THE CLOACA IN HUMAN EMBRYOS 13 



THE CLOACAL MEMBRANE 



While authorities seem to agree that the cloacal membrane 

 arises from the primitive streak, or at least that portion which 

 has been displaced ventrally in the formation of the tailfold, they 

 disagree on the point whether this part of the primitive streak 

 consists in applied layers of ecto- and entoderm, or whether it is 

 composed of all three germ layers. Stated in an simple manner, 

 is the cloacal membrane formed dorsally as an epithelial membrane 

 or is it formed ventrally through disappearance of intervening 

 mesodermal tissue? One of the earliest stages in the formation 

 of the tail fold is. found in the Spee reconstruction ('96) of embryo 

 Gle (2.0 mm.). A schematic sagittal section is presented in his 

 fig. 1, and emphasis is laid on the point that the primitive streak 

 is composed of all three germ layers. No. 1 of the present tabu- 

 lation was reconstructed by Mall ('97), and while a slightly older 

 stage in the development, shows the cloacal sac limited ventrally 

 by all three germ layers. No. 2 presents a similar condition. 

 The first embryo of this series to show a cloacal membrane is 

 no. 3 (3.5 mm.) which is quite a little farther advanced in the 

 development than no. 2; unfortunately the transition stages 

 are wanting. It is the writer's opinion therefore that the view 

 held by Keibel is the correct one, and that while the cloacal 

 membrane is derived from the primitive streak area, it is 

 formed in situ as an epithelial membrane on the ventral cloacal 

 surface, naturally after the formation of the tail fold. 



The sagittal width of the cloacal membrane varies somewhat 

 with the stage in the development. Born's statement that it 

 extends as far cephalward as the dermal navel is based on Keibel's 

 model of a 4.2 mm. embryo ('88). A somewhat similar condition 

 is noted in two of the embryos of this series. The writer does 

 not believe this to be the normal relation, but rather one which 

 through persistence may have a decided bearing on bladder ex- 

 strophy and epispadias. The epithelial plate, if it extends to the 

 dermal navel, and persists in this relation, separates the precloacal 

 tissue into two lateral halves, and accounts quite satisfactorily for 

 the deficience in the abdominal wall and for the gutter on the upper 



