Book - The development of the chick (1919) 10

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Lillie FR. The development of the chick. (1919) Henry Holt And Company New York, New York.

Lille 1919: Introduction | Part 1 - 1 The Egg | 2 Development Prior to Laying | 3 Outline of development, orientation, chronology | 4 From Laying to Formation of first somite | 5 Head-fold to twelve somites | 6 From twelve to thirty-six somites | Part 2 - 7 External form of embryo and embryonic membranes | 8 Nervous system | 9 Organs of special sense | 10 Alimentary tract and appendages | 11 The body-cavities, mesenteries and septum transversum | 12 Later development of the vascular system | 13 Urinogenital system | 14 Skeleton | Appendix

Part II The Forth Day to Hatching, Organogeny, Development of the Organs

Chapter X The Alimentary Tract and its Appendages

The origin of the alimentary canal and of its various main divisions and appendages has been considered in preceding chapters. The subsequent history will now be taken up in the following order:

  1. The mouth and oral cavity.
  2. The pharynx and its derivatives.
  3. The oesophagus, stomach and intestine.
  4. The liver and pancreas.
  5. The respiratory tract.

The history of the yolk-sac and allantois was considered with the embryonic membranes (Chap. VH); the detailed history of the mesenteries will be taken up in connection with the body cavities (Chap. XI).

I. Mouth and Oral Cavity

The oral cavity may be defined embryologically as that part of the alimentary canal formed on the outer side of the oral plate. Anatomically, however, such a definition is unsatisfactory both because it is impossible to determine the exact location of the oral plate in late stages, and also because of the difference in extent of the ectodermal component in roof and floor of the mouth; the definitive mouth cavity includes part of the floor of the embryonic pharynx. It is, however, of interest to determine as nearly as possible the limits of the ectodermal component of the oral cavity. In the roof this is not difficult because the hypophysis, which arises just in front of the oral plate, retains its connection with the mouth cavity until definitive landmarks are formed. The median sagittal section of an eight-day chick (Fig. 148) shows that this point is situated almost immediately opposite to the glottis, that is, between the palatine and tubal fissures in the roof (cf. Fig. 175). In the floor the extent of the ectodermal component is much less. If the tongue is entirely a pharyngeal structure (in the embryological sense) the limit of the ectoderm would lie near the angle between the tongue and the floor of the mouth. In the side walls the boundary must be near the lines uniting the dorsal and ventral points as thus determined.

Fig. 175. — Floor and roof of the mouth of the hen. The jaw muscles were cut through on one side, the lower jaw disarticulated and the entire floor drawn back. Corn. H., Cornu of the hyoid. Fis. pal., Palatine fissure. Fis. Tub.,

Tubal fissure. Mu., cut surface of jaw muscles.

We have already considered the formation of the boundaries of the mouth (Chap. VI and Chap. VII), and of the palate (Chap. IX, page 299). These data need not be repeated, so we have left to consider only the development of the beak, egg-tooth, tongue, and oral glands.

Beak and Egg-tooth

The beak is a horny structure formed by cornification of the epidermal cells around the margins of the mouth: the egg-tooth is a mammiform hard structure with pointed nipple (Figs. 176 and 177) situated on the dorsum of the upper jaw near its tip (cf. Fig. 150). Its function is to aid in breaking the shell-membrane and the shell itself at the time of hatching; shortly afterwards it is lost. It is, therefore, an organ concerned with a single critical event in the life of the individual; nevertheless fully developed like the instinct of its use, needed only for the same critical event. Though its structure is different from that of the beak, it develops in connection with the latter, and the two will, therefore, be considered together.

Fig. 176. — Outline of the upper jaw of a chick embryo of 18 days' incubation. (After Gardiner.)

E. T., Egg tooth. L. gr., Lip groove.

The formation of the egg-tooth begins on the sixth day from an area situated in the middle line near the tip of the upper jaw, distinguishable in the living embryo by its opacity, which contrasts with the translucency of the surrounding parts; in profile view, the area is seen to be slightly elevated. In sections the appearance is found to be due to an accumulation of rounded ectodermal cells lying between a superficial layer of periderm of several layers of cells, and the subjacent mucous layer of the epidermis (Fig. 177). Without losing their rounded shapes this mass of cells gradually assumes the form of the egg-tooth by the fourteenth day. The overlying layer of periderm is lost during the act of hatching. During their differentiation the cells of the egg-tooth secrete an intercellular substance of horny consistency in which intercellular protoplasmic connections are found. The protoplasm of the cell-bodies themselves becomes densely packed with granules, apparently also of a horny nature, and the boundaries of the cells and outlines of the nuclei become indistinct.

Fig. 177. — Transverse section through the upper jaw of a chick embryo of 11 days. (After Gardiner.)

E. T., Egg tooth. H. Horn. L. gr., Lip groove. Pd., Periderm. T. R., Tooth ridge.

Reptiles with a horny egg-shell are provided with a true dentinal tooth on the premaxilla, which has the same function as the egg-tooth of birds and of those reptiles that have a calcareous shell (crocodiles, turtles, and Trachydosaurus). The latter is, however, as we have seen, a horny structure, and therefore not a tooth morphologically. Rose therefore proposes the term Eischwiele" for the horny toothlike structure, to distinguish it sharply from the real egg-tooth.

The formation of the upper beak begins in the neighborhood of the egg-tooth and spreads towards the tip and the angle of the mouth. Similarly, in the lower jaw the differentiation begins near the tip. It is a true process of cornification, that takes place beneath the periderm and involves many layers of cells. It is therefore preceded by a rapid multiplication of cells of the mucous layer of the epidermis. Soon after the appearance of the horn a groove appears a little distance above and parallel to the margin of the upper beak, extending from the anterior end a short distance backwards (Fig. 176). In sections, this appears as an invagination of the epidermis; a similar but shallower invagination appears on the lower beak. In the upper beak the lips of the invagination fuse together and thus close the groove; in the lower beak the groove flattens out and disappears. These grooves correspond in many respects to the grooves that form the lips of other vertebrates, and they may be interpreted as a phylogenic reminiscence of lip-formation.


All existing species of birds are toothless, but some of the most ancient fossil birds possessed well-developed teeth; it is natural, therefore, to expect that rudiments of teeth might be found in the embryos of some existing birds. In the early part of the nineteenth century some observers interpreted papillae on the margin of the jaws of certain young birds as rudimentary teeth; these were, however, shown to be horny formations, and therefore not even remotely related to teeth. Gardiner was one of the first to call attention to a thickening of the ectoderm forming a ridge projecting slightly into the mesenchyme, just inside the margin of the jaw of chick embryos about six days old (Fig. 177). The ridge disappears shortly after cornification sets in. Gardiner discussed the possibility of this representing a stage in tooth formation, and rejected the interpretation. Rose, however, has found the same ridge still better developed in embryos of the tern and ostrich, and identifies it very positively with the tooth-ridge or first step in the formation of the enamel organ of other vertebrates. It seems probable that this is the case, and that in this ridge we have the very last stage of the disappearance of teeth.

The Tongue. The tongue develops from two primordia in the floor of the embryonic pharynx, one situated in front of, and the other behind the thyroid diverticulum. The former, or tuberculum impar, becomes manifest on the fourth day as a slight rounded swelling situated between the lower ends of the first and second visceral arches. The swelling is bounded behind by a groove that has the ductus thyreoglossus for its center, and in front by a shallow groove, that represents the frenulum, on the posterior margin of the mandibular arches. The second primordium, or jjars copularis, arises just behind the thyroid and includes the lower ends of the second visceral arches, a small part of the lower ends of the third, and the region between these arches. According to Kallius the tuberculum impar forms only the center of the fore part of the tongue, and the lateral parts arise from two folds that form right and left of it (lateral tonguefolds). The tuberculum impar thus expanded and the pars copularis constitute two very distinct components in the development of the tongue.

Soon after the closure of the thyroid duct the two tongue components become confluent, but the zone of junction remains visible for a long time as a groove (cf. Fig. 148). Moreover the epithelium of the forward component soon becomes thickened and stratified, while in the pars copularis the epithelium remains thin and simple for a long time. With the elongation of the jaws the tip of the tongue grows forward above the frenulum (Fig. 148) and the shape of the entire organ conforms itself to the shape of the mouth cavity.

Figure 175 shows the tongue of the adult fowl. The anterior half is pointed and horny and is bounded from the posterior half by a double crescent whose posterior convexity is beset with horny spines. It seems probable that the anterior portion is derived from the precopular part, though this has not been demonstrated by continuous observation. Cornification of the precopular part sets in about the eighth day, and the early thickening of the epitheUum of this part already referred to is undoubtedly the first stage in the process.

The development of the musculature of the tongue has not been followed. The development of the skeletal parts is considered under the head of the skeleton.

Oral Glands. The following oral glands occur in the hen: 1, lingual glands; 2, mandibular glands; 3, glands opening at the angle of the mouth; 4, palatine glands in the neighborhood of the choanse. The only account of their development known to me is the brief one of Reichel. All the glands begin as solid ingrowths of the mucosa, which may branch more or less, and secondarily acquire a lumen. Their development begins relatively late. The mandibular glands appear first on the eighth day as a series of solid ingrow^ths of the mucosa extending on both sides of the base of the tongue forward to near the mandibular symphysis. They are still mostly solid on the eleventh clay, and very slightly branched, if at all. The lingual glands arise beneath the lateral margin of the tongue and grow up on each side of the lingual cartilage towards the upper surface where they branch out. They begin to form on the eleventh day. No glands form on the upper surface of the tongue. The glands of the angle of the mouth appear on the eleventh day, in situ, as slight epithelial ingrowths. Their further history has not been followed. Anterior and posterior palatine glands can be distinguished; the first in front of the choanse, the latter at the sides of and behind the choanse. They begin to appear after the eleventh day.

II. Derivatives of the Embryonic Pharynx

The pharynx, which is such an extensive and important region of the early embryo owing to the development of the visceral arches and clefts, becomes relatively much reduced in the process of development, though of course it becomes much larger absolutely. In the adult it is a somewhat ill-defined cavity from which the oesophagus leads away posteriorly, and which is confluent with the mouth anteriorly. The tubal fissure opens in its roof and the glottis in its floor. During the course of development, however, certain more or less persistent structures form from its walls, or from the epithelium of the pouches. Although these are relatively inconspicuovis organs in the adult, they are of considerable morphological importance, being of very ancient origin and common to the whole series of vertebrates. They are the thyroid body or gland, the thymus, the postbranchial or suprapericardial bodies, and certain epithelial vestiges.

Fate of the Visceral Clefts

The times of opening and closing of the visceral clefts have been already given (pp. 176 and 177). The later history of the first visceral pouch has been described (p. 297). The second, third, and fourth pouches retain their connections with the corresponding ectodermal grooves for a long time during the thickening of the visceral arches. The consequence is, that not only the pouches, but also the ectodermal furrows, are drawn out into long epithelial tubes, and the original closing plate is thus deeply invaginated. In the case of the second cleft the tube ruptures and begins to degenerate on the sixth day, leaving no remnants. In the case of the third and fourth clefts the ectodermal components become solid on the sixth day, and form strands (funiculi prcecervicales) connecting the entodermal pouches with the sinus cervicalis. These strands are subsequently broken through and disappear. Parts of the entodermal pouches, however, persist in the thymus, suprapericardial bodies and other epithelial remains. (See below.)

Thyroid. The thyroid sac (median thyroid of authors) loses all connection with the pharyngeal epithelium on the fourth day, and on the seventh day it becomes divided in two massive lobes placed bilaterally (see Fig. 178). These then migrate backwards on each side of the trachea towards the hinder end of the derivatives of the third visceral pouch (Verdun) and become lodged in the junction of the subclavian and common carotid arteries, where they are found in the adult just internal to the jugular vein.

The so-called lateral rudiments of the thyroid, or postbranchial bodies, are histologically entirely different from the thyroid proper. They are described below.

Visceral Pouches

The second visceral pouch leaves no derivatives in the adult; during the fourth day, however, a considerable thickening of the epithelium appears on its dorsal and posterior aspect, near its opening into the pharynx; though this disappears very soon, it may be considered to represent the thymus II of Selachia and Anura.

The third visceral pouch loses its connection with the pharynx by atrophy of its internal portion between the seventh and eighth days, and its intermediate portion persists as an epithelial pocket on the ventral face of the jugular vein (Fig. 178). This pocket soon divides into dorsal and ventral moities of which the former develops into the chief part of the thymus (thymus III) and the latter into the so-called epithelial vestige III. (See below.)

The fourth visceral pouch likewise separates from the pharynx on the seventh day, and furnishes from its dorsal portion the thymus IV, and from its ventral portion epithelial vestige IV. (See below.)

Fig. 178. — The derivatives of the embryonic pharynx of the chick. (After Verdun from Maurer.) A. Of 7 days' incubation. B. Of 8 days' incubation.

Ep. 3, Ep. 4, Epithelial bodies derived from the third and fourth visceral pouches. J., Jugular vein, p'br (V)., Postbranchial bodies derived from the fifth visceral pouch. Ph., Pharynx. Th. 3, Th. 4, Parts of the thymus derived from the third and fourth visceral pouches respectively. T'r., Thyroid. Ill, IV, third and fourth visceral clefts.

The fifth pouch (postbranchial body) likewise becomes isolated on the seventh day as a closed vesicle; its differentiation is considered below.

The Thymus

According to the above, the thymus of the chick has a double origin on each side; the main portion (thymus III) is derived from the dorsal wall of the intermediate part of the third visceral pouch. This soon elongates to form an epithelial cord extending along the jugular vein; a smaller portion (thymus IV) of the thymus is derived from a corresponding part of the fourth visceral pouch, and fuses with thymus III (Fig. 178).

In the young chick the thymus forms a voluminous tract of lobulated aspect, extending the entire length of the neck; later it atrophies and in old subjects one finds only traces of it. (Verdun.)

Epithelial vestiges are formed from the ventral wall of the intermediate portions of the third and fourth visceral pouches; these come to lie together at the hinder end of the thymus in the base of the neck. They are found in the adult near the lower pole of the thyroid (Fig. 178).

The postbranchial bodies have been called lateral rudiments of the thyroid; in their differentiation, however, they do not form thyroid tissue, but two main kinds of epithelial tissues similar to the tissues of the thymus and epithelial vestiges respectively. They are to be regarded, therefore, as a fifth pair of visceral pouches, for which there are other reasons, as we have seen before. The constituent elements, however, do not separate as in the case of the third and fourth visceral pouches, but form a rather illdefined mass situated a short distance behind the thyroid (Fig. 178).

The epithelial derivatives of the embryonic pharynx in the chick are, therefore; 1. thyroid; 2. thymus (from III, IV); 3. epithelial vestiges (from III, IV); 4. postbranchial bodies, including thymus V and epithelial vestiges V. The thyroid develops in essentially the same manner in all vertebrates. In the case of the thymus it may be said in general that more visceral pouches are concerned in the lower than in the higher vertebrates.

III. The GEsophagus, Stomach and Intestine

During the third and fourth days a very pronounced lateral curvature of the alimentary canal develops, the convexity being turned to the left and the concavity therefore to the right. The part involved extends from the posterior portion of the oesophagus to the end of the duodenum. As the duodenum is at first very short, the stomach is the part principally affected at the start. The depth of the mesogastrium (dorsal mesentery of the stomach) is considerably increased by the displacement ; in the region of the greatest curvature it descends directly in the middle line, then bends sharply to the left and is attached to the dorsal wall of the stomach; the accessory mesentery arises at the bend. (See Chap. XL) The stomach does not rotate on its long axis so as to carry the attachment of the mesogastrium to the extreme left, as in mammals; in the chick the lateral bending of the stomach appears to be uncomplicated by any such rotation. The curvature leaves a large space within it to the right containing the meatus venosus and liver, in short, the entire median mass of the septum transversum.

The main divisions of the intestine are marked out by their position, size-relations and structure before the closure of the yolk-stalk; thus on the third day the oesophagus appears as a constricted portion immediately behind the pharynx, and the stomach as a spindle-shaped enlargement behind the oesophagus; the duodenum is indicated at the same time by the hepatic and pancreatic outgrowths. The form of the intestine on the sixth day is illustrated in Figure 179. Behind the stomach, the intestine forms two loops descending ventrally. The first or duodenal loop is relatively slightly developed at this time, and forms an open curve just beneath the right lobe of the liver. Its ascending limb rises to a high dorsal position just behind the liver, and bends sharph^ to enter the descending limb of the second loop. This bend or duodeno-jejunal flexure (X, Fig. 179) is a relatively fixed point in the growth of the intestine, and marks the boundary between the duodenum and succeeding parts of the small intestine. The second loop descends deep into the umbilical cord, and the yolk-stalk is attached to its lowermost portion. A bilateral swelling at the upper end of its ascending limb is the primordium of the caecal processes, and marks the anterior end of the large intestine, which passes in a slight curve to the cloaca. In the subsequent growth of the intestine the fixed point referred to above at the hinder end of the duodenum is held in its place, and the duodenal loop in front of it simply becomes longer without forming secondary convolutions; the pancreas comes to lie in this loop. The second loop, on the other hand, forms numerous secondary convolutions (Fig. 180) which lie at first in the umbilical cord, but which are gradually retracted (seventeenth to eighteenth day) into the abdominal cavity.

Fig. 179. — Viscera of a chick embryo of 6 days, seen from the right side. (After Duval.)

All., Allantois. Au. r., Right auricle. B.a., Bulbus arteriosus, c. pr., Csecal processes. D. L., Loop of the duodenum. Giz., Gizzard. Lg. r., Right lung. Li., Liver. R., Rectum, t. R., Tubal ridge. V., Ventricle. W. B., Wolffian body. Y. St., Yolk stalk. X., Duodcno-jejunal flexure.

Fig. 180. — Viscera of a chick embryo of 17 days' incubation from the right side. (After Duval.)

Am., Attachment of amnion to umbilical stalk. Li. r., 1., Right and left lobes of the liver. Pc, Pancreas. U. St., UmbiHcal stalk. Other abbreviations same as Fig. 179.

The two intestinal caeca begin to grow out as finger-shaped processes from the swelling already referred to, about the seventh day, and rapidly attain considerable length. The large intestine elongates only about in proportion to the growth of the entire embryo.

Having thus noted the general gross anatomy of the embryonic intestine, we may next note a few details concerning some of its divisions. The history of the mesenteries is considered in Chapter XI).

(Esophagus. Owing to the rapid elongation of the neck the oesophagus quickly becomes a long tube. On the sixth day its lumen becomes very narrow, and on the seventh day completely occluded immediately behind the glottis, owing to proliferation of the lining cells. On the eighth day the occluded portion extends only a short distance behind the glottis: it is compressed dorso- vent rally and extended laterally throughout the occluded region (Fig. 181). On the eleventh day it is open again along its entire length. The crop arises as a spindle-shaped dilatation of the fx^sophagus at the base of the neck; on the eighth day it is about double the diameter of the parts immediately in front of and behind it (Fig. 150). No detailed account of its development exists.

Fig. 181. — Photograph of a transverse section through the oesophagus and trachea of an 8-day chick. Cop. H., Copula of the hyoid. (Es., (Esophagus. Tr., Trachea. Ven. jug., Jugular vein.


It is well known that the stomach of birds exhibits two successive divisions, the pro vent riculus and the gizzard, the former of which has a digestive function and is richl}^ provided with glands, while the latter has a purely mechanical function, being provided with thick muscular walls, within which is the compressed cavity lined on each side by tendinous plates.

On the third day of incubation, the divisions of the stomach are not recognizable, either by the form of the entire organ or by the structure of the walls. On the fifth day, however, the first indications of the formation of the compound glands of the pro vent riculus may be seen in the cardiac end; the posterior or pyloric end occupies the extreme left of the gastric curve and forms the rudiment of a blind pouch projecting posteriorly, that develops into the gizzard. On the sixth and seventh days this pouch expands farther in the same direction (cf. Fig. 179), and a constriction forms between the anterior portion of the stomach, or pro vent riculus, and the gizzard, as thus marked out. The gizzard grows out farther, to the left and posteriorly, at the same time undergoing a dorso-ventral flattening, owing to the formation of the large muscle-masses. According to this account, therefore, the. greater curvature of the gizzard would represent the original left side of the portion of the embryonic stomach from which it is derived, and the original right side would be represented by the lesser curvature.

The large compound glands of the proventriculus are indicated on the fifth or sixth days as slight depressions of the entoderm towards the mesenchyme; on the seventh day these become converted into saccular glands with narrow necks (Fig. 182). Each sacculus becomes multilobed about the twelfth or thirteenth days, and each lobulus includes a small number of culs-de-sac, lined with a simple epithelium. The last subsequently become tul)ular, and the original sacculus then represents the common duct of a large compound gland. (See Cazin.)

The simple, tubular glands of the gizzard begin to form about the thirteenth or fourteenth day, and the lining of the gizzard is simply the hardened secretion of these glands; it is thus essentially different from cuticular and corneous structures of the surface of the body. According to Cazin, the glands of the gizzard are formed as folds and culs-de-sac excavated in the thickness of the original epithelial wall, by elevations of the subjacent connective tissue. It should be noted finallv, that from the eie:hth day on, the surface of the mucosa, both in the proventriculus and in the gizzard, is covered with a thick layer of secretion; subsequently replaced in the gizzard by the corneous lining.

Fig. 182. — Photograph of a transverse section of an 8-day chick through the region of the proventriculus and tip of the heart. A. coel., Coeliac artery. A. o. m., Omphalomesenteric artery. Cav. om., Cavum omenti. Cav. pc, Pericardial cavity. Coel., Coelome. Gon., Gonad. Lig. g-h., Gastro-hepatic ligament. M. D., Miillerian duct. Mtn., Metanephros. p'c, Membranous pericardium. Pr'v., Proventriculus. S'r., Suprarenal. V. c. i., Vena cava inferior. Ven., Ventricle of heart. V. h. 1., Left hepatic vem. V. s'c, Subcardinal vein. V. umb., Umbihcal vein.

Large Intestine, Cloaca, and Anus. The cloaca of the adult is a large chamber opening to the exterior by the anus; it consists of three divisions: the proctodseum or terminal chamber is capable of being clo.sed by the sphincter muscle, the bursa Fabricii opens into its dorsal wall, and it is separated by a strong circular fold from the intermediate section of the cloaca or iirodaeum; this is a relatively short division of the cloaca which receives the renal and reproductive ducts in its dorsal wall by two pairs of openings; it is bounded from the larger anterior division, coprodseum, by a rather low circular fold; the coprodaeum passes gradually, without a sharp line of division, into the rectum.

The early embryological history of these parts has been considered in the preceding chapters. The condition on the fourth day is shown in the accompanying figure (Fig. 183) representing a sagittal section of the hind end of the embryo. The cloaca is the large terminal cavity of the intestine, closed from the exterior by the cloacal membrane, in which the entoderm of the floor of the cloaca is fused to the superficial ectoderm at the base of the tail. The line of fusion is a long, narrow median strip, extending from just below the neck of the allantois to the hinder end of the cloaca. Leading out from the cloaca ventrally, in front of the cloacal membrane, is the neck of the allantois, and dorsal to this, the large intestine. Though not shown in the figure, it may be noted that the Wolffian ducts open into the cloaca behind and dorsal to the opening of the rectum.

Fig. 183. — Median sagittal section of the hind end of a chick embryo on the fourth day of incubation. (After Gasser from Maurer.)

All., Allantois. Am., Tail fold of amnion, cl. M., Cloacal membrane. CI., Cloaca. N'ch., Notochord. n. T., Neural tube. R., Rectum. Y. S ., Wall of yolk-sac.

The appearance of the cloaca in a longitudinal section does not, however, give an adequate idea of its form. The anterior portion of the cloaca which receives the rectum, stalk of the allantois and Wolffian ducts is expanded considerably in the lateral plane, and thus possesses a large cavity. The posterior portion, on the other hand, is greatly compressed laterally and the cavity is extremely narrow. During the fifth day the walls of this part of the cloaca become actually fused together, and its cavity obliterated, or rendered virtual only (Fig. 184). Thus the anterior part of the cloaca is prolonged backwards by a median plate which is continuous ventrally with the cloacal membrane.

Fig. 184. — Frontal section through the region of the cloaca of a 5Way chick embryo.

an. F., Anal fold. B. F., Bursa Fabricii. CI., Cloaca. Coel., Coelome. Rect., Rectum. W. D., Wolffian duct. X., Posterior angle of the body-cavity; the epithelium is invaginated and folded so as to simulate a glandular structure.

This plate was interpreted by all the earlier observers (up to Wenckebach) as the hypertrophied cloacal membrane. It is, however, not difficult to demonstrate in good series of sections, that this is not the case; the cloacal membrane forms only a small part of this plate, and its ectodermal component is thin.

During the fifth and sixth days, vacuoles appear in the posterior and dorsal part of the fused portion of the cloaca, and these soon run together in the uppermost part, but remain as a chain of vacuoles ventrally (Fig. 184). The vacuolated portion is the primordium of the bursa Fabricii and its duct. Its cavity, which is extremely narrow and ill-defined at this time, may be regarded as a re-establishment of the cavity of the posterior division of the embryonic cloaca; its communication with the anterior portion of the cloacal cavity is soon closed.

At this stage the lining epithelium of the rectum is much thickened, and the lumen has therefore become narrow (Fig. 184).

During the seventh day the conditions change very rapidly and on the eighth day the relations are as shown in Figure 185. The anterior portion of the original cloaca, or urodseum, has become compressed in an antero-posterior direction; the allantois leads off from it anteriorly and ventrally, and the rectum with its cavity now obliterated is attached to its anterior face; the dorsal extension, above the rectum (see Fig. 185), is related to the urinogenital ducts. The bursa Fabricii has now a well-defined cavity that no longer communicates with the urodseum. The tissues surrounding the cloacal membrane have grown out to form a large perianal papilla, and the cloacal membrane is therefore invaginated; its direction also is so altered that the invaginated cavity or proctodseum now lies behind it; the bursa Fabricii is on the point of opening into the highest point of the proctodseum. Vacuolization of the tissue between the cloacal membrane and the urodasum indicates its subsequent disappearance.

At eleven days (Fig. 186) the general arrangement is essentially the same, but there are important differences in detail. The bursa Fabricii has now become a long-stalked sac, opening into the proctodseum at the level of the urodseal membrane. The latter is still quite a thick plate, but the vacuoles in it foreshadow its final rupture. The lower end of the large intestine is perfectly solid, and higher up, somewhat vacuolated. (The solid stage begins on the seventh day.) The urinogenital ducts open into the urodseum above the solid end of the large intestine. It will be seen, therefore, that the urodseum is transformed into a passageway between the urinogenital ducts and the allantois, being closed anteriorly by the solid large intestine and posteriorly by the urodseal (cloacal) membrane.

Fig. 185. — Photograph of the region of the cloaca in a median sagittal section of an 8-day chick.

All., Allantois. An., Anus. B. F., Bursa Fabricii. caud. A., Caudal artery. Int., Intestine. N'ch., Notochord. p. P., Perianal papilla. Rect., Rectum. Ur'd., Urodseum.

During the twelfth and thirteenth days, the vacuoles in the upper part of the large intestine flow together and re-establish the cavity, but the lower end still remains closed by a solid plug of cells; immediately anterior to the latter the large intestine is dilated, and this apparently corresponds to the coprodaeum of the adult cloaca. Even on the seventeenth day the large intestine appears to be still closed at its lower end, and the urodseal membrane still persists as a plug of vacuolated cells. (Gasser.) Both plugs must, however, disappear soon after.

It would thus appear that the urodgeum only of the adult cloaca corresponds to the embryonic cloaca; the proctodjeum is certainly derived from an ectodermal pit, and it is probable that the coprodseum represents the enlarged lower extremity of the embryonic large intestine. The bursa Fabricii is an entodermal structure derived from the posterior portion of the embryonic cloaca.

Fig. 186. — Chick embryo of 11 days, sagittal section through the region of the cloaca. Reconstructed from several sections. (After Minot.)

All'., Ascending limb of the allantois. AH"., Descending limb of the allantois. An., Anal invagination., Urodeal membrane. Art., Umbihcal artery. B. F., Bursa Fabricii. b. f., Duct of the bursa. Clo., Cloaca. Ec, Ectoderm. Ent., Entoderm of the rectum. Ly., Nodules of crowded cells, probably primordia of lymphoid structures in the wall of the large intestine. W. D., Wolffian duct.

IV. The Development of the Liver axd Paxcreas

The Liver

The anterior and posterior liver diverticula, described in Chapter VI, constitute the rudiments from which the substance of the liver is derived. A third diverticulum is distinguished by Brouha as the right posterior diverticulum; this is an early outgrowth of the posterior diverticulum. Hepatic cylinders arise from both primary diverticula at an early stage, and these, branching and anastomosing, soon form a basket-work of liver tissue around the intermediate portion of the meatus venosus. The anterior diverticulum alone extends forward to the anterior end of the meatus, and it even encroaches on the sinus venosus, as we have already seen; in the posterior part of the meatus venosus, on the other hand, the liver tissue is derived entirely from the posterior diverticulum. The mesenchyme in the interstices of the hepatic framework is replaced almost immediately by blood vessels that empty into the meatus, and thus appear as branches of the latter.

Fig. 187. — Reconstruction of gizzard, duodenum, and hepato-pancreatic ducts of a chick embryo of 124 hours. (After Brouha.)

D. ch., Ductus choledochus. D. cy., Ductus cysticus. D. h. cy., Ductus hepato-cysticus. D. h. d., Dorsal or hepato-enteric duct. Du., Duodenum. G. bl., Gall bladder. Giz., Gizzard. Pa. d., Dorsal pancreas. Pa. v. d.. Right ventral pancreas. Pa. V. s., Left ventral pancreas.

The gall-bladder is a very early formation, arising from the hindermost portion of the posterior hepatic diverticulum, as a distinct bud about the stage of 68 hours (Fig. 103), and forming a pyriform appendage at 84 hours. It may reasonably be regarded as derived from the most posterior portion of the primitive hepatic gutter, an interpretation that agrees with the condition found in more primitive vertebrates.

At the stage of 68 hours (cf. Fig. 103B), the anterior and posterior diverticula proceed from a common depression of the ventral wall of the duodenum, the ductus choledochus. By means of an antero-posterior constriction, the latter becomes much more clearly defined as development proceeds (Fig. 187); there arise from it also the right and left ventral primordia of the pancreas (see below), so that it receives at this stage four main ducts, viz.: the right and left ventral pancreatic diverticula and the cephalic and caudal hepatic diverticula. On the sixth day these four ducts obtain independent openings into the duodenum and the common bile duct thus ceases to exist. The relations thus established are practically the same as in the adult.

As the caudal hepatic diverticulum grows out it carries the attachment of the gall-bladder with it, so that the latter is then attached to the caudal diverticulum, which is thus divided in two parts, a distal or ductus hepato-cysticus, and a proximal or ductus cystico-entericus. That portion of the liver arising from the cephalic diverticulum is thus without any connection with the gall-bladder. There seem, however, to be anastomoses between the ductus hepato-cysticus and the original cephalic duct (ductus hepato-entericus) in the adult, lying in the commissure of the liver; the embryological origin of these appears, however, to be unknown. In the course of the development, the openings of the two original ducts into the duodenum come to lie side by side instead of one behind the other, and the original cephalic duct (ductus hepato-entericus) appears to be derived mainly from the left lobe, and the ductus cystico-entericus mainly from the right lobe of the liver. The actual distribution is, however, by no means so simple; the mode of development of the lobes of the liver (see below) would explain a preponderant distribution of the cephalic duct to the left, and the caudal duct to the right lobe.

The liver is primarily an unpaired median organ. Its division into right and left lobes is therefore secondary and has no fundamental embryological significance. The factors that determine its definitive external form are the following: (a) the relative power of growth of its various parts; (6) limitation of its extension to the septum transversum and its connections; (c) the limitations of space in the coelome.

Bearing these principles in mind, the growth of the liver may be described as follows: three primary divisions succeeding one another in a cranio-caudal direction, may be distinguished at an early stage, viz., an antero-dorsal division, abutting on the postero-dorsal part of the sinus venosus, formed by the anterior end of the cephalic hepatic diverticulum; an intermediate division, surrounding the meatus venosus in which both cephalic and caudal hepatic diverticula are concerned; and a postero- ventral division, beneath the posterior end of the meatus venosus and the right omphalomesenteric vein, formed exclusively by the caudal diverticulum.

The growth of the liver causes expansion of the median mass of the septum transversum in all directions, excepting anteriorly, and the substance of the liver extends more or less into all the connections of the latter, viz., the lateral mesocardia, the lateral closing plates associated with the umbilical veins, the primary ventral ligament, the mesentery of the vena cava, the gastrohepatic ligament, and that part of the hepatic portal vein formed by the right omphalomesenteric vein.

At the stage of 96 hours the anterior division spreads out in the lateral mesocardia behind the Cuvierian ducts nearly to the lateral body-wall on each side. The intermediate division, on the other hand, lies largely on the right side of the middle line, owing to the displacement of the stomach to the left and the meatus venosus to the right. A small lobe is, however, pushing itself to the left beneath the gastro-hepatic ligament. The posterior division lies entirely on the right ventral side of the hinder end of the meatus venosus and right omphalomesenteric vein, as far back as the dorsal anastomosis. There are, of course, no sharp lines of demarcation between the divisions, so that in general it may be said that the liver substance tends more and more to the right side of the body from its fairly symmetrical anterior end backwards.

The lines of development of the liver are thus marked out. On the sixth day the anterior division is larger on the left than on the right side, owing no doubt to the incorporation of the sinus venosus into the right auricle, thus leaving more room for the liver on the left side. Passing backwards in a series of sections to the region of the center of the meatus venosus, we find the liver larger on the right than on the left side, being centered around the meatus, but a small lobe extends over to the left side A^entral to the stomach. The posterior division, again, is confined to the right side and ends in a free right lobe projecting caudally to the region of the umbilicus. The division of the liver into right and left lobes thus takes place on each side of its primary median ligaments, dorsal or gastrohepatic, and primary ventral; expansion being inhibited in the median line by the stomach above and heart below, it takes place on both sides, but particularly on the right side where there is more space.

The reader is referred to Chapter XI for description of the origin of the ligaments of the liver and the relations of the liver to the pericardium and other structures; also to Chapter XII for description of its blood-vessels.

The histogenesis of the liver should be finally referred to. This organ is remarkable in possessing no mesenchyme in the embryonic stages (Minot, 1900); but from the start the hepatic cylinders are directly clothed with the endothelium of the bloodvessels, so that only the thickness of the endothelial wall separates the hepatic cells from the blood in the sinusoids. The hepatic cylinders have been described as arising in the form of solid buds from the primary diverticula; the buds first formed branch repeatedly, forming solid buds of the second, third, etc., orders, and wherever buds come in contact they unite, forming thus a network of solid cylinders of hepatic cells. The solid stage does not, however, last very long, for on the fifth day it can be seen that many of them have developed a small central lumen by displacement of the cells. Thus there gradually arises a network of thick-walled tubes instead of solid cylinders, and the whole system opens into the primary diverticula from which it arose.

The Pancreas

The pancreas arises as three distinct entodermal diverticula, the origin of which has been already described, and has correspondingly in the adult three separate ducts oj^ening into the duodenum. (Two pancreatic ducts is the rule in Gallus, according to Gadow in Bronn's Thierreich.) Of the three pancreatic diverticula, the dorsal one arises first (about 72 hours) then the right ventral slightly earlier than the left ventral (about 96 hours). The two latter arise from the common hepatic diverticulum near its jimction with the duodenum (Fig. 188). The differentiation of the three parts is essentially similar, and proceeds naturally in the order of their origin. Solid buds arise from the ends of the diverticula, and these branch repeatedly in the surrounding mesenchyme, but do not anastomose; the final terminations of the buds form the secreting and the intermediate portions the various intercalated and excretory ducts that form a branching system opening into the main ducts.

Fig. 188. — Transverse section through the duodenum and hepatopancreatic ducts of a chick embryo of 5 days. (After Choronschitzky.) Ao., Aorta, cav. F., Caval fold. Coel., Coelome. D. hep. 2, 2 a, 2 b, Posterior hepatic diverticulvun and branches of same. Du., Duodenum. Li., Substance of Hver. M'st., Dorsal mesentery. Pa. d., Dorsal pancreas. Pa. v. d.. Right ventral pancreas. Pa. v. s., Left ventral pancreas. Spl., Spleen. V. c. p., Postcardinal vein. V. H., Vena lienalis. V. o. m. d.. Right omphalomesenteric vein. V. o. m. s., Left omphalomesenteric vein.

The successive stages in the development of the pancreas may be stated thus (following Brouha): At 124 hours the two ventral pancreatic ducts pass anteriorly and a little to the left, crossing the cephalic hepatic duct which lies between them. They are continued into ramified pancreatic tubes which already form two considera])le glandular masses. The right ventral pancreas is united by a very narrow bridge to the dorsal pancreas, and the latter is moulded on the left wall of the portal vein, while its excretory duct has shifted on the left side of the duodenum nearer the ductus choledochus. At 154 hours the duct of the dorsal pancreas is still nearer to the others, and the three pancreatic ducts enter a single glandular mass, the dorsal portion of which, derived from the primitive dorsal pancreas, is moulded on the left wall of the portal vein, and is continued into a smaller ventral portion formed by the fusion of the two ventral pancreases..

Subsequently, the pancreatic lobes fill up the duodenal loop (Figs. 179 and 180), and elongate with this so as to extend from one end of it to the other in the adult; the three ducts open near the termination of the duodenum (end of distal limb) beside the two bile ducts.

V. The Respiratory Tract

The origin of the laryngotracheal groove and the paired primordia of the lungs w^as described in Chapter VI. At the stage of 36 somites the laryngotracheal groove includes the ventral division of the post branchial portion of the pharynx, which is much contracted laterally so as to convert its cavity into a deep and narrow groove. This communicates posteriorly with right and left finger-shaped entodermal diverticula (the entodermal lung-primordia) extending into the base of the massive pearshaped mesodermal lung-primordia attached to the lateral walls of the oesophagus. The mesodermal lung-primordia are continuous with the accessory mesenteries, as described in Chapter XI; and by them attached to the septum transversum.

Bronchi, Lungs and Air-sacs

The primitive entodermal tubes form the primary bronchi, in which two divisions may be distinguished on each side, viz: a part leading from the end of the trachea to the hilum of the lung (extra-puhiionary bronchus), and its continuation within the lung, extending its entire length (mesobronchus) . All the air passages of the lung, and the airsacs, arise from the mesobronchi by processes of budding and branching, enlargement of buds to form air-sacs, and by various secondary anastomoses of branches. The mesobronchi are surrounded from the first by a thick mass of mesenchyme, covered of course towards the body cavity by a layer of mesot helium. In the early development the mesenchyme of the lung-primordia grows so rapidly as to provide adequate space for the branching of the mesobronchi entirely within the mesenchymal tissue.

Although the development of the lungs of the chick was studied by several earlier investigators, our principal reliance in this subject rests on the beautiful and complete study by Locy and his students.

We may note the general topographical development as follows: The expansion of the lungs takes place into the pleural cavities; they therefore raise themselves from their surfaces of attachment, oesophagus and pleuroperitoneal membrane, and project in all directions, but especially dorsally and anteriorly (Fig. 189). We may thus distinguish free and attached surfaces; the latter is nearly a plane surface and on the whole ventral in position, and the free arched surfaces are dorsal. However, it should be remembered that the pleuroperitoneal membrane which forms the attached surface, lies at first in a sagittal plane, and only secondarily becomes frontal. In successive stages, the attached surface of the lung (pleuroperitoneal membrane) rotates from a sagittal to an approximately frontal plane (Chap. XI). An anterior lung lobe grows out in front and dorsal to the mesobronchus, beginning at six days, and the extra-pulmonary bronchus thus acquires a ventral insertion into the lung.

Stages in the development may be described as follows: At 96 hours, the bronchi arise from the end of the trachea, ventral to the oesophagus and pass back on either side of the latter, describing near their centers a rather sharp curve that brings the dorsal ends to a higher level than the oesophagus. A very slight dilatation at the extreme end of the mesobronchus is usually interpreted as the beginning of the abdominal air-sac.

At six days the mesobronchus within the hmg describes a course nearly parallel to the oesophagus as far as the middle of the lung; in this part of its course it lies near the median surface and ascends very slightly. About the middle of the lung it makes a sharp bend, and passes toward the lateral and dorsal surface of the lung; here it enters a considerable thin-walled dilatation from which it is continued straight backwards by means of a second curve, and ends in the same slight thick-walled dilatation that we noted on the fourth day. There are thus three very distinct divisions of the mesobronchus which we may name the anterior, the middle, and the posterior.

Fig. 189. — Photograph of transverse section through the lungs of an 8-day chick embryo. A. A. d., Right aortic (systemic) arch. D. art. d., s., Right and left ductus arteriosi. Ent'b.l., Branches of first entobronchus. M. ph pc, Pleuropericardial membrane. Mes'b. d., s., Right and left mesobronchia. (Es., (Esophagus. Pc, Pericardial cavity, pi. Cav., Pleural cavity. Rec. p. e. s., Left pneumato-enteric recess. V. c. a., Anterior vense cavse.

Four evaginations arise on the sixth day from the mesial wall of the anterior division of the mesobronchus, which is otherwise unbranched. These represent the entobronchi; they arise in antero-posterior order, and the first is therefore the largest. The part of the mesobronchus from which they arise will form the vestibulum of the adult lung.

Later on the same day the ectobronchi, six in number, begin to arise from the dorsal surface of the dilated portion of the middle division of the mesobronchus. Other independent outgrowths of the same division of the mesobronchus are the so-called laterobronchi and dorsobronchi (Locy). These four groups of out-growths may be classed as secondary bronchi (Fig. 191).

On the ninth day (Fig. 191) the first entobronchus has formed a number of branches in the anterior lobe of the lung, and two of its terminal twigs, one in the antero-dorsal, the other in the anteroventral tip of the lung, are slightly dilated and project as primordia of the cervical and interclavicular air-sacs respectively. The second entobronchus is also subdivided several times; its terminal branches extending to the dorsal surface of the lung. The third entobronchus bends ventrally, and from its base a narrow canal extends into the pleuroperitoneal membrane, where it expands into the anterior thoracic air-sac, which is much the largest of the air-sacs at this time.

Between the eighth and eleventh days, numerous tertiary bronchi (parabronchi) arise from the secondary bronchi (Fig. 190). These are considerably smaller than the tubes from which they arise, and are extremely numerous, radiating from all parts of the secondary bronchi towards the free surfaces and interior of the lungs. They are embedded in the mesenchyme of the lung, which is already marked out into areas hexagonal in cross-section, with the parabronchi in the centers, by the developing pulmonary blood-vessels.

From the twelfth to the eighteenth days parabronchi of different origin meet and fuse in a most extensive fashion, thus forming an intercommunicating net-work of tubes throughout the lung. Air-capillaries finally arise from the parabronchi in the centers of the hexagonal areas and form an anastomotic net-work arising from and surrounding the parabronchi. This completes the system of tubes arising from the secondary bronchi; but another system, that of the recurrent bronchi, develops from the air-sacs which we now go on to consider.

Fig. 190. — Transverse section through the lungs of a chick embryo of 11

a. til. A. S., Anterior thoracic air-sac. Ao., Aorta. Aur. d., s., Right and left auricles. B. d., s., Right and left ducts of Botallus. F., Feather germs. Li., Liver. P. C, Pericardial cavity, p. p. M., Pleuroperitoneal membrane. P V , Pulmonary vein. Par'b., Parabronchi. PI. C, Pleural cavity. Pt. C, Peritoneal cavity. R., Rib. Sc, Scapula. V. d., s., Right and left ventricles.

The expanding hmgs nearly fill the pleural cavities on the eleventh day. Subsequently, the pleural cavity is obliterated by fusion of the free surfaces of the lungs with the wall of the pleural cavities. Thus it happens that the dorsal surfaces of the lungs of the adult have no peritoneal covering," although this is denied by other authors.

The air-sacs are terminal expansions of entobronchi or of the mesobronchus (Fig. 191). From all of them with the exception of the cervical sac there grow bronchial tubes which connect with parabronchi secondarily within the lung proper. Owing to their method of origin, and also to the fact that the current of air through them in the functional lung is from the air-sacs, these tubes are known as recurrent bronchi. The lungs of birds thus differ from those of other vertebrates in having no terminal alveoli, containing residual air; there is instead a system of communicating tubes through which the air flows.

Fig. 191. — The air passages of the limg of the chick early on the ninth day of incubation. A Lateral view; B. Mesial view. (After Locy and Larsell.) Abd. S., Abdominal Air-sac. Ant. Th. S., Anterior thoracic air-sac. Br., Bronchi. Cerv. S., Cervical air-sac. Dors., Dorsibronchi. Ect. 1, Ect. 2, etc., First to fourth Ectobronchi. Ent. 1, Ent. 2, etc., First to fourth Entobronchi. Lat. 3, Third laterobronchus. Lat. moi.; Mes. moi.. Lateral and mesial moieties of the interclavicular air-sac. Rec. Br., Recurrent bronchi.

The abdominal air-sacs do not undergo any considerable expansion until after the eighth day (cf. Fig. 191). Then they push through the hinder end of the pleuroperitoneal membrane, now fused with the lateral body- wall, and penetrate the latter just beneath the peritoneum. About the tenth day they begin to expand into the abdominal cavity just behind the liver, thus evaginating the peritoneum. The left sac is somewhat larger than the right. The expansion goes on lapidly and by the thirteenth to the fifteenth day they have reached the hinder end of the body cavity, and have akeady expanded into it so far as to form fusions with the mesentery. Recurrent bronchi begin to develop from their base about the ninth day.

The cervical sacs appear early from an anterior branch of the first entobronchus (Fig. 191). They form no recurrent bronchi (Locy).

The interclavicular sac, which is single in the adult, arises from two sacs on each side, a lateral moiety from the first entobronchus, and a mesial moiety from the third. These four parts fuse to form the single sac of the adult (Locy). These sacs form recurrent entobranchi.

The anterior thoracic sac forms about the seventh day as a dilatation of the ventral wall of the third entobronchus projecting into the pleuroperitoneal membrane near its median edge; it thus lies just lateral to the pneumato-enteric recesses. From this position it expands laterally and posteriorly in the pleuroperitoneal membrane and thus gradually splits it in two layers (Fig. 190, 11 days).

The posterior thoracic air-sac arises from the third laterobronchus somewhat later than the others, and grows at first through the hinder portion of the pleuroperitoneal membrane to enter the lateral body wall. In its subsequent expansion, it splits the posterior portion of the pleuroperitoneal membrane, as the anterior thoracic air-sac does the anterior portion of the same membrane. Anterior and posterior thoracic air-sacs then come into contact, forming a septum. Both form recurrent bronchi.

The lower layer of the pleuroperitoneal membrane, split off from the upper layer by expansion of anterior and posterior thoracic air-sacs, constitutes the oblique septmn. The most posterior portion of the oblique septum, however, is derived from the peritoneum of the lateral body wall by expansion of the posterior thoracic air-sacs behind the pleuroperitoneal membrane.

Like the abdominal air-sacs, the remainder expand rapidly, particularly from the fourteenth day on, among the thoracic viscera, and fuse intimately with these and the walls of the body cavity in a few days, the coelomatic fluid being in the meantime absorbed. The interclavicular air-sac grows out to form the subscapular air-sac and at the time of hatching has approached close to the humerus." (Selenka.)

The Laryngotracheal Groove

The embryonic primordium of the larynx and trachea communicates at first along its entire length with the postbranchial division of the pharynx (72 hours). At 96 hours the hinder portion of the groove is already converted into a tube lying beneath the anterior end of the oesophagus; this is the beginning of the trachea; the anterior part of the original groove represents the larynx, and its opening into the pharynx the glottis. It is not clear whether the trachea arises as an outgrowth of the hinder end of the laryngotracheal groove, or from the hinder portion of the groove itself, by constriction from the pharynx. At 96 hours the lumen of the lower end of the trachea and adjoining portion of the two bronchi is obliterated by thickening of the walls; this is, however, a very transitory condition.

The growth of the trachea in length is extremely rapid, keeping pace, of course, with the elongation of the neck. At six days the trachea is a long epithelial tube with thick wahs branching into the two bronchi at its lower end. At its cephahc end the lumen opens into a considerable cavity, representing the larnyx; the glottis appears to be closed by a plug of epithelial cells continuous with the sohd wall of the oesophagus. At eight days the lumen of both larynx and glottis is completely closed by the thickened epithehum; at eleven days the cavity of the lower end of the larynx is re-established, and the cell mass at the upper end is converted into a mesh-work by vacuoUzation; the lips of the glottis still show a complete epithelial fusion. Thus it is apparent that the cavity of the larynx is estabhshed by the formation of vacuoles within the soUd cell-mass, and by their expansion and fusion. I cannot say how soon the glottis becomes open.

The development of the laryngotracheal apparatus, including the cartilages and muscles, has not been specially investigated in the chick. In general, it can be said that the parts external to the epithehum arise from the mesenchyme, which begins to condense around the epithelial tube on the fifth day. On the eighth day the glottis forms a decided projection into the pharynx. Distinct cartilaginous rings in the trachea are not visible on the eighth day, but are well formed on the eleventh day. As regards the syrinx it has been established by Wunderhch for Fringilla domestica that the tympanic cartilage arises from the lower tracheal rings. The origin of the musculature of the syrinx is not known.

Lille 1919: Introduction | Part 1 - 1 The Egg | 2 Development Prior to Laying | 3 Outline of development, orientation, chronology | 4 From Laying to Formation of first somite | 5 Head-fold to twelve somites | 6 From twelve to thirty-six somites | Part 2 - 7 External form of embryo and embryonic membranes | 8 Nervous system | 9 Organs of special sense | 10 Alimentary tract and appendages | 11 The body-cavities, mesenteries and septum transversum | 12 Later development of the vascular system | 13 Urinogenital system | 14 Skeleton | Appendix

Cite this page: Hill, M.A. (2019, October 18) Embryology Book - The development of the chick (1919) 10. Retrieved from

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