13 



I thiiik WiLLEY is riglit in eonsidering the abundancc of yolk 

 in the eg'gs of the Indian and Australian Onychopliora (Ecrperi- 

 patus^ Fen'pafoides) as a later acquisition. The food snbstances, 

 which in Peripafopsis probably are socrcted permanontly during 

 the development of the embryo, now reach the egg at once and 

 fill it with yolk; in fact Siieldon in Peripatoides novae- zealmidiae 

 Ilutt. has seen yolk material reaching the yoiing, stalked egg 

 from the cavity of the ovary. Abiindance of yolk, finally, has 

 enabled the species distinguished as Ooperipatus and Sympen- 

 patus to return to oviparity. 



The American species of Peripatus are modified in an entirely 

 different manner. Their development is certaiuly not primitive, as 

 are their external characters according to Bouvier a. o. In these 

 species the intra-uterine nutrition has become more perfect by the 

 existence of a more intimate connection between the embryo and 

 the v^all of the uterus. By this more intensive nutrition not only 

 the egg could become still more reduced in size than already is 

 the case in Penpatopsis^ but the trophic vesicle also has become 

 smaller. The researches of VoN Kennel, in some points improved 

 by ScLATER, allow a complete comparison with Peripatopsis. Most 

 of the peculiarities in the development of the American species 

 can be explained by taking in consideration that the embryonic 

 area, from which afterwards the whole embryo will be developed, 

 at an early period invaginates into the embryonic vesicle (see 

 fig. C). The cause of this phenomenon probably is want of 

 space in the uterus, which is much narrower than it is in the 

 African species. By this invagination the image of "a gastrula, 

 the „pseudogastrula" of Sclater, arises. YoN Kennel, by the lack 

 of some of the youngest stages, has made an error in eonsidering 

 only the inner layer of the pseudogastrula as belonging to the 

 young embryo; the outer layer he mistook for the uterine epithelium. 



This invagination of the embryonic area has an analogon in 

 Paraperipatus novae-hritamiiae^ in which also the embryo sinks 

 in in the however much larger trophic vesicle. Willey compares 

 this phenomenon with the forming of the amnion in Insects. 



