18 AUGUSTUS G. POHLMAN 



in the apparent length of the urogenital membrane is due to pro- 

 liferation of the ectodermal cells or to inclusion of the surface 

 ectoderm as Born states it. The difference in opinion on this 

 point will be brought out in a later article. 



The development of the ventral cloacal segment interests us 

 because it has to do with the formation of the bladder. Detailed 

 measurements of this region have been suppressed because of the 

 variable size of the cloaca proper, but the union of the allantois to 

 the cloaca seems to be fairly definite in Models C, D and E. In 

 all of these embryos the Wolffian ducts open a little above the 

 center of a line drawn from the caudal extremity of the cloacal 

 membrane to the apparent line of union of allantois with cloaca. 

 We have therefore no material reason for not believing, inasmuch 

 as this is the primitive relation of the Wolffian orifices and there 

 is no evidence of downward displacement of the allantois, that the 

 gradually constricting area above the Wolffian orifices is ventral 

 cloacal segment (see Model B). 



The active widening of the ventral cloacal segment, particularly 

 at the level of the Wolffian orifices, is evident to every observer 

 of the cross sections in this region, and in the dorsal convergence 

 of the ureters — a fact first noted by Lieberkiihn and Keibel. 

 It is evident also, as suggested by them, that the distal segment 

 of the Wolffian ducts may become incorporated into the anlage 

 for the bladder. This alone does not however account for the 

 lateral rotation of the ureter to gain the position noted in Model 

 F, nor does it explain the later upper displacement of the ureter 

 to its normal position in the bladder. 



It is seen in our models that the renal buds appear on the dorsal 

 aspect of the Wolffian ducts at some distance from the cloaca, and 

 the intervening segment of duct may be termed the ' cloacal seg- 

 ment' merely for convenience in the description. It is well 

 known that this gradual shifting of the ureter is not due alone 

 to the resorption of the cloacal segment of the Wolffian duct, for 

 when two ureters arise from the same duct, the one with the ori- 

 fice nearest the cloaca assumes the more lateral position. In 

 this region we have to deal with the relations as they appear from 



