ON THE OVIPAROUS SPECIES OP ONYCHOPBOliA. 391 



lays stress upon the large size ^ of tlie egg* combined witli 

 the almost complete absence of yolk. He assumes " that the 

 ovum of the Cape species has only recently lost its yolk, and 

 tliat it may be compared to an ovum of the New Zealand form 

 [P. novee-zealandire] from which the yolk has been almost 

 completely dissolved out by some reagent. As a matter of 

 fact [he adds] it is impossible, with our present methods, to 

 effect this complete solution of yolk and leave its proto- 

 plasmic framework; but what we cannot effect has been done 

 by nature in the most complete manner, leaving an ovum 

 which is little more than a loose protoplasmic spongework, 

 excepting at one point where the protoplasm is more dense." 

 Willey (1), in discussing this question, observes that " the 

 view that the egg of Peripatus capensis exhibits a stage 

 in the process of acquiring yolk, instead of being a stage in 

 the loss of yolk, could be maintained with equal force." 

 Surely, however, it is hardly likely that the protoplasm would 

 acquire a vesicular structure in anticipation of the formation 

 of yolk; in order to justify us in accepting this view we 

 should be obliged to show that the vesicular structure has 

 some value altogether apart from yolk formation. 



Writing on the development of a South American species 

 (P. Imthurni) Sclater (1) also comes to the conclusion that 

 the alecithal condition of the egg is secondary. 



In this species the ovum appears to be much smaller than 

 in P. capensis. The segmentation is complete, and there 

 is no appearance of sponginess, such as occurs in P. capen- 

 sis; " nor would one suspect, from the nature and size of the 

 ovum, that it had been derived from a meroblastic ovum 

 and had only comparatively recently lost its yolk. . . . Now, 

 it seems to me that the loss of yolk has had precisely the 

 same effect in the ovum of Peripatus that it has on the ovum 

 of placental mammals, i.e. (1) diminution of the size of the 

 ovum; (2) total segmentation, and (3) the formation of what 

 I have termed the embryonic vesicle, which appears to me 



' Sedgwick gives the lengtli of llie youngest ovum found in tlie oviduct, as 

 O'i mm. 



