INTERRELATIONS OF THE MESONEPHROS, ETC. 201 



approximately the same as that of the appearance of the other. 

 Moreover, the placentae of those animals studied which are able 

 to utilize the Wolffian body until the kidney is ready for action 

 show no such modification of the ectoderm. There are two 

 objections, it seems to me, which might stand in the way of an 

 immediate acceptance of these facts as proof that the placental 

 plates surely represent the lost glomerular ones. The first has 

 already been mentioned ; since the ' breathing epithelium' of the 

 lungs and of the gills is also characterized by membranous plates, 

 may not the placental plates represent the apparatus for fetal 

 oxygenation instead of for fetal excretion? As was pointed out 

 previously, it is possible that the same apparatus may serve for 

 both purposes. The strongest argument for considering the 

 plates excretory instead of respiratory is the fact that all em- 

 bryos from very early periods require oxygen, yet many classes of 

 mammals never develop the placental plates, and in many others 

 they appear relatively late, long after the need for oxygen must 

 have been felt. In the rat alone, of those animals studied, 

 would the development of the apparatus keep pace with the needs 

 of the embryo. On the other hand, the fact that in the i)ig, 

 sheep, and cat placental respiration must in all probability be 

 an active secretory process does not eliminate the possibility of 

 an osmotic respiration for other embryos at certain periods. 



The second objection is perhaps a little more puzzling. If in 

 certain cases the kidney becomes functional during fetal life, 

 as we must suppose in animals provided with a large allantoic 

 reservoir, and no placental osmotic apparatus, why should it 

 not also become active in all mammals? In other words, why 

 should excretion in certain animals take place through the pla- 

 centa instead of through the kidney in the later periods of preg- 

 nancy, when the kidney is apparently capable of activity? Of 

 the readiness of the older fetal kidneys of various classes of ani- 

 mals to perform the excretory function there is hardly a doubt. 

 The kidney glomerulus of a new-born rabbit is quite similar to 

 that of a new-born cat, though the one has not yet presumably 

 been functional and the other has been active for manv davs. 



