20 AUGUSTUS G. POHLMAN 



In conclusion we may sum up the points that have been investi- 

 gated in this paper in a review of the development of the cloaca 

 and of the structures in relation with it. The article has been 

 wi'itten largely as a check on the earlier stages in the development 

 in order that the writer might have first hand information regard- 

 ing this important segment of the enteron. 



SUMMARY 



The cloaca is a closed entodermal sac and is never normalh' 

 opened to the outside (amniotic cavity) through rupture of the 

 cloacal membrane. It represents that portion of the enteron 

 which lies caudal to the opening of the allantois and may not be 

 separated from the tail gut until the formation of the cloacal mem- 

 brane, when the inferior border of the membrane forms an arbi- 

 trary limit between the cloaca and the tail gut. The cloacal mem- 

 brane arises from the primitive streak area. The primitive streak 

 at the time of its ventral displacement in the formation of the tail 

 fold, is composed of three germ layers, and the cloacal membrane 

 is formed in situ through disappearance of the intervening meso- 

 derm. Normally, the cloacal membrane occupies the caudal 

 half of the ventral cloacal border, but in some cases, the membrane 

 may extend as far cephalward as the dermal navel.. This latter 

 condition has an important bearing on the embryology of bladder 

 exstrophy and epispadias. The increase in width and thickness of 

 the cloacal membrane is concomitant with the corresponding 

 changes in the ventral segment of the cloaca, and is due to ecto- 

 dermic proliferation. The membrane is displaced in the process 

 of cloacal division through development of the precloacal meso- 

 dermic tissue to form the genital eminence, and at the same time, 

 the original membrane comes to lie deeper in the substance of the 

 embryo. The division of the cloaca begins before the arrival of 

 the Wolffian ducts, and is effected by the downgrowth of a coronal 

 septum. The division, when completed, results in a ventral, (or 

 bladder-urogenital sinus,) segment and a dorsal, (or rectal,) segment 

 which are about of equal size as far as their antero-posterior meas- 

 urements are concerned. The same holds true of the resultants 



