520 RICHARD EVANS. 



V. The Structure op the Ovum. 



In no group of comparatively small size does the structure 

 of the ovum vary as in the Onychophora. In Peri- 

 patoides and Eoperipatus the ovum is large and full of 

 yolk; in Peripatopsis it is large and devoid of yolk; in 

 Peripatus and Paraperipatus it is small and free from 

 yolk. 



With regard to the primitive condition of the ovum there 

 are two views — the one put forward by von Kennel (9), and 

 recently supported by Dr. Willey; the other held by Mr. 

 Sedgwick (15), and adopted by Korschelt and Heider in their 

 ' Text-book of Embryology.' Von Kennel's view, so frequently 

 adopted by Dr. Willey, is that the ancestral Peripatus had 

 a small yolkless egg, which it laid in water; Mr. Sedgwick's 

 view, adopted by Korschelt and Heider, is that the ancesti'al 

 form had a large egg full of yolk. Kennel, with whom the 

 first view originated, thought that the course of embryonic 

 development in the Peripatidte had followed two divergent 

 lines of evolution, the one leading towards the type of 

 development occurring in the genus Peripatus, the othc 

 towards that found in the genus Peripatoides ; the ovum 

 remaining yolkless in the former, but developing yolk in the 

 latter — a feature which is considered by Willey to be a 

 secondary one, which culminates in the oviparity of P. 

 oviparus (Dendy). Sedgwick, disagreeing with Kennel, 

 sees only one line of evolution; according to him the yolk- 

 bearing egg of Peripatoides is primitive, the vesicular 

 egg of Peripatopsis is intermediate, while that of Peri- 

 patus represents the end result of a series of modifications 

 (15, p. 463). 



Dr. Willey has recently expressed himself very strongly on 

 these questions. Even with regard to the condition found in P. 

 capensis, he declares that the opposite view to that of Sedg- 

 wick could be sustained with equal force, though he admits 

 "there is no means at present known of deciding between the 

 two views in this particular case," and " both of them seem to 



