YOUNG TWIN HUMAN EMBRYOS WITH 17 L9 PAIRED SOMITES. 33 



in the ninth segment, 2 caudad of the first on the right. Following it are 1 in the tenth, 

 2 each in the eleventh and twelfth, 1 in the beginning of the thirteenth. There is a aephro- 

 stome present for all except the last tubule. The Wolffian duct is similar to that of the right 

 side and extends caudally to exactly the same level. In front of the first tubule, for two 

 segments, doubtful remains of tubules are seen. The nephrogenic cord beginning in the 

 thirteenth segment contains 7 mesonephric vesicles, 2 more than on the right, the first being 

 in the thirteenth segment, the last in the eighteenth. 



The embryos standing nearest to these two in development of the urinogenital system 

 show many interesting points of comparison. Embryo Pfannenstiel III, modeled by Low, 

 the urinogenital system of which is specially described by Felix (1912), possesses 13-14 

 somites and the pronephros is just beginning its development. There are rudiments of 

 it in the second, fourth, and sixth segments, a separate tubule in the seventh, 2 united in 

 the eighth and ninth, and two pronephric ridges on each side of the body, united end to 

 end and connected to the tubules as far as the fourteenth somite. There is no Wolffian 

 duct. The presence of this pronephric anlage back as far as the fourteenth somite is inter- 

 esting and leads to the question whether pronephric tubules may not be found caudad of the 

 twelfth segment, which Felix has stated to be the extreme limit of their occurrence. This 

 would give a reasonable explanation of the conditions found by me, for although Embryo 

 VI has no tubules beyond the twelfth segment, Embryo V has one on each side in the 

 thirteenth. -There is a gap in the series described by Felix, in the very middle of which 

 come the two embryos here described, and then comes Felix's second example, the Meyer 

 embryo of 23 paired somites, modeled by Peter Thompson, Felix studying specially the 

 urinogenital system. Here there are 7 pronephric tubules, lying opposite the ninth to 

 twelfth somites, the first separate, the others united, forming the Wolffian duct, which 

 extends caudad to the twenty-second somite. From the thirteenth to the beginning of 

 the sixteenth somite lie 8 mesonephric vesicles in a chain, continuous from the latter point 

 to the unsegmented mesoderm with the nephrogenic cord. 



The above-mentioned embryo, although advanced 4-5 somites over the two described 

 in this paper, shows a nephric system intermediate between the two, more tubules and 

 fewer vesicles than Embryo VI, fewer tubules and more vesicles than Embryo V. The 

 most noticeable difference is the caudal extension of the Wolffian duct over 6 additional 

 segments, it reaching only to the sixteenth segment in both the twins. It will be noticed 

 that from a condition with no Wolffian duct at the 14-somite stage we pass to one in the 

 18-19 somite stage with a duct extending over from 7 to 9 segments and in the 23-somite 

 stage extending over 14 segments. There is thus a very rapid caudal growth of the free end 

 of the Wolffian duct, it soon reaching the unsegmented mesoderm, and from now on, as 

 records show, it keeps pace with the formation of the somites and reaches the cloaca at the 

 27-somite stage, although it does not penetrate until later. 



The urinogenital system has been described by MacCallum (1902) for an embryo of 

 19 somites, length 3.5 mm., which is thus almost exactly comparable in its general develop- 

 ment to the two here described. In this embryo MacCallum found a single duct lying in 

 the sixth to ninth segments, connected to the coelom at a depression opposite the sixth 

 somite. There was then a break, and opposite the tenth segment the Wolffian body began 

 and extended to the unsegmented mesoderm, having 13 thickenings on it, regarded as 

 developing mesonephric tubules. MacCallum interprets the small anterior mass as a 

 rudimentary pronephros. Evidently there is here one pronephric tubule and a short 

 -tret eh of Wolffian duct. This Wolffian duct has not half the length shown in the two 



