THE CLOACA IN HUMAN EMBRYOS 19 



their topography for it is not possible as yet to differentiate 

 mesodermal epithelium from that of entodermal origin. 



The widening of the ventral cloacal segment is made evident 

 from the following measurements. A comparison of the inter- 

 Wolfiian width and the width of the dorsal segment gives a pro- 

 portion of 20 : 9 in B; 28 : 8 in C ; 36 : 9 in D ; and 46 : 9 in E. 

 This means that the ventral cloacal segment increases markedly 

 in width while the dorsal segment remains about the same. Ob- 

 served in transverse sections, this active widening gives the im- 

 pression of two lateral mesodermic folds encroaching upon the 

 cloacal lumen (Retterer's idea.) The widening of the ventral 

 cloacal segment is most marked at the level of the Wolffian orifices 

 and progresses equally both upward and downward giving the idea 

 coronal section of the cloaca in Model E a diamond shape. 



Born has stated that the trigonum may be of mesodermic origin. 

 We have measured Model F and compared it wdth Model 30 and 

 find the following: that the inter-ureteral distance (between 

 the orifices) is greater in F (0.55 mm.) than in no. 30 (0.34) ; while 

 the Wolffian ducts in the former are almost twice as far apart as 

 in the latter. This implies that the ureter is not displaced lateral- 

 ward from the duct but that both are displaced medianward, and 

 that if there is any part of the bladder to which the mesoderm has 

 definitely not contributed it is the trigonum. 



We are of the opinion that the ventral segment of the cloaca 

 probably forms the anlage for the bladder including the urachus, 

 as was stated in Born's article, and that the cloacal segment of 

 the Wolffian ducts (mesodermic) contributes some part. How 

 much is impossible to determine. When the cloacal membrane 

 extends as far upward as the dermal navel and persists as an epi- 

 thelial membrane, we have the origin of the cases of complete 

 bladder exstrophy which take in the whole ventral wall of the blad- 

 der and urachus. The present champion of the allantoigen origin 

 of the bladder is not supported by our investigations. Rather we 

 agree with Disse ('02) who states that Nagel misunderstood the 

 relations found in the earlier stages through examination of em- 

 bryos too far advanced in the development. 



A/^ 



