INTERRELATIONS OF THE MESONEPHROS, ETC. 187 



nate the probability of their use for fetal respiration; but even 

 if this cannot be done definitely, the possibility should be borne 

 in mind that both processes, if each is considered a purely physi- 

 cal diffusion, might go on simultaneously through the same 

 thin plates. 



THE ME80NEPHR0S 



1 have long been interested in the relative size of the Wolffian 

 body in different types of embryos, because of its influence 

 on their future development; as 1 have pointed out in earlier 

 papers it seems to govern the time of the connection of the 

 rete cords and the testis cords, the position of the spermatic or 

 ovarian arteries, and the development of the renal artery with 

 the consequent differences in the range of anomalies to be ex- 

 pected in these vessels. In this regard the commoner embryos 

 may be grouped as follows; pig and rabbit, large Wolffian bodies; 

 sheep, medium size to small; cat, man, guinea pig, and opossum, 

 small; mouse and rat, practically none at all. It was perhaps 

 not an unnatural mistake to suppose that the larger the Wolf- 

 fian body, the longer it would remain active, yet this is not at 

 all the case. The pig, as pointed out by MacCallum, has an 

 increasingly large mesonephros up to the 40.0 mm. stage, and no 

 reduction in its size until 95.0 mm. The rabbit, on the other 

 hand, while possessing at first almost as large an organ as the 

 pig, begins to show mesonephric degeneration at about 20.0 mm. 

 In the sheep, though the gland is never large, it is retained as 

 in the pig, increasing up to about 25.0 mm., and with no re- 

 duction at 48.0 mm. It is for this reason that the sheep was 

 classed among those animals with large Wolffian body in my 

 study of the testis and rete cords; the presence in late embryonic 

 life of this organ was erroneously thought to prove its great size 

 in earlier periods. The oposssum retains an active Wolffian 

 body after birth, and it is not replaced by the kidney for a con- 

 siderable time. This in itself would prove that for this class 

 of mammals, as in the lizards, the mesonephros is certainly 

 functional as a urinary organ. The cat, the guinea pig, and 

 man all have small Wolffian bodies, yet in the cat they increase 



