the common cardinals) behind. Around the endocardial lining there develops a coat of muscle fiber which later becomes striated to form the myocardium. Outside this is a lining of splanchnic mesoderm which forms the epicardium, continuous with the lining of a part of the coelom surrounding the heart, which will later be cut off by the septum transversum to form the pericardium. In this the heart is suspended by a dorsal and a veéritral mesentery known respectively as the dorsal and ventral mesocardia.

The later history of the heart is one of growth and subdivision into special chambers. Because the local growth of the heart is limited by the anterior and posterior walls of the pericardium


Fie. 141. — Diagrams to show early stages in development of vertebrate heart. A, paired heart tubes. B, same fused. C, primary flexure. D, later ‘‘S”’ stage. E, after antero-dorsal displacement of atrium.

and by the mesocardia in which it is suspended, any extension in length must be accompanied by coiling. The primary flexure of the heart is toward the right, thus changing the shape of the organ from a straight tube to a C-shaped one. Further growth results in the twisting of the heart into the shape of an S. Still later, the original posterior loop of the S is pushed forward and dorsad so that it comes to lie above the morphologically anterior end (Fig. 141).

The original chambers of the heart are produced by local dilations, of which the most posterior is the sinus venosus; next to this is the atrium; in front of this, the ventricle; and finally, the bulbus arteriosus. The sinus is the chamber into which the primitive veins enter; the atrium is a thin-walled distensile THE ARTERIES 219

chamber; the ventricle is a thick-walled, muscular, pulsating pump; and the bulbus is the chamber from which the blood efitéts the primitive arteries.

These chambers undergo different changes in the various types of vertebrates. Of these, the most important is a progressive differentiation, completed in the mammals and birds, of the atrium and ventricle into separate right and left halves, of which the right side receives venous blood from all parts of the body and transmits it to the lungs for respiratory exchange. From the lungs the blood is returned to the left side of the heart and thence conveyed to all parts of the body.

The arteries (Fig. 142). — The ventral aortae fuse into a single median tube sending branches into each of the visceral arches.

Anterior mesenteric

Fig. 142. — Diagrams to show principal arteries; A, in side view, B, cross section through mesenteric.

These branches, which unite with the dorsal aortae, are usually six in number and are known as the aortic arches. Anterior to these the ventral aortae continue forward as the external carotid arteries. Similar forward extensions of the dorsal aortae are known as the internal carotid arteries. In the region of the aortic arches the dorsal arteries remain separate as the aortic roots (radices aortae). Behind them, as has been mentioned, the paired vessels fuse as the median dorsal aorta. 220 MESODERMAL DERIVATIVES

The aortic arches. — In larvae breathing by means of external gills, a loop from each aortic arch grows out into the gill developing on the visceral arch with which it is associated. These loops are short-circuited when the external gills disappear.

In forms with internal gills, each aortic arch breaks up into capillaries in the demibranch and becomes divided into a ventral afferent branchial artery and a dorsal efferent branchial artery.

In vertebrates with a pulmonary respiration, aortic arches I and II, in the mandibular and hyoid arches, respectively, disappear. Arch III, in the first branchial arch, persists as the connection between the internal and external carotid arteries,

Internal External carotid carotid

I clgy TT Il


Vv Subclavian VI

Pulmonary arteries


B Cc D

Fig. 143. — Diagrams of aortic arches. A, hypothetical primitive type. B, in frog. C, in chick. D, in man. (After Kingsley.)

while the dorsal aorta between arches III and IV disappears. Arch IV becomes the systemic arch connecting the dorsal and ventral aortae (Fig. 143B). In birds (Fig. 143C), the arch on the left side disappears; in mammals (Fig. 143D), that on the right degenerates. Arch V is greatly reduced and frequently disappears or has at most a vestigial and transitory existence. From arch VI there grow back to the lungs the pulmonary arteries. The portion of the sixth arch distal to the pulmonary arteries is reduced in caliber and is known as the ductus arteriosus. It becomes occluded and degenerates in all the amniotes except some few reptiles.

Intersegmental arteries. — From the dorsal aortae are given off small branches between the myotomes (Fig. 142B). Some of these intersegmental arteries persist as the intervertebral arteries. The more anterior ones becomes united on either side by THE VITELLINE VEINS 221

a dorsal longitudinal vertebral artery. These vertebrals subsequently fuse to form an anterior basilar artery which divides behind the pituitary, the two halves uniting with the internal carotid on either side. The posterior halves of the vertebral arteries fuse to form the spinal artery which runs back beneath the spinal cord. In the region where the anterior limb buds are developing, intersegmental arteries grow out, to give rise to the subclavian arteries. Similarly, in the region of the pelvic limb buds, intersegmental arteries give rise to the iliac arteries. In the amniota, the allantoic arteries grow out from the iliac arteries into the walls of the allantois. These become so important that for some time it appears as though the iliac arteries were derived from the allantois instead of the reverse. These allantoic arteries, which degenerate at the time of birth, are known as the umbilical arteries in mammals as they traverse the umbilical cord and supply the placenta.

Other important intersegmental arteries become the renal arteries of the kidneys and the genital arteries of the gonads.

Mesenteric arteries. — From the dorsal aorta, a number of ventral branches, originally paired, but soon fused to become median vessels, pass down the dorsal mesentery. They unite with the capillaries of the yolk sac which they supply with blood. Later, some of them develop branches over the alimentary canal which persist after the loss of the yolk sac as the coeliac and mesenteric artcries.

The veins. — There are two primitive venous systems: the somatic system, comprising the cardinal veins; and the splanchnic, including the vitelline (omphalomesenteric) and, in amniotes, the allantoic (umbilical) veins. The cardinal veins are replaced by caval veins; the vitelline veins become transformed into a hepatic-portal system. The allantoics disappear at hatching (or birth). Finally, there are the pulmonary veins. In general, the history of these transformations may be summed up in the statement that the primitive independent venous systems become transformed into a system wherein an accompanying vein is developed for every artery.

The vitelline veins (Fig. 144). — These vessels, and their continuation, the subintestinal vein (in small-yolked forms), are the first vessels formed in the embryo. In the amniotes, two veins 222 MESODERMAL DERIVATIVES

grow out from these into the wall of the allantois to become the allantoic veins of the sauropsida (umbilicals of mammals). In man, however, the umbilical veins actually appear before the vitelline veins.

It has been noted previously that the vitelline veins pass around the liver on their way to the heart. As the liver enlarges, it surrounds the vitelline veins, and these become broken up in the liver tissue to form a great capillary network. In the amniota, the allantoic (umbilical) veins are similarly absorbed. The proximal portions of the vitelline veins, from the liver to the sinus venosus, are now known as the hepatic veins; the distal portions

64 Common ardinal p

Ductus { venosus ‘

Fig. 144. — Diagrams to show three stages in the development of the hepatic-portal venous system, based on conditions in man. (After Hochstetter.)

are called the portal veins. Of the umbilical veins, the right degenerates; the left for a time maintains a direct connection through the liver to the hepatic veins, known as the ductus venosus. This connection disappears at the time of birth. After the disappearance of the yolk, the portal vein and its tributaries, of which the most important is the mesenteric vein, carry blood from the digestive canal to the liver.

The anterior cardinal veins. — The original plan of the cardinal system is that of an H in which the upper limbs represent the anterior cardinals; the cross-bar the common cardinals, with the heart in the middle of the cross-bar; and the lower limbs represent the posterior cardinals (Fig. 145). The anterior cardinals arise as a drainage system for the blood passing into the head from the carotid arteries.

The anterior cardinals are often called the internal jugular THE POSTERIOR CARDINALS 223

veins. From these, parallel veins, known as the external jugular veins, branch off in the ventral region of the head. Veins from the vertebral region (vertebral veins) and from the pectoral appendages (subclavian veins) soon develop. In most vertebrates the common cardinals and the proximal portion of the anterior cardinals, i.e., up to the point where these tributary veins diverge, persist as the precaval veins. In some mammals, a crossconnection is formed between the anterior cardinals, after which the portion of the left anterior cardinal, proximal to the anasto

Anterior. cardinal Anterior


Pre cava ot} Coronary

sinus Common cardinal Post

Sub - cava


Post cardinal]



Fia. 145. — Diagrams to show three stages in the development of the caval venous system. Generalized (supra-cardinals omitted).

mosis, and the left common cardinal become the coronary vein draining the wall of the heart. The corresponding vessels on the right side persist as the precaval (anterior caval) vein.

The posterior cardinals. — Each posterior cardinal lies dorsal to the mesonephros which it drains. Beneath each mesonephros is developed a subcardinal vein. In the anamniotes these veins arise as tributaries of the posterior cardinals, returning blood from the tail where they are united to form the caudal vein. Later, they lose direct connection with the parent vessels and return blood from the tail region to the mesonephros as the renal-portal veins. The posterior portions of the subcardinals fuse as the interrenal vein, which acquires a secondary connection with the 224 MESODERMAL DERIVATIVES

hepatic vein, and persists as the postcaval vein. In the amniotes the postcaval vein is a complex which arises partly from the hepatic veins, partly from appropriated portions of the posterior cardinals and subcardinals, and partly from the supracardinals, a pair of vessels dorso-mesial to the posterior cardinals. It eventually replaces the posterior cardinals, so that the only blood vessels entering the right side of the heart are (1) the precaval vein returning blood from the head, pectoral region, and appendages; and (2) the postcaval vein returning blood from the trunk and pelvic appendages as well as all blood from the digestive canal conveyed by way of the hepatic-portal system.

The pulmonary veins. — These enter the left atrium and are new vessels which grow backward from the heart to the developing lungs.

The lymphatic system. — This system serves to return to the veins the blood plasma which has escaped from the capillaries (Fig. 146). It contains white blood corpuscles of the ameboid type (lymphocytes) which have the power of making their way through the capillary walls. The lymphatics apparently originate as intercellular spaces in mesenchyme which later become confluent and acquire a limiting endothelium. Like the blood vessels, the lymphatic capillaries anastomose and form larger vessels which drain into the veins. The walls of these central vessels are often muscular, and localized areas known as lymph hearts are found. So, too, localized distensible sacs, the lymph sacs, are notunknown. Some of these become lymph glands. The spleen, already alluded to in the section on mesenteries (page 198), is a hemolymph gland in which both lymphocytes and erythrocytes are proliferated.

THE FROG (SEE ALSO CHAPTER XI). — In the frog (Fig. 147), the primordia of the vitelline veins first appear and grow together as a loose aggregate of cells in front of the liver. Around this the coelom grows in from right and left to form the pericardium. Meantime the primordium of the heart endocardium develops from the loose aggregate of cells referred to above. The inner wall of the coelom (splanchnic mesoderm) becomes the myocardium. The atrium is divided by an interatrial septum into two auricles, right and left. The ventricle remains a single chamber. THE FROG, : 225

Superficial lym phatics

Jugular lymph sac

Subclavian lymph sac oly as ao] Lymph gland

yi) Deep lymphatics 4 /// Thoracic duct ‘ i Retroperitoneal lymph sac YA Cisterna chyli

Posterior lymph sac Superficial lymphatics Lymph gland

Fig. 146. — Reconstruction of primitive lymphatic vessels in human fetus of two months. (From Arey after Sabin.)

Arteries External Aortic

Ventral carotid __ arches

fiver 2orta Dorsal Internal; | WIWIVV VI - aorta

carotid ; ;_ Vitelline + Caudal

‘ Caudal

Heart :,. ’

Anterior > ey : cardia Posterior. *Vitelline Veins cardinal cardinal"


Fie. 147. — Diagram of embryonic vascular system of early tadpole. (After Kingsley.) 226 MESODERMAL DERIVATIVES

Aortic loops develop in the external gills, corresponding to aortic arches III, IV, and V. After the appearance of the internal gills, the ventral limb of the loop becomes the afferent branchial artery, while the dorsal limb becomes the efferent branchial artery. A similar differentiation takes place in arch VI. With the loss of branchial respiration, arch III becomes the proximal portion of the carotid arteries, arch IV the systemic arch which persists on both sides, and arch V disappears, while from arch VI arise vessels which carry blood to both the lungs (pulmonary arteries, Fig. 143B) and skin (cutaneous arteries).

The vitelline veins anterior to the liver fuse to become the hepatic vein: posterior to the liver, the right vitelline vein disappears, the left becomes the hepatic-portal vein. The anterior cardinal veins become the internal jugular veins; the common cardinals become the precaval veins. The posterior cardinal veins fuse between the mesonephroi, and a new vein grows back from the hepatic vein to the right posterior cardinal, to form the postcaval vein. The posterior cardinals, anterior to their junction with the postcaval, degenerate. Posterior to this junction they persist as the renal-portal veins carrying blood from the iliac veins to the kidneys.

THE CHICK (SEE ALSO CHAPTER XII). — In the chick (Fig. 148), the endocardium of the heart arises as the forward extension of the vitelline veins, which soon fuse as the pericardial | primordia are brought together beneath the head. The myocardium is formed as in the-frog. The right and left halves of the heart are completely separated by three septa: the septum aorticopulmonale, which divides the bulbus into a chamber on the right ‘leading to the pulmonary arteries and one to the left leading to the dorsal aorta; the interventricular septum, which divides the ventricle; and the interatrial septum, which divides the atrium into two auricles. This separation is completed at the end of the first week of incubation. The sinus venosus is incorporated in the right auricle.

Six aortic arches are formed: I and II disappear on the third and fourth days of incubation; IIT forms the proximal portion of the internal carotid artery; IV disappears on the left side but persists as the systemic arch on the right; V disappears; the pulmonary arteries arise from VI, but the distal portion of the MAN 227

right arch remains as the ductus arteriosus until the chick hatches (Fig. 143C).

The vitelline veins unite behind the sinus venosus to form the meatus venosus which later becomes the hepatic vein. The mesenteric vein becomes the portal vein, and the vitelline veins disappear at hatching. The allantoic veins grow backward from the common cardinals to join the capillaries of the allantois; the right allantoic degenerates on the fourth day, and the left acquires a new connection with the meatus venosus, by way of the


$432 Aortic _ _ g 2

EZES arches Eg gs s 3 a <Timmivvvr § 2 a

arene EERIUNNVT SS 88 5 2 3



Common ‘ Allantoie oornne


Posterior ardinal “PON ON Vitelline | “NS

Fie. 148. — Diagram of embryonic vascular system of chick. (After Kingsley.)

left hepatic vein. The allantoic vein degenerates at hatching. Two precaval veins are formed from the proximal portions of the anterior cardinals and common cardinals. The posterior caval vein arises from (1) a branch of the meatus venosus which grows back to meet the right subcardinal vein, (2) the fused subcardinals which carry blood from the mesonephros, and (38) the renal veins which develop in connection with the metanephros. The anterior ends of the posterior cardinals disappear, while the posterior ends supply the mesonephros and, after its degeneration, the common iliac veins, which pass directly to the postcaval vein.

MAN (SEE ALSO CHAPTER XIII). The heart arises in man (Fig. 149) much as in the chick; but the subsequent partition228 MESODERMAL DERIVATIVES

ing of this organ into right and left halves is more complicated, for two atrial septa are formed. The ventricle is separated by an interventricular septum, and the bulbus is divided by two septa which unite to form the septum aortico-pulmonale. The sinus venosus is incorporated in the right atrium.

The aortic arches are formed and have the same history as those of the chick, with the exception that it is the left fourth aortic arch which becomes the systemic arch (Fig. 143D).

The anterior portion of the right vitelline vein becomes the hepatic vein; the hepatic-portal arises from the posterior portion of the vitelline veins anterior to their junction with the mesenteric

Postcardinal veins Precardinal veins Descending aorte

Sinus venosus Vitelline veins Fia. 149. — Diagram of embryonic vascular system in man: (From Arey after Felix.)

vein. The anterior cardinals are united by an anastomosis (left innominate vein), and the left common cardinal disappears with the exception of the coronary vein. The right common cardinal, together with that portion of the anterior cardinal as far as the branching of the left innominate, becomes the precaval vein. The postcaval vein is a complex formed from (1) a branch of the hepatic vein, (2) the anterior portion of the fused subcardinals, (3) part of the fused supracardinals, and (4) the fused posterior portion of the posterior cardinals. The anterior portions of the posterior cardinals separate from these veins, unite by means of an anastomosis, and drain into the right precaval vein. They are then known as the azygos (right) and hemiazygos (left) veins. Of the umbilical veins, the left only persists, with a SKELETOGENOUS REGIONS 229

direct connection through the liver by means of the ductus venosus. At birth this duct closes and the umbilical vein dis appears. F. THE SKELETON

The skeleton of vertebrates consists of a system of supporting and protecting elements developed from mesenchyme. These elements pass through several conditions in later development. The primordia of the skeletal elements are preformed in connective tissue. These become transformed into cartilage, a process known as chondrification, through the activities of specialized cells, the chondrioblasts. Cartilage in turn is transformed into bone, through the action of osteoblasts, the process being known as ossification. Bones that pass through these three stages are known as cartilage bones. In the formation of some bones, the cartilaginous stage is omitted; these are known as membrane bones.| Both cartilage and bone are typically surrounded by a membrane of mesenchyme which is called the perichondrium or periosteum, as the case may be. The separate elements of the skeleton are connected with each other by ligaments, by cartilage, or in a bony union.

Transverse septum

Sagittal septum

Fia. 150. — Diagram to show the skeleton-forming regions as seen in the tail region of a vertebrate. (After Kingsley.)

Skeletogenous regions. — The principal regions where skeleton may be formed in the vertebrate body (Fig. 150) are (1) the 230 MESODERMAL DERIVATIVES

dermis of the skin, (2) the median sagittal planes between the myotomes on the dorsal and ventral sides of the body, (3) the right and left frontal planes between the dorsal and ventral muscle masses, (4) the transverse planes between the myotomes, (5) around the notochord, neural tube, and axial blood vessels, (6) in the visceral arches, and (7) in the paired appendages. Skeletal elements formed in (1) are called the dermal skeleton; those formed in (2) to (5), the axial skeleton; those formed in (6), the visceral skeleton; and those formed in (7), the appendicular skeleton. The skull contains elements from all but the appendicular skeleton.

The dermal skeleton. — Among living vertebrates the most primitive example of derm bones are the placoid scales (Tig. 151) of the eartilage fish which are formed in exactly the same way as teeth (Chapter VIIT). In the dermal skeleton two types of bones are distinguished. The investing bones (dermal plates) serve to envelop regions of the head

scale (Squalus acanthias) to show originof and trunk. The substituting

primitive dermal bone. Compare Tig. bones become so closely allied

119. (After Kingsley.) . . .

with the cartilaginous bones as to become fused with them or even to replace them in ontogeny. Many of the cranial bones are of this type. They may be distinguished by the fact that they

x * : Seles ee ORS IS SS Ectoderm do not pass through a cartilagi- SaaS EE Dermatomé


nous stage in development. The axial skeleton. — The primitive axial skeleton is the notochord, whose origin has been discussed in Chapter V. 1 NTA Around this a connective tissue Fig. 152. — Section through sclerotome heath is f db h of lizard (Scleporus) to show arcualia. snea 1s forme y mesenchy- (After Kingsley.) mal cells. The mesenchyme from each sclerotome now forms four little blocks, the arcualia (Fig. 152), two dorsal to the notochord and two ventral, from which the arches and centra of the vertebrae are formed, as well THE STERNUM 231

as the primordia of the ribs. The posterior arcualia of each somite unite with the anterior arcualia of the succeeding myotome to form the definitive vertebra, which thus comes to lie at the point of separation between two myotomes. Eight elements are thus concerned with a single vertebra: right and left dorsal arcualia from the anterior half sclerotome, and from the posterior half sclerotome, and the corresponding ventral elements.

The vertebrae. — In the prevertebral masses so formed appear centers of chondrification, one on each side of the spinal cord and one or more below the cord. These form, respectively, the neural arch and the centrum of the vertebrae (Fig. 153). In the tail region, two centers of chondrification arise below the centrum,


Fia. 153. — Section to show ossification centers in human vertebra and ribs. (After JXollman.)

enclosing the caudal prolongation of the dorsal aorta, and form a hemal arch. With the chondrification of the vertebrae the notochord disappears in all but the most primitive vertebrates, persisting only between the vertebrae as nuclei pulposi of the intervertebral discs. Finally the vertebrae become ossified, and the spines, zygapophyses, and other differentiations are developed.

The ribs. — Except in the caudal region, lateral processes arise from the vertebral primordia and grow out into the myosepta. They later become cartilaginous, and finally true bone. These are the ribs, of which there are two types, dorsal and ventral, distinguished according to the part of the vertebra from which they originate.

The sternum. — The sternum, or breast bone, arises in the amphibians from the coalescence of two longitudinal bars of cartilage, which later articulate with the coracoids of the pectoral girdle, but do not come in contact with the ribs. In the amniota, 232 MESODERMAL DERIVATIVES

the sternum arises from the fusion of the ventral ends of the anterior rib rudiments. In this way there arise two longitudinal bars, from which the unpaired sternum , <—_Chviele is formed by fusion along the mesial line (Fig. 154). Ends The skull. — The skull is a complex or of skeletal elements, arising from the chondrocranium, or primitive cranium of cartilage bones, which is derived in part from the protective covering of the brain and sense organs (neurocranium), and in part from the supporting elements of the visceral arches Fia. 154. — Diagram to show ori- (gnlanchnocranium). This is supplegin of mammalian sternum. . . (After Kingsley.) mented by numerous investing and substituting bones from the original dermal skeleton (dermocranium).

Neurocranium.— The neurocranium arises from the head mesenchyme which, as has been said, cannot be traced to any definite somites. In this mass, which completely invests the brain and sense organs, definite centers of chondrification appear. These masses unite to form the chondrocranium of the cartilage fish (Fig. 155). If the notochord be used as a point of orientation,




Otic capsule a 4. Occipital


Nasal capsule

Visceral arches

Fig. 155. — Diagram showing components of chondrocranium (Squalus acanthias). (After Kingsley.)

on either side of it is found a parachordal bar. In front of each of these is a separate rod; these are the trabeculae. Between the two parachordals and around the notochord, the basilar plate arises as the support of the epichordal brain. The trabeculae also fuse in front, to form the ethmoid plate which supports OSSIFICATION OF THE CHONDROCRANIUM 233

the prechordal brain, but remain separate at their posterior ends to form an opening through which the pituitary projects downward. In front of the ethmoid plate the trabeculae grow forward as the cornua. Dorsal to each trabecula, another longitudinal bar, the sphenolateral, arises. Between these two bars the cranial nerves make their way to the exterior.

Around each of the major sense organs a cartilaginous capsule develops. The olfactory capsules unite with the cornua, ethmoid, and sphenolaterals. The optic capsule rarely develops fully, usually persisting in the conncctive tissue stage as the sclera of the eyeball. The otic capsule, however, becomes completely chondrified and unites with the parachordals and the latero-sphenoids. Between the two otic capsules and sphenolaterals arises a dorsal plate which forms a roof for the brain. In the amniotes, one or more neck vertebrae are consolidated with the occipital region.

The splanchnocranium.— The digestive canal in the head region consists of the mouth, oral cavity, and pharynx, the walls of the pharynx being penetrated by the visceral clefts. As there is no coelom in this region, the lateral mesoderm is not divided but gives rise to mesenchyme which foreshadows the cartilaginous bars supporting the wall of this part of the body. These visceral arches are the mandibular, hyoid, and four (or more) branchial arches. The mandibular arch divides into dorsal and ventral portions, of which the dorsal portion becomes the pterygoquadrate cartilage (upper jaw of cartilage fish) while the ventral portion becomes the meckelian cartilage (lower jaw). The hyoid arch divides into a dorsal hyomandibular cartilage, and a ventral hyoid cartilage which is usually divided into several centers of chondrification. The hyomandibular acts as a suspensory element for the jaws in the fish. It is homologized with a bone of the middle ear, the columella, in amphibians, and the stapes of mammals (see page 269). The hyoid gives rise to the support of the tongue. The branchial arches are usually divided into four parts and act as gill supports in the anamniota and disappear or become laryngeal cartilages in amniota.

Ossification of the chondrocranium. — The limits of this text will not permit of an enumeration of all the bones formed from the chondrocranium (Figs. 156, 157, 158). They may be grouped, 234 MESODERMAL DERIVATIVES

however, as follows: (1) the occipitals, formed from the occipital vertebrae; (2) the sphenoids, arising from the parachordals, basilar plate, trabeculae, and latero-sphenoids; (3) the ethmoids,


Vomer Maxilla

Ethmoid Palatine Parasphenoid Orbito sphenoid Pterygoid Jugal Alisphenoid Squamosal


Quadrate + Prootic ~ A 5 “ Opisthotic

Supratemporal oo Basioccipital

Fia. 156. —— Diagram showing components of vertebrate skull, generalized. Ventral view. Chondrocranium stippled, dermal elements in outline. (After Kingsley.)

from the ethmoid plate and nasal capsule; (4) the otics, from the otic capsule. The pterygoquadrate bar gives rise to the pterygoid bones and the quadrate (which in mammals becomes the incus of the middle ear). The mcckelian cartilage gives rise to the





Fig. 157. — Dorsal view of skull diagrammed in Fig. 156.

articular bone at its distal extremity. This becomes the malleus, another ear-bone, of the mammals. The remainder of the meckelian persists as cartilage. In the hyoids and the branchials, bones are formed which retain the names of their cartilaginous predecessors. THE GIRDLES 235

The dermocranium (Figs. 156, 157, 158). — The derm bones which invest and, to some extent, supplant the elements of the

chondrocranium are too numerous to be more than mentioned Premaxilla here. The dorsal derm bones

are, from front to rear, the nasals, Maxilla

frontals, and parietals, together

with a number of smaller bones Jugal

which appear in variable quantity in the different classes. The

Postorbital —}

Nasal Lachrymal




principal lateral elements, from Squamosal front to rear, are the premaxillae, #4708"!


Tibia Ulna


Tarsals 09520 Carpals ‘0

oO oO Metatarsals Ui it Metacarpals

f ¥V Phalanges WY 0

Fig. 159. — Diagram of appendicular skeleton, tetrapod type, showing homologies of pectoral elements above and to left; pelvic elements below and _ to right. (After Kingsley.)

Parietal Supratemporal Interoccipital


Via. 158. — Lateral view of skull diagrammed in Figs. 156, 157.

maxillae, jugals, quadratojugals, and squamosals. The floor of the chondrocranium is invested by the parasphenoids, palatines, and vomer. The lower jaw is invested by a series of bones of which the most important is the dentary.

The appendicular skeleton. — The simplest forms of appendages, the unpaired and paired fins of fish, contain a skeleton consisting of parallel cartilaginous rods, which are divided into proximal portions, basalia, embedded in the body, the distal portions, radialia, extending into the free appendages. The paired appendages of fish are paddle-like fins; in tetrapods they are jointed legs. In both, the skeleton is divided into a basal girdle and a free appendicular skeleton (Fig. 159).

The girdles. — The girdles are in the form of inverted arches, of which the pectoral girdle is united to the axial skeleton in fish and free in the tetrapods, while the pelvic girdle, usually free in

fish, is united to the axial skeleton in the tetrapods. Each arch 236 MESODERMAL DERIVATIVES

typically consists of three portions. The dorsal one in the pectoral girdle is the scapula; in the pelvic girdle it is called the ilium. The two ventral elements of the pectoral girdle are the precoracoid (anterior) and the coracoid (posterior), while the corresponding elements of the pelvic girdle are the pubis and ischium. In the shoulder region, the clavicle, a derm bone, becomes associated with the pectoral girdle.

The free appendages. — The pectoral and pelvic appendages are very similar. Each has three segments: proximal, intermediate, and distal. The proximal segment of the pectoral appendage contains one bone, the humerus, while the corresponding bone of the pelvic limb is called the femur. The intermediate portion of the pectoral limb possesses two bones, the radius and ulna; while the corresponding bones of the pelvic appendage are the tibia and fibula. The distal segment is divided into three regions of which the proximal portion contains nine or ten bones, the carpalia of the pectoral appendage, tarsalia of the pelvic. The intermediate portion contains five metacarpalia or metatarsalia, respectively. The distal portion contains the free phalanges of the fingers or toes.



Pectoral Gencral Pelvic

Girdle Seapula | lium Procoracoid Pubis Coracoid Ischium

Free appendage

Humerus Femur

Radius Tibia

Ulna Fibula Carpalia Tarsalia Metacarpalia Metatarsalia Phalanges (I-V) Phalanges (I-V)

Origin of the appendicular skeleton. — All the bones of the appendicular skeleton, with the exception of the clavicle, are formed from a mesenchymal blastema in the limb buds by the appearance of centers of chondrification. The origin of this ORIGIN OF THE APPENDICULAR SKELETON 237

mesenchyme is probably from the somites, but the details of the process are still imperfectly understood.

THE FROG.! — Nine vertebrae are formed, of which the first is known as the cervical vertebra, or atlas, the succeeding seven are the abdominal vertcbrac, and the last is called the sacral vertebra as it is to this that the pelvic girdle is attached. No caudal vertebrac are formed, but thrce strips of cartilage enclose the notochord and form the primordium of the adult urostyle. Dorsal ribs are differentiated, but these remain rudimentary and fuse with the transverse processes of the vertebrae. The sternum arises from the fusion of two longitudinal bars of cartilage which never attain connection with the ribs. It persists anterior and posterior to the pectoral girdle.

The cartilage bones of the skull are the exoccipitals, prodtics, stapes, ethmoids, and the pterygoquadrate (in part), articulare, mentomeckelian, hyoid, and branchials. The derm bones are the fronto-parictal, nasals, premaxillac, maxillac, quadratojugals, squamosals, parasphenoid, palatines, vomers, and dentaries.

In the pectoral girdle develop the scapula, coracoid, and precoracoid, the last of which is replaced by the clavicle. In the pelvic girdle only the ilium and ischium ossify. Only four digits are present in the hand, the thumb (pollex) being absent.

THE CHICK. — There are sixteen cervical vertebrae, of which the first is the atlas, and the second, which has appropriated the centrum of the first, is the axis; five thoracic vertebrae; about six lumbar vertebrac; two sacrals; and about fifteen caudals. The last thoracic, all lumbars, and sacrals and five caudals are fused to the pelvic girdle. The last four caudals are fused into a pygostyle. Dorsal ribs are formed by the cervical and the thoracic vertebrae. The sternum arises from two longitudinal bars of cartilage which unite in the median line. It is distinguished by the development of a large keel (carina) for the attachment of the pectoral muscles.

The cartilage bones of the skull are the basioccipital, exoccipitals and supraoccipitals; prodtics, epiotics, and opisthotics; basisphenoid, orbitosphenoids, and alisphenoids; the ethmoid; quadrate, articular, meckelian cartilage; stapes, hyoid, and branchials. The derm bones are the frontals, parictals, nasals, lachrymals, premaxillac, maxillae, jugals, quadratojugals, squamosals, pterygoids, palatines, parasphenoids, vomer, angular, supra-angular, opercular, and dentary.

The pectoral girdle devclops a scapula and coracoid, together with a dermal clavicle. Ilium, ischium, and pubis ossify separatcly in the pelvic girdle. Five digits are performed in the pectoral appendage; of these the first and fifth fail to develop further. Five also appear in the embryonic skeleton of the pelvic appendage; the fifth soon disappears, and the first is extremely short and develops no phalanges.

MAN. — Seven cervical vertebrae, including the axis and atlas, twelve thoracic, five lumbar, five sacral, and four caudal vertebrae are formed. Of these, the sacral vertebrae are united to the pelvis, and the caudal vertebrae are frequently fused to form the coccyx. Primordia of ribs are formed by all vertebrae except those following the first caudal. Only the thoracic segments, however, develop complete ribs. The sternum arises from two longitudinal primordia with which the first eight or nine ribs acquire cartilaginous connections.

The cartilage bones of the skull are the occipital (in part), the sphenoid, the ethmoid, the turbinates, temporals (in part), the stapes, malleus, incus, and hyoid. The malleus and incus are the representatives of the articular and quadrate. The

1 The details of the skeleton in this and succeeding paragraphs arc for reference only. 238 MESODERMAL DERIVATIVES

derm bones are the occipital (in part), temporals (in part), frontal, parictals, lachrymals, nasals, vomer, maxillae, zygomatics, palatines, and mandible, the last-named bone representing the fused dentaries. It is apparent that many of the bones of the human skull are the result of the fusion of separate centers of ossification which represent skull elements of the lower vertebrates. The second and third visceral arches contribute to the formation of the hyoid, the others to the laryngeal cartilage.

The pectoral girdle consists of the scapula, with which is fused the coracoid. There is no precoracoid, but a dermal clavicle is present. The centers of ossification that represent the pubis, ischium, and ilium fuse to form an innominate bone. The free appendages terminate in five digits. In conclusion, it should be mentioned that the adult condition of the human skeleton is not attained until the age of twenty-five.


The musculature of the vertebrate is derived from mesenchyme (Fig. 160), of which the greater part originates from the myotomes and gives rise to striated muscle cells, controlled by the central nervous system, the skeletal musculature. A portion,


Neural tube

Notochord Aorta

Dorsal appendicular muscle mass

Ventral appendicular Gut muscle mass Splanchnie mesoderm

Somatic mesoderm

Fig. 160. — Diagram of transverse section through vertebrate embryo in region of limb bud, to show origin of appendicular muscles. (After Corning.)

however, originates from splanchnic mesoderm and gives rise to non-striated (smooth) muscle cells(found in the skin, surrounding the alimentary canal, blood vessels, and the urogenital organs. They are/controlled by the autonomic nervous system (page 254), and make up the visceral musculature. Several exceptions to these general statements should be noted. The muscle cells of the heart are striated; the muscles derived from the visceral arches are both)striated and controlled by the central nervous CRANIAL MUSCLES 239

system, although derived from lateral mesoderm. It will be noted later that the muscles of the iris of the eye (page 266) and of the sweat glands (page 246) are apparently ectodermal in origin.

Dermal musculature. — In the skin are found striped muscles which are derived from skeletal musculature (see below) but which have lost their attachment to the skeleton. The dermal musculature is best developed in the amniotes. The muscles of expression in man are dermal muscles supplied by the seventh cranial nerve (see Chapter X).

Axial musculature. In this section are included all the muscles arising from the myotomes and attached to parts of the axial skeleton, which they move. They are originally metameric, but their later history is obscured by subsequent migration, fusion, splitting, budding, and degencration. The intercostals, between the ribs, however, preserve their original metamerism, which in the others may be traced to some extent by the innervation, since the connection between a spinal nerve and the muscle mass it supplies is established early in organogeny and remains constant. Thus it can be shown that the musculature of the diaphragm, supplied by the phrenic nerve, arises from a cervical myotome.

Cranial muscles. — Like the cranium, the associated muscles are derived from different sources and consist of skeletal and _ visceral muscles. The muscles

of the eyeball arise from Fia. 161.— Head of embryo dogfish (Squalus

_ _ acanthias) showing preotic somites (A, B, C) the three preotic myo and cranial nerves (V, VII, [X, X). (After tomes (Fig. 161), of which — jingsley.) 7

the first supplies all the

muscles of the eyeball except the superior oblique, derived from the second myotome, and the lateral rectus, supplied by the third head myotome. These are innervated by the third, fourth, and sixth cranial nerves, respectively. The tongue musculature is 240 MESODERMAL DERIVATIVES

derived from the myotomes associated with the occipital vertebrae and supplied by the twelfth cranial nerve. The muscles of mastication, the facial muscle, and the laryngeal muscles, together with those of the ear bones, arise from the visceral arches (Fig. 162),

Glossopharyngeal _, Facial



ma \i Me


. id Mandibular Branchial arches Fiyel arch

Fia. 162. — Diagram to show primitive visceral muscles in relation to visceral skeleton and cranial nerves. (Hypothetical after Wilder.)

Vagus \\\ i

and are supplied by cranial nerves V, VII, [X, X, and XI (see Chapter X).

Appendicular muscles. — In the anamniotes, these muscles arise from the myotomes; among the amniotes, their origin is doubtful, as the limb bud develops as an undifferentiated mass of mesenchyme surrounded by ectoderm. In this blastemal mass,

Dorso - medial muscle primordia


Humerus Ventro - lateral muscle primordia

Fig. 163. — Reconstruction of the pectoral muscle masses in a 17-mm. Necturus: (Prepared by H. F. DeBruine.)

muscles and bones are laid down, the differentiation proceeding from the proximal toward the distal end. The pectoral muscles differentiate before those of the pelvic appendage. The appenSUMMARY 241

dicular muscles are found in antagonistic groups: protractors, which move the limb forward; and retractors, which have the opposite effect; levators, which raise the limb; and depressors, which contract in the opposite direction. Like the axial muscles, these have become highly modified and specialized among the tetrapods (Fig. 163).

Visceral muscles. — Under this head are included the muscles lining the alimentary tract, lungs, vascular organs, and urogenital system. All arise in the mesoderm which surrounds the endothelial lining of the organs concerned. The muscle cells of the heart arise as smooth muscle cells which become striated in later development. It is interesting in this connection that the smooth muscle cells of the bladder of the dog have been transformed into what are apparently striate muscles when this organ is made to pulsate rhythmically by continued irrigation.


The following structures are derived from the middle germ layer:

A. The notochord

B. The mesoderm

I. The lateral mesoderm Epithelium of the coelom Pericardial cavity Pleural cavity Peritoneal cavity

Mesenteries Dorsal mesentery Ventral mesentery Mesocardia Mesohepares

II. The intermediate mesoderm

Kidneys Pronephros Mesonephros Metanephros 242 MESODERMAL DERIVATIVES

Genitalia Gonads Genital ducts Wolffian (mesonephric) duct Miillerian (oviducal) duct External genitalia (also ectodermal)

Adrenal glands

Interrenals (Suprarenals from ectoderm)

C. The mesenchyme III. (Principally from splanchnic mesoderm)

The blood corpuscles Blood plasma Blood vessels Heart Arteries Veins The lymphatics

IV. (Principally from the axial mesoderm)

Connective tissue Skeleton Dermal Axial _ Cranial Chondrocranium Neurocranium Splanchnocranium (or visceral skeleton) Dermocranium Appendicular Musculature Dermal Axial Cranial Appendicular Visceral (from splanchnic mesoderm)


Allen, E. 1932. Sex and Internal Seerction.

Arey, L. B. 1934. Developmental Anatomy, 3rd Ed., Chaps. 9-13.

Brachet, A. 1921. Traité d’embryologie des vertébrés, Part II, Bk. 1, Chap. 2; Bk. 2, Chaps. 1+4.

Hertwig, O. 1906. Handbuch, Vol. 1, Chap. 5; Vol. 3, Chaps. 1-7.

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

Keibel and Mall. 1910-1912. Human Embryology, Chaps. 11-13, 15, 18, and 19.

Kellicott, W. E. 1913. Chordate Development.

Kerr, J.G. 1919. Textbook of Embryology, Chaps. 4-6.

Kingsley, J.S. 1926. Comparative Anatomy of Vertebrates, 3rd Ed.

—— 1925. The Vertebrate Skeleton.

Lillie, F. R. 1919. The Development of the Chick, 2nd Ed.

MeMurrich, J. P. 1923. The Development of the Human Body, 7th Ed.

Vialleton, L. 1924. Membres et ceintures des vertébrés tétrapodes. CHAPTER X ECTODERMAL DERIVATIVES

The ectoderm, being the external germ layer, gives rise to the outer layer of the skin, the epidermis, which continues into all the openings of the body} Of these, the development of the mouth, the cloaca and its derivatives, and the visceral clefts has been discussed. There remain for consideration the openings of the nostrils, the chamber of the eye, and the external auditory meatus. These will be taken up in connection with the sense organs, which, together with the nervous system, form in development a sensory-nervous complex.


The integument consists of two parts, the ectodermal epidermis, and the mesodermal dermis. The epidermis soon de

after Prentiss.)

laminates into two layers, the deeper germinativum, from which new strata are proliferated towards the exterior, and an outer periderm or embryonic skin (Fig. 164). Beneath the periderm, the outer cells of the germinativum are transformed into a horny layer, the corneum. The underlying dermis is essentially a supporting layer of mesenchyme cells derived largely from the outer side of the myotome, a region which is sometimes known as the dermatome. In the dermis are formed blood vessels, connective tissue, bone, and muscle. The bony scales of fish are dermal in origin.

Derivatives of the corneum. — In the amniotes the horny layer of the epidermis is frequently fragmented to form horny scales

Fig. 165.— Diagrams showing similar development in A, scale; B, feather; and C,

hair. (After Kingsley.)

(Fig. 165A), such as those of reptiles, or those found on the legs of birds, or the tails of rats, ete. Among the birds, scales are largely replaced by feathers which originate in much the same

Fig. 166. — Diagrams to show ectodermal primordia of A, nail; B, claw; and C, hoof. Above in sagittal sections; below ventral view. (After Kingsley.)

manner as scales. The epidermal plate, however, grows down like a cup to enclose a core of dermal origin (Fig. 165B). The epidermal sheath gives rise to the quill and barbs, while the core gives rise to the pulp, by means of which nutriment is supplied to the developing feather. Among the mammals, hair arises in a 246 ECTODERMAL DERIVATIVES

very similar fashion. An epidermal plate grows down into the dermis to form the hair bulb, the proximal end of which invaginates to receive a mesodermal core, the hair papilla, while around the whole is a mesodermal hair sheath (Fig. 165C). The hair papilla, however, does not grow out into the center of the hair as does the pulp of the feather. Claws, nails, and hoofs arise from the union of two epidermal primordia like those of scales, a dorsal unguis and a ventral subunguis (Fig. 166). Derivatives of the germinativum. — The germinativum, in addition to producing the more superficial layers of the epidermis, gives rise to the glands of the skin Unicellular Multicellular (Fig. 167). Among the anamnigland gland . — — otes, these glands are usually unicellular and produce the mucus which serves to diminish the friction of the skin against the water while swimming. Unicellular glands frequently aggregate to

3 Epi 2 | dermis eo:

Se esa £ produce multicellular glands, such RS Sars Chromatophore 7 as the flask glands and cement

glands of the anamniotes, or the sebaceous (oil) and sudoriparous (sweat) glands of the mammals. The mammary glands of mammals are modified sudoriparous glands secreting the milk by which the new born are nourished through infancy.

Derivatives of the dermis. — Two types of pigmentation are to be distinguished in the integument. The first is produced by pigment secreted in the ectodermal epidermis, i.e., the melanin, of the frog tadpole. The second is produced by chromatophores, which are mesenchyme cells of the dermis. These secrete pigment granules and move toward the light to form a layer immediately below the epidermis, some even wandering into the epidermis itself.

THE FROG. — The ectoderm of the frog embryo is ciliated at 6-mm. body length and remains so until the length of 20 mm. is attained, when the cilia disappear except on the tail which remains ciliated until metamorphosis. The jaws and oral combs of the tadpole are derivatives of the corneum and consist of rows

Fig. 167. — Section of Protopterus skin to show glands. (After Kingsley.) THE NERVOUS. SYSTEM 247

of horny denticles forming replacement series. The oral gland, or sucker, is a multicellular mucous gland derived from the germinativum and elevated by the elongation of its gland cells. It arises as a crescentic groove posterior and ventral to the point where the stomodeum will appear, then becomes V-shaped, and finally divides by the degeneration of the middle portion. The cement gland atrophies soon after the opening of the mouth. The pigmentation of the skin is derived from two sources, the melanin of the egg which is distributed to the epidermis, and the mesenchymal chromatophores (Fig. 199) which develop in the dermis.

THE CHICK. — The scales on the legs are typical reptilian scales and are derived from the corncum; they sometimes bear feathers in the young’bird and so form a transition between scales and the characteristic avian feathers. The claws arise in the corneum from two primordia, a dorsal “ claw-plate ” and a softer “ clawsole.” To prevent the sharp claws tearing the embryonic membranes, the concavity of the claw is filled with a pad known as the neonychium, derived from the corneum, which is lost after hatching. The beak arises from the corneum around the upper and lower margins of the jaws. The egg tooth is a horny prominence on the dorsal side of the upper jaw, appearing on the sixth day of incubation but not taking on its ultimate shape until the fourteenth. It serves to aid in breaking the shell and is lost after hatching.

MAN. — The nails arise from nail-plates and sole-plates, of which the latter are rudimentary structures. They are covered during fetal life by the eponychium, consisting of the periderm and outer layers of the corneum. The hairs are arranged in patterns which have been’ interpreted as reminiscences of the ancestral scalés. The first growth of hair is called the lanugo; it is cast off, except over the face, soon after birth. The mammary glands arise from two longitudinal thickenings of the epidermis, known as the milk ridge. In later development the gland resembles an aggregation of sudoriparous glands.


Although the nervous system and sense organs arise together and remain in functional continuity, it has become customary to distinguish the sense organs (receptors) from the nerves (trans248 ECTODERMAL DERIVATIVES

mittors) by which stimuli are passed on to the muscles or glands (effectors). | Both the nervous system and the sense organs arise from specialized regions of the dorsal ectoderm, knowy respectively as the neural plate and the sense plates (placodes)4 These represent an inward growth from the germinativum as opposed to the outward growth which produces the epidermis. In the frog this division is clearly indicated by a line of cleavage between the outer epidermal ectoderm and the inner nervous ectoderm. Both the neural plate and the sensory placodes withdraw from the surface and become subepidermal by a process of invagination. In this connection it is interesting to note that the optic placode is incorporated and invaginates with the neural plate so that when the retina of the eye develops, it does so from the brain. |

The neural tube. —'The neural plate is an elongate structure, extending from the blastopore to the head region.! Local growth results in the incurving of this plate to produce a neural groove with conspicuous lips, the neural folds. As this growth continues the groove sinks inward and the lips meet above it, thus converting the groove into a neural tube, which breaks away from the overlying epidermis and sinks into the interior. The cells at the margin of the neural plate form, at each dorso-lateral angle of the neural tube, a bar known as the neural crest, which subsequently segments into the ganglia.

The neurons. — The inner lining of the neural tube, corresponding to the outer layer of the neural plate, is called the ependyma. This is the center of cell proliferation (Fig. 170). Two types of cells are formed: the supporting cells, or spongioblasts; and the embryonic nerve cells, or neuroblasts. The neuroblasts migrate out of the ependyma and form an intermediate mantle layer in which they become transformed into neurons. These nerve elements have a prolongation at one end known as the axon or nerve fiber, while at the other are branched projections called dendrites. The axons grow out from the mantle layer into the outer layer of the cord, known as the marginal layer, where they secrete the medullary sheaths which act as insulating coats. Not all axons become medullated. Similar changes take place in the ganglia, whereby neurons and supporting cells are differentiated. TYPES OF NEURONS 249

Types of neurons. — We may distinguish four types of neurons (Fig. 168), as follows: (1) Afferent neurons arising in the ganglia and sending theiraxons to the dorsal region of theneuraltube. These convey excitations from the sensory receptors to the neural tube. Two sub-types are distinguished: (a) the somatic sensory neurons, conveying excitations from the exterior; and (b) splanchnic sensory neurons, conveying excitations from the viscera. (2) Efferent neurons, with their bodies in the ventral region of the neural tube, sending their axons to effectors (muscles or glands). Two sub-types are recognized: (a) somatic motor and (b) splanchnic motor. These af- Fig. 168.— Diagram to show cross-sections of the ferent and efferent neu- spinal cord at three levels, the posterior level above. rons form the periph- The dotted lines indicate the paths of neurons whose

bodies lie wholly within the cord, suprasegmental to the left.


eral nervous system. (3) The intersegmental neurons have their bodies in the ventral portion of the neural tube and their axons are usually directed towards its posterior end. They serve to connect efferent neurons in the different segments of the body. (4) The suprasegmental neurons have their bodies usually in the dorsal portion of the neural tube and their axons are directed toward the anterior end of the tube, i.e., the brain. They serve to convey afferent excitations toward the brain and in that organ give rise to the great brain centers. The axons of 250 ECTODERMAL DERIVATIVES

these last two types of neurons form the descending and ascending bundles of the brain and cord.

The spinal cord. —4 The spinal cord, or neural tube exclusive of the brain, retains its primitive characteristics, - The cavity, or neurocoel, persists as the central canal. (Between each pair of vertebrae/ the afferent and efferent neurons’ form a pair of spinal nerves which run out into the myotomes and hence have a metamerism equivalent to that of the myotomes, an important point in considering the homologies of the muscles. { In the region of the pectoral and pelvic appendages, several of the segmental nerves combine to form the brachial and the sacral plexus, respectively. The cord becomes surrounded by an envelope of mesenchyme known as the meninx, which in the higher vertebrates becomes divided into an inner pia mater and an outer dura mater. The development of the nerves will be taken up in a later section.

The brain. — Whereas the cord is largely composed of afferent, efferent, and intersegmental neurons, by which certain reflex actions are directed, the anterior end of the neural tube enlarges and differentiates into the complex brain (Fig. 169). Here arise several centers in which the impulses received mainly from the major sense organs, nose, eye, and ear, are correlated. The brain may be divided into two major regions: the archencephalon, or prechordal brain; and the deutencephalon, or epichordal brain. With continued local growth, the archencephalon grows down in front of the notochord, thus forming the first or cranial flexure. At the same time, three dilations appear: the prosencephalon from the archencephalon; the mesencephalon at the point of the flexure; and the rhombencephalon from the deutencephalon. It is convenient to associate the future history of the prosencephalon with that of the nose, the mesencephalon with that of the eye, and the rhombencephalon with that of the ear.

The prosencephalon. — The later history of the prosencephalon is complicated by the fact that\the optic placode is included in the neural tube at this point. Accordingly, we find he prosencephalon dividing into an anterior telencephalon and a posterior diencephalon.

The telencephalon. — The anterior part of the telencephalon becomes the olfactory lobe, which receives the afferent neurons from the nose. From the roof develops the cerebrum, ‘which beTHE DIENCEPHALON 251

comes the most complex and important center of association’. From the floor arises the optic part of the hypothalamus.* There are two cavities, or telocoels (also known as the lateral ventricles).

- Deutencephalon


Neyrenterie EX




Fig. 169. — Diagrams to show early development of the vertebrate brain in sagittal sections. A, prechordal and epichordal divisions. B, primary brain vesicles. C, definitive vesicles. The longitudinal broken line indicates division between roof and floor regions. (After von Kuppfer.)

The diencephalon. — The roof of the diencephalon gives rise to the thalamus in front, and the metathalamus behind; from the latter springs a dorsal diverticulum, the epithalamus. This structure, often known as the epiphysis, gives rise to something very much resembling an unpaired eye in early embryonic life; this later becomes the pineal gland of the adult, one of the so252 ECTODERMAL DERIVATIVES

called endocrine glands. / The eyes take their origin from the side of the diencephalon. The floor of the diencephalon gives rise to a ventral diverticulum — the infundibulum, which grows downward to meet the advancing hypophysis from the stomodeum (see page 181). The two later fuse to form the pituitary glandJanother of the endocrine series. Behind the infundibulum, the floor of the diencephalon forms the mammillary part of the hypothalamus. It is evident that the thalamencephalon, often used as a synonym of the diencephalon, differs from it by the inclusion of the optic part of the hypothalamus, which is derived from the telencephalon although indistinguishable from the mammillary part of the hypothalamus in the adult. The thalami contain nuclei (masses of neurons) which receive afferent impulses from the optic, general sensory, and acoustic organs, and transmit impulses to and from the other centers of the brain. The cavity of the diencephalon persists as the diacoel (third ventricle).

The mesencephalon. — The roof of the mesencephalon gives rise to the corpora bigemina (quadrigemina in mammals), or optic lobes, the centers which receive afferent impulses from the eyes transmitted through the diencephalon.| The floor of the mesencephalon is the anterior portion of the brain stem, from which the motor neurons of the cranial nerves depart. The third and fourth cranial nerves originate from the mesencephalon. Its cavity is the mesocoel (or aqueduct).

The rhombencephalon. —‘The hind-brain, like the fore-brain, is divided into two regions, metencephalon and myelencephalon, respectively. ~

The metencephalon. — The roof of the metencephalon gives rise to the cerebellum, the center associated with hearing except in mammals), the lateral line organs of anamnidtes, and the sense of equilibrium. The floor of the metencephalon is part of the brain stem, and from it arises the pons, a bundle of axons connecting the two sides of the cerebellum. The cavity is the metacoel.

The myelencephalon. — The roof of the myelencephalon is covered by a thin roof plate, the choroid plexus. Its floor forms the posterior portion of the brain stem. (The cranial nerves, from V to XII inclusive, depart from this portion of the stem, which merges imperceptibly into the spinal cord. Its cavity, THE SPINAL NERVES 253

hardly distinguishable from that of the metencephalon, is called the myelocoel (fourth- ventricle).

The spinal nerves. af The nerves are segmentally arranged bundles of afferent and efferent neurons originally associated with the myotomes. The afferent neurons arise in the ganglia, the efferent in the floor of the spinal cord. Accordingly, a typical spinal nerve has two roots in the cord: a dorsal afferent root uniting with the ganglion; and a ventral efferent root which unites with the dorsal root after the other has attached itself to the ganglion ¥Fig. 170). The nerve trunk then divides into branches, each containing afferent and efferent neurons, which are called rami and supply the body wall, although one (the com

Dorsal root Marginal layer Somatic sensory neuron foi rs Ependymal layer Visceral sensory neuron fd i \\\ Manile layer

Spinal ganglion Visceral motor neuror

Somatic motor neuro

Dorsal ramus


Lat. terminal . O division CV\’\A ( Ventral terminal division of , Aorta teen A Spinal nerve . Ramus communicans Sympathetic ganglion

Fia. 170. — Diagram to show the neuron components of a spinal nerve. Transverse section of 10 mm. human embryo. (From Arey after Prentiss.)

municating ramus) connects with a sympathetic ganglion, derived from a spinal ganglion, through which the splanchnic afferent and efferent neurons serve the viscera.

It has been shown by Coghill that the development of behavior is closely paralleled by the development of the connections (synapses) between the neurons. Thus in the urodele, Ambystoma, the first reflex of the embryo, a bending away from a light touch on the skin, does not take place until an intermediate neuron in the spinal cord has established synaptic relations with the sensory tract on one hand and a floor plate cell which already has established a synaptic relation to the motor tract on the opposite side of the spinal cord (Fig. 171).

Fig. 171. — Diagram to show in transverse section of Ambystoma larva, neurons concerned in earliest reflex. (From Coghill, ‘““Anatomy and the Problem of Behavior.’’)

The autonomic nerves. — The brain, spinal cord, and cranial and spinal nerves are grouped by anatomists as the central nervous system. Associated with this is the autonomic nervous system, consisting of nerves and ganglia and controlling the smooth muscles of the viscera and blood vessels, and some glands. This system arises from the neural plate, like the central nervous system, but from the lateral margins which become the neural crests. At the time when the neural crests are dividing into the cerebrospinal ganglia, some of the cells migrate inward toward the dorsal aorta, where they aggregate and multiply to form the chain ganglia. The chain ganglia on each side become connected by fore and aft extensions which form the sympathetic trunks. They retain a connection with the cranial and spinal ganglia by means of the communicating rami, and send out nerves along the principal blood vessels. From the chain ganglia, by secondary and tertiary migrations, arise the prevertebral and visceral ganglia. In the head the four sympathetic ganglia (ciliary, sphenopalatine, otic, and submaxillary) arise from the semilunar ganglion of the fifth cranial nerve, and later acquire connections with the chain ganglia (Fig. 172).

Fig. 172. — Diagram to show migrations of autonomic ganglia in human develop ment. (After Streeter.)

It has already been noted (page 214) that some of the cells from the autonomic ganglia (chromaffin cells) migrate to the vicinity of the mesonephros to form the suprarenal gland.

The cranial nerves. — The cranial nerves, or nerves of the head regions, contain not only splanchnic and somatic afferent and efferent neurons comparable to those of the spinal nerves, but also special afferent neurons from the nose, eye, ear and lateral line system. There are ten cranial nerves in the anamniotes, twelve in the amniotes (Figs. 173, 174). To these should be added in all cases the terminal nerve, unknown when the cranial nerves were first classified.

O. Terminal, a ganglionated nerve from the organ of Jacobson entering the cerebral lobe with functions unknown, probably sensory.

Fig. 173. — Diagram to show origin of cranial I. Olfactory, a non-gangli nerves in man. (After His.) onated sensory nerve from the olfactory sensory region of the nose to the olfactory lobe.

II. Optic (ophthalmic), a non-ganglionated sensory nerve from the retina of the eye to the floor of the diencephalon where the fibers from the two eyes cross (optic chiasma). Each set of fibers then enters the brain and runs to the optic lobe on the opposite side of the brain to that on which the eye is located.

Il. Oculomotor (motor oculi), a motor nerve, somatic with some sensory elements, from the floor of the mesencephalon to all muscles of the eyeball except the superior oblique and the lateral rectus.

IV. Trochlear, a motor nerve, somatic with some sensory elements, from the roof of the mid-brain to the superior oblique muscle of the eyeball. THE CRANIAL NERVES 257

V. Trigeminal, a mixed nerve. Its somatic sensory neurons arise in the semilunar ganglion, the motor elements in the floor of the myelencephalon. The sensory neurons are somatic (general cutaneous). The motor neurons supply the jaws (mandibular arch).

VI. Abducens (pathetic), a somatic motor nerve with some sensory elements, arising from the myelencephalon and supplying the external rectus muscle of the eyeball.

VII. Facial, a mixed nerve. The afferent neurons arise in the geniculate ganglion and are splanchnic in nature, supplying the hyoid arch, and also the tongue of mammals. In the anamniotes, an associated ganglion gives rise to a lateral branch with afferent components from the lateral line organs. The efferent neurons supply the hyoid arch in the lower vertebrates and the facial region in the amniotes. The rami of the fifth and seventh nerves are closely associated.

Fig. 174. — Diagram showing relationships between cranial nerves and parts supplied. A, B, C, head somites. Arabic numerals, visceral arches. Roman numerals, nerves.

VIII. Acoustic (auditory), a ganglionated sensory nerve arising from the acoustic ganglion and bearing afferent neurons from the ear. In higher vertebrates it becomes differentiated into the vestibular and cochlear nerves, each with its own ganglion produced by the division of the acoustic ganglion.

IX. Glossopharyngeal, a mixed nerve. The afferent neurons arise in the petrosal and the superior ganglion and are principally splanchnic. They divide into a prebranchial branch running into the hyoid arch and a postbranchial branch into the first branchial arch. The efferent components are principally found in the postbranchial branch.

X. Vagus, a mixed nerve arising by the fusion of several primitive cranial nerves, which supplied the arches with afferent (from the jugular ganglion) and efferent neurons. In addition, the vagus gives off a visceral branch to the stomach, lungs, etc., and in the anamniotes a lateral branch to the lateral line organs of the trunk (from the nodosum ganglion).

XI. Accessory, a motor nerve which innervates the muscles of the shoulder girdle and is found only in the amniotes. A ganglion (of Froriep) disappears before the embryo becomes adult.

XII. Hypoglossal, also a motor nerve, which innervates the tongue in the amniotes. In the anamniotes the tongue is innervated by so-called “ occipital’? nerves which possibly are the fore-runners of the hypoglossal, prior to the appropriation of the occipital region by the head.

Metamerism of the nervous system.— The metameric arrangement of the nerves, like that of the segmental arteries, is purely secondary and dependent upon the primary metamerism of the mesoderm. The nerves, however, are more conservative than the vascular organs or myotomic derivatives. For example, the diaphragm of mammals is supplied by muscles from one of the cervical myotomes, and the innervation of the diaphragm (phrenic nerve) still arises from the cervical region. Many attempts have been made to reconstruct the metamerism of the head, by a study of the cranial nerves, following Bell’s law: that every original cranial nerve has, like a spinal nerve, a dorsal sensory and ventral motor root.

This problem has been complicated by the fact that in the head there are two types of metamerism, (1) primary as indicated by the head myotomes in the elasmobranch embryo, and (2) secondary (branchiomeric) as indicated by the visceral arches (Fig. 174). Accordingly, there are two types of musculature, (1) somatic as represented by the muscles of the eyeball, and (2) splanchnic as represented by the muscles of the jaws and visceral arches. Two types of efferent neurons, therefore, are present, (1) somatic and (2) splanchnic. The splanchnic motor neurons of the cranial nerves differ from those of the trunk, however, in that no sympathetic neurons intervene between them and the muscles which they supply. There are altogether three sets of afferent neurons: (1) the general sensory or cutaneous, which correspond to the somatic sensory neurons of the trunk; (2) splanchnic sensory, which correspond to those of the trunk; and (3) lateral, belonging to the lateral line system. The cranial nerves are evidently not serially homologous, as can be seen from Table 11.


Nerve Afferent Afferent Efferent Efferent erv Somatic Splanchnic Somatic Splanchnie I Smell IT Vision III Movement of eyeball IV Movement of eyeball Vv General Movement cutaneous of jaw VI Movement of eyeball VII Taste Hyoid and facial movement and salivation Vill Hearing and equilibration Ix Taste and Salivation, pharyngeal pharyngeal sensation movement x Visceral Movement of sensation viscera and pharynx XI Movement of pharynx and shoulder XII Movement of tongue

Finally, we must mention the neuromeres which have been reported in various vertebrate embryos. These are formed by constrictions in the cranial portion of the neural tube and interpreted by some authors as the remains of a neural metamerism. They seem in many forms to correspond with the cranial nerves and more probably represent areas of local growth prior to the outgrowth of the nerves themselves.

The general problem of the metamerism of the head still awaits solution. The latest summary, that of Brachet, indicates the probable number of segments in the primitive head as six. Three of these are ephemeral, and their somites give rise to mesenchyme. The three posterior segments are associated with the first three visceral clefts bounded by the first four arches, each of which has its own cranial nerve: the trigeminal of the mandibular arch; the facial of the hyoid; the glossopharyngeal of the first branchial; the vagus of the second branchial arch. According to this interpretation, the posterior clefts and arches are reduplications supplied by new branches of the vagus, while the accessory and hypoglossal are secondarily acquired spinal nerves.

THE FROG (SEE ALSO CHAPTER XI). — The prechordal and epichordal divisions of the brain are demarcated by the notochord, and the division into the three primary vesicles is but slightly indicated. The brain of the frog never develops neuromeres. The optic lobes are corpora bigemina. The division into myelencephalon and metencephalon is incomplete, and no pons is formed. There are forty pairs of spinal nerves in the tadpole, reduced to ten in the adult. There are but ten of the cranial nerves (XI and XII not included). The sympathetic ganglia originate from the cranial and spinal ganglia by the emigration of ganglion cells.

THE CHICK (SEE ALSO CHAPTER XII). — The divisions of the brain into the three primary and five secondary vesicles is well marked. Eleven neuromeres are formed, of which three are found ‘in the prosencephalon, two in the mesencephalon, the remainder in the rhombencephalon. “Three flexures are formed: (1) cranial in the floor of the mesencephalon; (2) cervical at the junction of the myclencephalon and the spinal cord; and (3) pontine in the floor of the myclencephalon. A pons is formed. There are fifty pairs of nerves developed in the chick of eight days (Lillie), of which thirty-cight are spinal and twelve cranial, including the eleventh and twelfth which are not incorporated in the head of the frog.

MAN (SEE ALSO CHAPTER XIII). — The particular feature of importance in the development of the human brain is the great increase in size and complexity of the cerebral hemispheres of the telencephalon. The optic lobes are quadripartite (corpora quadrigemina), of which the two anterior lobes are especially associated with vision, the two posterior ones with hearing.


The nervous system receives stimuli not only from outside the body but also from within, such as those concerning the tension of the muscles. For the reception of stimuli, special organs — the sense organs — are developed. Of these the most conspicuous are the eyes, the ears, and the nose. In addition, it must be remembered that the entire skin functions as a sense organ by means of special receptors, and that stimuli are received from the viscera and other internal structures by means of free nerve terminations.

Of the special sense organs, the eye is most specialized in its mode of development. It is responsive to photic stimuli. ‘The nose represents a concentration of chemical sense receptors, more highly developed than the scattered taste buds of the head, which are confined in adult mammals to the cavity of the mouth. The ear, responsive to slower vibrations (pressure, sound) in the surrounding medium, originates in a manner similar to that of the lateral line system. This system is highly developed in the aquatic anamniotes, vestigial or absent in the amniotes. The ear, on the other hand, is more highly developed in the amniotes.

Fig. 175. — Diagrams showing early stages in development of nose. A, nasal placodes (in black). B, same now on ventral surface of head. C, nasal pits. D, nasal grooves, anterior-ventral view. E, nasal tubes, ventral view, lower jaw removed.

The nose. -+ The nose arises as a pair of local thickenings of the ectoderm at the anterior end of the head (Fig. 175). These thickenings are hereafter known as the nasal (olfactory) placodes. Later they invaginate to form the nasal (olfactory) pits,)which persist as the nose of all fish except the air-breathing dipnoi. Here also should be noted the fact that the cyclostomes are peculiar in the possession of a single median nasal pit. Among the tetrapods(the nasal pits elongate to become oro-nasal grooves, the anterior ends of which become connected with the developing mouth into which they are carricd. The original anterior ends of the nasal pits, therefore, come to lie at the posterior end of the mouth and open into the pharynx as the internal nares, while the original posterior ends become the external nares (Fig. 175E). The nasal cavity is later separated from the oral cavity by the ingrowth of the maxillary, palatine, and pterygoid bones, which form the hard palate YFig. 176). Jacobson’s organ arises as a pocket of the olfactory epithelium. Its function is unknown. The olfactory epithelium contains ciliated cells connected to the olfactory lobe by means of the first cranial nerve) which is peculiar in that its ncurons run directly to the brain. without the interposition of ganglion cells. Jacobson’s organ receives a branch of the trigeminal nerve.

Fig. 176. — Sagittal hemi-section through human nose. (After Howden.)

The eye. — The optic placodes are incorporated into the neural plate, where they can be distinguished as lateral thickenings of the margin at points which will later be included in the diencephalon. (Fig. 177). When the tube is formed, the relation of the sensory epithelial cells to the exterior is, of course, reversed. The optic placodes “ invaginate,” but, owing to their relation to the neural tube, the result is an apparent “ evagination ” from the tube towards the exterior. This produces the outgrowths which later, by constriction, give rise to the proximal optic stalks and distal optic vesicles. At the point where the optic vesicle touches the ectoderm, two reactions take place: (1) a local thickening of the ectoderm, called the lens placode, from which the lens of the eye develops; and (2) an invagination of the optic vesicle whereby this vesicle is transformed into a two-layered optie cup This invagination continues into the optic stalk to produce a groove called the choroid fissure.

The lens. —< The lens placode invaginates to form the lens pit, which then withdraws still further from the surface and becomes closed in by the union of its external lip to form the lens vesicle. The lens‘vesicle becomes solid by the elongation of the cells on the internal side which assume a clear transparent appearance.)

The optic cup. — The inner layer of the cup becomes the sensory portion of the retina, the outer layer the pigmented portion. Jt will be recalled that the sensory epithelium of the eye is inverted, and as a result the rods and cones, or sensory elements, of the retina are pointed away from the light.’ In the pigmented layer of the retina, melanin is secreted. Meantime the cavity of the optic cup becomes filled with a clear fluid secreted by the surrounding cells, which later becomes viscous and forms the vitreous humor.

The envelopes of the eyeball (Fig. 178). — Around the optic cup and stalk, a layer of mesenchyme accumulates, which later differentiates into an inner delicate layer called the choroid which contains pigment and capillaries and: may be compared with the pia mater of the brain, and an outer dense layer known as the sclera, which may be compared with the dura mater of the brain. THE ENVELOPES OF THE EYEBALL 265

The external portion of the sclera over the lens makes contact with the epidermis-and becomes transparent to form the cornea.

Fig. 177. — Diagrams showing early stages in development of vertebrate eye. A, optic placodes (in black). B, same after formation of neural tube. C, optic vesicles and lens placodes. D, optic cups and lens pits. E, optic cups and lens vesicles.

The epidermis over the eye forms the conjunctiva. In some vertebrates, sclerotic cartilage, or even bone, is formed, the vestige of an optic capsule. ) The edge of the choroid, together with 266 ECTODERMAL DERIVATIVES

the marginal retina, gives rise to the iris, a circular curtain surrounding the opening of the cup which is called the pupil of the eye. The muscles of the.iris are apparently of ectodermal origin. The iris divides the space between the lens and the cornea into two chambers, an anterior and a posterior chamber, which are filled with a fluid, the aqueous humor. The muscles of the eyeball are six in number, arising from the three head myotomes. They are innervated by the oculomotor, trochlear, and abducens nerves.

Fig. 178. — Horizontal section of human eye. (After Howden.)

The optic nerve. — The afferent neurons pass from the retina into the optic cup and form a bundle which passes out through the choroid fissure and into the optic stalk, and so to the optic chiasma on the floor of the diencephalon, where they cross and make their way to the optic lobes on the opposite side.

The lateral line system. — This is a diffuse sensory organ consisting of sense buds arranged in rows over the head and body of aquatic anamniotes. Its function apparently is to detect slow vibrations in the water. The origin of the lateral line system is a lateral thickening of the sensory ectoderm which later breaks up into separate suprabranchial placodes. These are found in the THE INNER EAR 267

embryos of the amniotes but soon degenerate. The lateral line system is of particular interest inasmuch as the lateral thickening referred to is in some cases continuous with the otic placode which gives rise to the ear. The principal nerve supplying the lateral system is the facial, although trigeminal, glossopharyngeal, and vagus often contain lateral line components.

The ear. — The ear becomes differentiated into the vestibule or equilibratory organ and the cochlea or organ of hearing. Three parts of the ear are distinguished (Fig. 180). The inner ear, giving rise to the vestibule and the cochlea, arises from an ectodermal otic (auditory) placode. The middle ear appears in the amphibians, and it is derived from the endodermal first visceral pouch. The outer ear, found only in the amniotes, is an ectodermal derivative of the first visceral groove and an outgrowth from the mandibular and hyoid arches

The inner ear. — This originates from the otic placode, which invaginates to form an otic (auditory) pit (Fig. 179) and later closes over to withdraw from the epidermis as the otic (auditory) vesicle or otocyst., In some vertebrates (elasmobranchs) the vesicle retains its connection with the exterior by means of a hollow stalk, the endolymphatic duct. Usually this connection is lost and the endolymph duct of the adult is a new formation. The vesicle divides into a ventral saccule and a dorsal vestibule or utricle. The saccule gives rise to the éndolymph duct and the lagena, which in mammals becomes th@ coiled cochlea or organ of hearing, while the utricle gives rise by constriction to three semicircular canals, each with a dilation at one endf the ampulla. The sensory epithelium is restricted to the lagena and ampullae} The cavity of these structures is known as the membranous labyrinth, and contains a fluid, the endolymph. Concretions, the otoliths, may appear in the endolymph of the vestibular portion. Around this labyrinth ‘the otic capsule, Jor skeletal labyrinth} is formed. ‘This later ossifies to give rise to the otic bones. (The skeletal labyrinth contains a fluid known as the perilymph. In vertebrates with a middle ear) two openings are formed in the skeletal labyrinth, the fenestra rotunda, closed by a membrane, and the fenestra ovale, into which the stapes projects.g The acoustic nerve, whieh is ganglionated, divides into a vestibular and a cochlear nerve, each with its separate ganglion.

Fig. 180. — Frontal section of human ear. Semi-diagrammatic. (After Howden.) THE FROG 269

The middle ear. — The middle ear arises from the first visceral pouch, which constricts into a proximal auditory (Eustachian) tube and a distal tympanic cavity which is separated from the exterior by the tympanic membrane,(a persistent closing plate formed from ectoderm and endoderm. Through the tympanic cavity there is a chain of bones (auditory ossicles) connecting the tympanum with the fenestra ovalis. In anurans and sauropsids, this chain of auditory ossicles consists of the columella and stapes (hyomandibular). In the mammals, the columella is replaced by the incus and malleus, equivalent to two other jaw bones, the quadrate and articulare, respectively. The muscles of the middle ear, tensor tympani and stapedial muscles, arise from the mesoderm of the mandibular and hyoid arches, respectively, and are innervated by the facial and glossopharyngeal nerves.

The outer ear.—— The external ear consists of the external auditory meatus, derived from the first visceral groove, and the pinna, which arises from tubercles on the mandibular and hyoid arches. It is composed of mesoderm and ectoderm, contains muscles, and is strengthened by cartilage. The innervation is from the facial nerve.

THE FROG (SEE ALSO CHAPTER XI).— In the development of the nose, the nasal groove stage is suppressed. Instead, a thickening develops from the olfactory pit into the mouth as far as the pharynx. This acquires a lumen which connects the olfactory pit to the pharynx. The development of the eye presents no especial peculiarities. The endolymph duct is a dorsal evagination from the otocyst. The semicircular canals are each formed by the appearance of a pair of ridges in the cavity of the utricle which fuse to enclose the cavity of the canal. The saccule gives rise to two ventral diverticula, the cochlea and basilar chamber. The function of the latterisunknown. The tubo-tympanic cavity arises from the first visceral pouch, which in the frog is vestigial and has no cavity. From this rudiment a strand of cells grows dorsad and later acquires a lumen. It loses its connection with the pharynx and moves backward to the ear region where it acquires a secondary connection with the pharynx (Fig. 181). The tympanic membrane is apparently entirely ectodermal. The columella, which connects the tympanum with the inner ear, arises from two primordia: the inner stapedial plate, which is a part of the otic capsule; and a cartilage derived from the palatoquadrate bar. This cartilage is thought to be homologous with the hyomandibular bone of fishes. The lateral line organs arise from the fragmentation of a placode known as the placode of the tenth cranial nerve, which innervates this series. Similar epibranchial placodes appear on the head and are innervated by the seventh and ninth nerves. They are larval sense organs and disappear at metamorphosis.

Fig. 181. — Rana pipiens, diagram to show the parts of the ear. Schematic crosssection through head.

THE CHICK (SEE ALSO CHAPTER XII). — The chick has a cleft palate due to the incomplete fusion of the palatine processes of the maxillae. Jacobson’s organ makes a short appearance as a vestigial organ but disappears before hatching. The eye possesses three eyelids, the third (nictitating membrane) arising from a separate fold inside that which forms the upper and lower lids. The pecten is a vascular plate in the vitreous humor, from mesenchyme which enters the choroid fissure. Its function is unknown. \The endolymphatic duct arises from the dorsal wall of the otocyst. The semicircular canals arise as outpocketings of the otocyst prior to its separation into utricle and saccule. The cochlea is more highly developed than in the frog. The tubo-tympanic cavity arises from the first pharyngeal pouch. The tympanum is formed from ectoderm and endoderm and includes a middle layer of mesenchyme. The columella arises from a stapedial plate and hyomandibular cartilage. The external auditory meatus is short, and no pinna is developed.

MAN (SEE ALSO CHAPTER XIII). — The organ of Jacobson is rudimentary and may completely disappear in the adult. A small fold (plica semilunaris) is the representative of the nictitating membrane. The cochlea is highly differentiated. The tube and tympanic cavity form from the first visceral pouch. The tympanum apparently is composed of all three germ layers. There are three auditory ossicles. The stapes is derived from the second visceral arch, while the malleus and incus arise from the first visceral arch. They are thought to represent the quadrate and articular bones of reptiles, respectively. The pinna arises from elevations on the mandibular and hyoid arches.


The ectoderm gives rise to the epithelial linings of the following structures:

A. The epidermis, with the apertures of Oral cavity Visceral clefts Cloaca

B. The neural plate 1. Neural tube Brain and cranial nerves Prosencephalon Telencephalon Diencephalon Mesencephalon Rhombencephalon Metencephalon Myelencephalon Cord and spinal nerves

2. Neural crest Ganglia Cerebrospinal Autonomic Suprarenal gland 272 ECTODERMAL DERIVATIVES

C. Sensory placodes

1. Nose

2. Eye (choroid and sclera from mesoderm)

3. Ear (middle ear from endoderm, ossicles from mesoderm)

4. Lateral line organs


Arey, L. B. 1934. Developmental Anatomy, 3rd Ed., Chaps. 14-17.

Brachet, A. 1921. Traité d’embryologie des vertébrés, Part H, Bk. 1, Chap. 4.

ACoghill, G. E. 1929. Anatomy and the Problem of Behavior.

Hertwig, O. 1906. Handbuch, Book II, Chaps. 5-10.

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

Keibel and Mall. 1910-1912. Human Embryology, Chaps. 14 and 16.

Kerr, J. G. 1919. Textbook of Embryology, Chap. 2.

Kingsley, J.S. 1926. Comparative Anatomy of Vertebrates.

Lillie, F. R. 1919. The Development of the Chick, 2nd Ed.

MeMurrich, J. P. 1923. The Development of the Human Body.

Strong, O. S. 1921. The Nervous System, being Chap. 17 of Bailey and Miller, Textbook of Embryology, 4th Ed.

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

Cite this page: Hill, M.A. (2024, April 15) Embryology Book - Introduction to Vertebrate Embryology 1935-3. Retrieved from

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