194 JOHN LEWIS BREMER 



found in the amount of excreting surface developed in these 

 embryos and the correlated differences in their intra-uterine 

 growth rates it seems probable that great variations exist, 

 which, if persistent in the adult, should certainly be carefully 

 considered in animal experimentation. 



THE ALLANTOIS 



If certain embryos excrete urine from the Wolffian body or 

 kidney during their entire intra-uterine life, and others do not, 

 there should be easily recognizable differences in the size of the 

 receptacle for this urine in the two classes. In spite of the inter- 

 est formerly taken in the time of the opening of the cloacal 

 membrane, as a possible passage for the urine into the amniotic 

 cavity, repeated chemical analyses of the amniotic fluid show 

 that it may contain only traces of urinary matter, and thus 

 make it extremely doubtful if this passage is ever used normally 

 for any length of time. Failing this external outlet, the urine 

 from both the Wolffian body and the kidney must pass to the 

 allantois. In birds and reptiles the allantois is a large sac 

 capable of containing a considerable quantity of fluid. The 

 same is true of certain mammals. According to Grosser, Hert- 

 wig, and others, the allantois is an enormous vesicle in the pig 

 and sheep; in the cat it is again of large size, though smaller 

 than in the former two animals. In the rabbit, on the other 

 hand, it "reaches no special expansion; it is here limited to the 

 area of the placenta."* In man and in the guinea pig, ''that is, 

 in those embryos which form no hollow allantois,"* only the 

 slender allantoic stalk is found; yet, as was mentioned above, 

 this narrow cavity seems to Keibel capable of storing a small 

 amount of fluid. Minot states that ''the allantois in man in- 

 creases very little in diameter after the second month, "^ the 

 time when the Wolffian body, as we have seen, ceases to be func- 

 tional. In the rat and mouse, even this small receptacle is 

 absent, as an entodermal allantois never develops. 



* Hertwig, loc. cit., vol. 2, p. 250. 

 "Minot, loc. cit., p. 355. 



