THE CLOACA IN HUMAN EMBRYOS 11 



mality suggested itself but on careful study, it was decided that 

 the first evidences of cloacal division were at hand. 



Model B, other than a better developed tail gut, a slightly higher 

 position of the Wolffian ducts and a shorter cloacal membrane 

 agrees with Keibel's model of the 4.2 mm. embryo. Keibel's 

 model shows this same evidence of division; a frontal septum be- 

 tween the allantois and hind gut slipping down to a level approach- 

 ing the upper limit of the cloacal membrane. The writer has 

 found no embryo where the cloacal membrane approaches the 

 level of the dermal navel, and will consider this point later. 



Model C. {No. 9; Mall no. 80; 5.0 mm..) This model shows 

 marked degeneration of the tail gut. The allantois is narrow 

 lumened, while the widely lumened ventral cloacal segment lies 

 ventral to the hind gut and dorsal segment, and is separated from 

 them by peritoneum. The Wolffian ducts open in the same situ- 

 ation as in the foregoing model, and present dorsal diverticulae, 

 the renal buds. The ventral portion of the cloaca, especially the 

 area above the cloacal membrane, is markedly widened and flat- 

 tened. The lateral furrow of Model A was not present. This 

 model conforms to the Keibel model of a 6.5 mm. embryo (a con^ 

 siderably older stage if we go by the greatest length rule). The 

 cloaca has lost its more or less even width and attains its greatest 

 lateral diameter at the level of the Wolffian orifices. There is no 

 evidence of displacement of the Wolffian ducts from their primitive 

 position in relation with the upper limit of the cloacal membrane. 



Model D {No. 12; Mall no. 2; 7.0 mm.) This model fills in a 

 gap in the Keibel series which may be considered quite an im- 

 portant one. The hind gut and all that pertains to the dorsal 

 segment of the cloaca has retained an even caliber, while the ven- 

 tral segment has widened progressively. The division of the clo- 

 aca may be traced to the level of the Wolffian orifices. The Wolf- 

 fian ducts are much better developed and the renal anlage has 

 resolved itself into distinct ureter and kidney segments. The seg- 

 ment of Wolffian duct between the orifice of the ureter and the 

 cloaca is relatively shorter. The renal anlagen have assumed the 

 position of dorsal convergence (mentioned by Keibel) but this is 



