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



tact with the latter at the beginning of the sixteenth segment. Whether there is actual 

 fusion can not be stated, but it may be emphasized that immediately in this vicinity, around 

 the end of the Wolffian duct, the ectoderm shows absolutely no change and no preparation 

 for further formation of the Wolffian duct and its growth caudad. The evidence here 

 corresponds exactly with that obtained by Felix (1912) and supports his view of the forma- 

 tion of the Wolffian duct by fusion of the ends of pronephric tubules anteriorly, and the 

 caudal extension of it to the cloaca by growth of the tip, or splitting off of cells from the 

 nephrogenic cord; it also strengthens materially his statement that he would deny any 

 participation of ectoderm in the formation of the duct. This embryo comes midway 

 between two described by him, one of 13-14 segments, having no duct yet formed, and one 

 of 23 somites, where the duct nearly reaches the cloaca. 



From the thirteenth to eighteenth segment, inclusive, the character of the nephro- 

 genic tissues is similar, forming a large rounded mass of cells in each segment, connected 

 on one side by a narrow string of cells to the somite, on the other side to the ccelomic epi- 

 thelium. On them are small protuberances in the thirteenth and fourteenth segments, 

 suggesting the origin of tubules from them. These masses (text figs. 3 and 5) are all con- 

 tinuous, forming an unbroken nephrogenic cord, but there are at intervals small central 

 lumina, so that the cord is practically a fused series of vesicles. It is rather difficult to 

 determine the exact number of mesonephric vesicles present, as the lumina are not all 

 distinct, but there are either 10 or 11. There is thus no numerical agreement with 

 the number of segments in which they lie, a fact universally true in all human embiyos 

 described. Fusion with the ccelomic epithelium is very extensive, beginning at the seven- 

 teenth somite, and the line of division between the epithelium and nephrogenic cord is 

 marked by a furrow which gradually disappears by the time the unsegmented mesoderm 

 is reached. Marked fusion also occurs with the nineteenth somite, which is not entirely 

 formed, and the line of division between somite and nephrogenic cord is very soon lost, and 

 the intermediate cell mass, which here represents the nephrogenic cord, very soon also loses 

 its identity — the somitic plate, intermediate cell mass, and lateral plate all blending in one 

 common, undifferentiated mass of mesoderm. 



The fully developed tubules of the ninth to twelfth segments are called pronephric, 

 principally because the Wolffian duct is just in process of formation and is being formed by 

 their union — according to Felix (1912) mesonephric tubules never appear until after the 

 formation of the Wolffian duct — and also because other authors have found pronephric 

 tubules at this same level in young embryos possessing 23 paired somites or less. The 

 abrupt change from pronephric tubules developed as far as the twelfth segment to meso- 

 nephric vesicles beginning in the thirteenth is in thorough accord with the emphatic state- 

 ment of Felix that in the human embryo pronephric tubules are never developed caudal 

 to the twelfth segment, which is the first thoracic. 



As to the presence of openings of the nephric tubules to the ccelom (the nephrostomes 

 or trichter) there is some doubt, but it may be mentioned that in several segments there is 

 a cleft in the ccelomic epithelium at the point where the intermediate cell mass fuses with 

 it, and in some cases this even opens into the cavity of the intermediate cell mass. But 

 this may be an artifact, and indeed it is simulated by occasional breaks in the ccelomic 

 epithelium in other places, so giving rise to doubt as to its actual significance. There are, 

 however, two undoubted openings to the ccelom, each situated in a depression of the ccelo- 

 mic wall. These openings (text fig. 5) are the nephrostomes of the first two pronephric 

 tubules, one in the ninth segment, the other in the tenth. The connection of the inter- 



