14 AUGUSTUS G. POHLMAN 



surface of the phallus. In practically all of the embrj'os exam- 

 ined, the cloacal membrane did not extend higher than the level 

 of the Wolffian orifices. The membrane is normally shortest in 

 length at the time of its formation but attains its greatest length 

 in Model C. (0.29 mm.); in Model D (0.27 mm.); in Model E 

 (0.34 mm.). This increase is not marked when we consider that 

 these stages represent 5, 7 and 12 mm. embryos respectively. 



The width of the cloacal membrane is a rather indefinite quan- 

 tity to measure ; it widens progressively however with the widen- 

 ing of the ventral cloacal segment, and in the proportion of 5 in 

 B to 8 in C to 12 in D or in other words in proportion to the great- 

 est length measurement of the embryo. 



The position of the cloacal membrane is about parallel to the 

 long axis of the embryo in Models A, B and C. With the develop- 

 ment of the precloacal tissue, the cloacal membrane is displaced 

 so that its upper end is pushed caudalward — beginning in D; 

 more marked in E ; and culminating in F where the upper end of 

 the membrane has been displaced through practically 90° with the 

 caudal end as the fixed point of the radius. The displacement is 

 probably due to the active lengthening of the ventral cloacal wall 

 and to the development of the genital eminence. The membrane 

 is divided into two parts in Model F — a ventral or urogenital 

 membrane and a dorsal or anal membrane; both of which have 

 been displaced from the surface in the manner suggested by Born. 

 The anal membrane persists beyond the stages described in this 

 paper, and is dealt with in sufficient detail in Keibels' article. 

 The urogenital membrane ruptures with its formation and is of 

 particular interest in its relation to the position of the bulboves- 

 tibular glands ; a point to be brought in a paper which is to appear 

 shortly. It may be noted that in Model D the cloacal membrane 

 is much thicker in its midarea, and not infrequently one finds a 

 heaping up of cells in this region and a corresponding eminence 

 on the inner surface of the cloacal (intracloacal epithelial plug) . 

 The significance of this is not understood but it may be due to 

 the more active proliferation of the ectodermal cells which con- 

 tribute to the formation of the cloacal membrane. 



