DEVELOPMENT OF THE URINARY AND SEXUAL ORGANS. 995 



the mesogastrium in the second month; the latter are at first larger than the kid- 

 neys. (3) The pancreas develops in the same way as the salivary glands, and 

 originally in two rudimentary parts, a dorsal and a ventral. It is, however, 

 not yet indicated in the fourth week. (4) The liver appears early, beginning as 

 an evagination by means of two hollow, primitive ducts, which break up to form 

 the biliary passages. At their periphery they exhibit solid cellular masses, the 

 liver-cells, which thus also are derived from the hypoblast. As early as the second 

 month the liver is large; it secretes as early as the third month. According 

 to Kupffer a single large gland, which in lower animals extends throughout the 

 length of the mid-bowel, corresponds to the liver, spleen and pancreas. From 

 this the three organs are subsequently differentiated. (5) In birds two 

 small sacs are formed from the hind-gut. (6) The fetal respiratory organ, the 

 allantois, has already been considered (p. 974). The inner surface of the ccelom, 

 the surface of the bowel and of the mesentery become covered with a serous mem- 

 brane, the peritoneum. This encloses the bowel, which for a time remains simple, 

 in a duplicature or fold. On the stomach, which, at first, occupies a vertical 

 position as a spindle-shaped dilatation of the digestive tract, this fold is known 

 as the mesogastrium. Subsequently the stomach comes to lie upon its side, and 

 in such a way that the left aspect becomes the anterior, the right the posterior. 

 In this way, the insertion of the mesogastrium, which at first was directed pos- 

 teriorly, toward the vertebral column, becomes directed toward the left. The 

 line of insertion is formed by the region of the greater curvature, which subse- 

 quently becomes more markedly curved. From the greater curvature, the meso- 

 gastrium is prolonged as a pouch-shaped appendage (Fig. 390,7 and //, 5 *) , the omen- 

 tal bursa, so far downward as to extend over the transverse colon and the coils of 

 small intestine ///, TV). As the mesogastrium consisted originally of two layers, 

 the duplicature formed from it, the omental bursa, must consist of four layers. 

 In the fourth month the posterior surface of the omental bursa becomes adherent 

 to the surface of the transverse colon. 



DEVELOPMENT OF THE URINARY AND SEXUAL ORGANS. 



Urinary Organs. The urine-forming gland originates developmentally from 

 three organs, wnich succeed one another in function: (i) The rudimentary kid- 

 ney (pronephros) . (2) The primitive kidney (mesonephros) . (3) Definitive 

 kidney (metanephros) . 



1. The rudimentary kidney is in the amniota (and salachians) only a rudi- 

 mentary embryonal organ; in the remaining vertebrates, it still exhibits some 

 functional activity in the embryonal or larval period; it is here the provisional 

 embryonal kidney (as the Wolffian body is for the amniota). In bony fish 

 canals can be differentiated that begin anteriorly within the abdominal cavity 

 by means of funnel-shaped openings and unite to form a common excretory duct 

 that empties into the cloaca. In front of the funnels lies the glomerulus, whose 

 secretion is carried outward in the canals. 



2. In the amniota the primitive kidney (mesonephros) is the fetal uriniferous 

 organ. From this there arises as the first formation, in the chick on the second 

 day, in rabbits on the ninth day, the duct of the primitive kidney or Wolffian 

 duct (Fig. 392, /, W), which is formed from cells of the ectoderm and at first is 

 solid, to the side of and somewhat behind the primitive vertebrae and extending 

 from the fifth to the last primitive vertebras. Seated within this duct, there arise 

 in the mesoblast from the level of the liver downward a series of tubules, w r hich 

 in the chick are believed at first to open free at their other extremities into the 

 peritoneal cavity, and which become transformed into structures similar to the 

 glomerulus of the kidney by the ingrowth of vascular convolutions into their 

 extremities. The tubules elongate, become twisted into convolutions, and increase 

 by the addition of newly formed communicating accessory tubes. The caudal 

 extremity of the Wolffian duct is at first closed; its lower extremity, which lies 

 in a fold projecting into the abdominal cavity (plica urogenitalis), opens (in the 

 rabbit on the eleventh day) into the urogenital sinus. In the anamnia the 

 primitive kidney is the permanent uriniferous gland. 



3. Just above the outlet of the Wolffian duct the definitive kidney (meta- 

 nephros) arises in an upward direction as the renal duct. The elongated duct 

 divides at its upper extremity like a shrub. These accessory branches finally 

 form convolutions. Each canal at its extremity assumes the form of a pedun- 

 culated hollow rubber-ball, with a cup-shaped depression, into which the vascular 



