THE CLOACA IN HUMAN EMBRYOS 15 



THE ECTODERMAL CLOACA 



An ectodermal cloaca has been stated by Keibel to be present 

 only in traces, if at all. None of the embryos examined showed 

 a surface depression, and we may therefore critically examine 

 Nagel's fig. 1 and his contention concerning the cloacal membrane. 

 It will be seen that in fig. 1, while the embryo shows a cloacal 

 division comparable to our Model E; the phallus is developed to 

 the extent of that found in Model F ; w^hile the cloacal membrane 

 is ruptured. None of the embryos of this stage, or younger than 

 this stage, showed rupture of the cloacal membrane, and we may 

 therefore agree with Keibel that the cloacal membrane remains 

 intact normally as long as it persists as such. Nagel believes that 

 the rupture of the membrane is essential or where would the secre- 

 tion of the mesonephros be passed? We may answer this ques- 

 tion. The ventral cloacal segment and the allantois do not show 

 the degree of widening necessary to accommodate the urinary excre- 

 tion, and we call to mind Hill's work on the pig embryo ('04) and 

 our own ('04) on the human embrj'o, in both of which it is con- 

 clusively shown that the mesonephros is actively degenerate long 

 before the kidney is developmentally fitted to take up a urinary 

 function. Cases of atresia urethrae commonly occur without 

 bladder distension ; monstrosities may go to term without kid- 

 neys and with no undue persistence of the mesonephros; and ex- 

 perimentally, it has been shown that there is no evidence of urinary 

 excretion into the amniotic fluid even in term children. We do 

 not believe it necessary to assume that the mesonephros function- 

 ates as an organ of excretion in the placental mammals, and do 

 not consider Nagel's contention for a functional rupture of the 

 cloacal membrane as a serious one. 



THE TAIL GUT 



The tail gut is that portion of the enteron that lies caudal to the 

 lower limit of the cloacal membrane and before the formation of 

 the cloacal membrane has no definite line of demarkation from the 

 cloaca proper. The tail gut is represented by a conical sac in 



