30 YOUNG TWIN HUMAN EMBRYOS WITH 17-19 PAIRED SOMITES. 



THE URINOGENITAL SYSTEM. 



The urinogenital system is in a very early stage of its development. There is a defi- 

 nite area of the coelomic wall, the middle plate of the body, between splanchnopleure and 

 somatopleure, which extends back as far as the sixteenth somite on each side, forming the 

 Wolffian or urinogenital fold. Beyond the sixteenth somite this portion of the wall is 

 not formed, the splanchnopleure and somatopleure meeting at a narrow angle. All the 

 nephrogenic tissue is found in the urinogenital fold or located at the angle behind it. This 

 fold begins gradually anteriorly, increasing in size as it proceeds caudad and then decreasing 

 again rapidly in the last few segments. 



The description following is for the right side (text figs. 3 and 5) in Embryo VI, which 

 shows the greatest development in this embryo. The structures exhibit a close correspond- 

 ence to those described by Felix in Keibel and Mall's Human Embryology. 



In the first three somitic segments no trace whatever of nephric tissue can be identified, 

 although tubules have been described in these segments, Dandy (1910) finding them in the 

 first two in an embryo of 7 segments. If they were ever present in Embryo VI they have 

 completely degenerated. In segments 4 and 5 there are small, compact masses of cells 

 embedded in loose tissue in the nephric region, their appearance very much suggesting parts 

 of tubules, and in the sixth, seventh, and eighth segments remains of tubules even more 

 distinct are to be found, and in these segments is also to be found a portion of the inter- 

 mediate cell mass from which these tubules arose. In the sixth segment this is attached 

 only to the coelomic epithelium, and in all the remaining segments it is connected with both 

 the somite and the coelomic epithelium. Mrs. Gage (1905) , describing an embryo of 4.3 mm. 

 with 28-29 somites, says: "The connection which probably existed between the myotome 

 and the coelom through the intermediate cell mass which gives rise to the blastema forming 

 the tubules is of an earlier stage than these here considered." She says she has observed 

 this in the chick, and it has been found in the rabbit. I can confirm her supposition of 

 its presence in the human embryo of early stages, as it is quite evident in a few of the seg- 

 ments in this present embryo and also in its twin. 



At the cephalic boundary of the ninth segment the Wolffian duct begins and also a 

 well-developed pronephric tubule, which arises from the dorsal or lateral wall of the inter- 

 mediate cell mass, passes dorsally, and then turns caudad and opens into the Wolffian duct. 

 Starting from the intermediate cell mass of the tenth segment are two large pronephric 

 tubules, one beginning at the cephalic edge of the segment, the other in the caudal half. 

 Projecting into the ccelom in this region, just ventral to the attachment of the intermediate 

 cell mass to the coelomic epithelium, is a fairly solid, club-shaped protuberance with a 

 slightly narrowed stalk, in which can be seen a blood-vessel. I think that this may unhesi- 

 tatingly be called an external glomerulus. It is the only one occurring in this embryo. 

 Janosik (1887) also found a single external glomerulus in a 3 mm. embryo. In the eleventh 

 segment there is a pronephric tubule, not so large as those in the tenth, and in the twelfth 

 is a still smaller one. These tubules all run caudally under the Wolffian duct, which lies 

 dorsal and lateral to them, and they open into the duct in the segment behind the one in 

 which they arise. The Wolffian duct and the tubules, and the intermediate cell masses 

 of these segments, all contain lumina. 



Behind the twelfth segment there is a sudden change in the nephric system. The 

 Wolffian duct (text fig. 5) receives no tubules and rapidly decreases in size, loses its lumen, 

 and becomes flattened and crescentic in outline, lying applied to the dorsolateral surface 

 of the nephrogenic cord. It approaches the ectoderm and ends just beneath and in con- 



