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



mediate cell mass in segments caudad to this occurs at the point of a slight depression in 

 the wall of the coelom, but there is no definite, funnel-shaped opening present. 



The left side of this embryo (text figs. 3 and 5) is very similar to the right, but slightly 

 less developed. In the first 5 somitic segments tubal remains are very few and are small 

 and hard to distinguish from the surrounding mesenchyme. The intermediate cell mass 

 is recognizable from the sixth segment back to the unsegmented mesoderm. The first 

 recognizable tubule is in the caudal portion of the ninth segment, starting from the ccelomic 

 epithelium, and at this point there is a funnel-shaped depression forming the nephrostome. 

 The tubule soon separates from the epithelium and runs dorsally and then caudally, form- 

 ing the beginning of the Wolffian duct, there being no little blind end of the duct in front 

 of it, as on the right side. There are 5 pronephric tubules in all, the first being the only 

 one possessed of a nephrostome. The others are found, 2 in the tenth segment, 1 each in 

 the eleventh and twelfth; each has a lumen and unites with the Wolffian duct. The duct 

 is very large here, but rapidly decreases in size, becoming crescentic and solid, as it runs 

 caudally, and ending as a rapidly narrowing string of cells. There are three cells in the 

 section before the end, and one cell finally at the end, in contact with the ectoderm, at 

 exactly the same level as on the right side, namely, between the fifteenth and sixteenth 

 somites. 



Embryo V shows even much better than Embryo VI the development of the prone- 

 phros, which is nearly double the size of that found in Embryo VI, while the mesonephric 

 anlage contains only about half the number of vesicles found in the latter. This surprising 

 difference is the more remarkable because in all other organs the two embryos present an 

 almost completely equal and parallel development. In the matter of somites formed it 

 will be remembered Embryo VI has 18-19 and Embryo V has 17-18, and it would not be 

 expected that a difference in development of only one somite would mean so great a differ- 

 ence in the urinogenital system as is here manifest. 



The urinogenital folds extend from the third somitic segment, on the dorsal wall of the 

 pleuroperitoneal passage, to the caudal end of the sixteenth segment. The intermediate 

 cell mass is present from the fourth segment on the left and the fifth on the right, back to 

 the unsegmented mesoderm, and is connected to the somite and to the ccelomic epithelium 

 in each segment. Remains of tubules in front of the pronephros can not be seen on the 

 right side. The pronephros (text figs. 4 and 6) begins on this side with two fully formed 

 tubules in the seventh segment. Following this tubules occur as follows: 1 in the eighth, 

 2 in the ninth, 1 in the tenth, 2 in the eleventh, 2 in the twelfth, and 1 at the beginning of 

 the thirteenth segment, making a total of 11 tubules. In each case there is a distinct 

 trichter or nephrostome by which the tubule communicates with the ccelom. Each tubule 

 is budded dorsally from the intermediate cell mass, forming the principal tubule as 

 described by Felix (1912), the intermediate cell mass forming the supplementary tubule, 

 and by the fusion of the ends of the principal tubules the Wolffian duct is formed, which 

 therefore extends from the seventh segment. It receives no connections beyond the last 

 pronephric tubule, but extending caudally, overlying the nephrogenic cord, ends at exactly 

 the same point as the duct of each side in Embryo VI, namely, between the fifteenth and 

 sixteenth segments. 



Posterior to the pronephros the intermediate cell masses have fused to form a solid 

 string, the nephrogenic cord, which contains small cavities at intervals, forming meso- 

 nephric vesicles, of which there are 5, the last being in the seventeenth segment. 



On the left side, as in Embryo VI, development is not quite the same as the right, 

 being a little less (text figs. 4 and 6). There are 7 pronephric tubules, the first appearing 



