THE CLOACA IN HUMAN EMBRYOS 21 



of the division of the cloacal membrane — the urogenital and anal 

 membranes ; both of which are about of the same length and both 

 of which are displaced from the surface through upgrowth of the 

 surrounding mesoderm. The ventral cloacal segment enlarges pro- 

 gressively in length and in width during the division of the cloaca, 

 and as the widening is most marked at the orifices of the Wolffian 

 ducts, the cloacal segment of the ducts may contribute to this 

 active enlargement, or in other words, some of the epithelium of the 

 bladder may be of mesodermic origin. The displacement of the 

 ureter on the Wolffian duct, and the later upward displacement of 

 its orifice from that of the duct, is due in part to the disappear- 

 ance of the cloacal segment of the duct. The orifices of both ure- 

 ter and Wolffian duct are however shifted medianward after the 

 completion of the cloacal division, so that the trigonum may be 

 spoken of as distinctly of entodermic origin. The greater part 

 of the bladder and urachus are developed from the ventral segment 

 including the mesodermic contributions from the Wolffian ducts. 

 The allantois probably contributes no part in the formation of the 

 bladder and retains its original relation to the lower border of 

 the dermal navel. The urogenital membrane ruptures normally 

 when the division of the cloaca is completed, or in other words, 

 with its formation, and before the arrival of the Miillerian ducts 

 at the urogenital sinus. The contribution of the entoderm and 

 ectoderm in the formation of the urethra, together with the devel- 

 opmental relations of the bulbovestibular glands, will be considered 

 in a later paper. 



