INTERRELATIONS OF THE MESONEPHROS, ETC. 203 



that the kidney of these animals also is capable of activity, but, 

 as Wertheinier points out, this does not prove that excretion does 

 normally take place, as in all of these experiments some dis- 

 turbance of the fetal circulation is inevitable. The question, it 

 seems to me, may be left open to further investigation, under- 

 taken with a knowledge of the possibility of a placental excretion 

 in certain animals. 



CONCLUSIONS 



The AVolfhan jjody or mesonephros is a gland of urinary 

 excretion. 



Mammalian embryos may be divided into two classes; those 

 which retain functional Wolffian bodies until the kidneys are 

 sufficiently developed to excrete urine, as is the case in birds and 

 reptiles, and those in which the Wolffian bodies degenerate be- 

 fore the kidneys reach functional ability. The first class includes 

 the pig, sheep, and cat; the second, the rabbit, guinea pig, man, 

 and rat. 



Within each of these classes individual animals show great 

 differences in the size and presumable excretory ability of the 

 Wolffian l;)odies, without regard to the length of their duration. 



The allantois is the receptacle of the urine formed within the 

 body of the embryo; it is present as a reservoir only in those 

 animals with an embryonic excretion, and its size varies with the 

 size of the Wolffian bodies and with their duration. The urethral 

 opening, though present, is not normally used for the passage of 

 fetal urine. 



In those animals without the i)ossibility of a continuous uri- 

 nary excretion within the embryo, i.e., with an early degeneration 

 of the Wolffian body, the placenta is provided with an apparatus 

 similar to that found in the glomeruli of the Wolffian body or the 

 kidney, thin plates of epithelium overlying the fetal capillaries. 

 These appear in the placenta at about the time when the Wolf- 

 fian bod}^ commences to degenerate, or in the case of the rat, 

 which never develops mesonephric glomeruli, at about the time 

 of the normal development of the glomeruli in other embryos. 

 These plates continue and increase in number till term. They 



