NO. 6 ANNELIDA, ONYCHOPIIORA, AND ARTHROPODA SNODGRASS 53 



lies beneath a superficial layer of yolk, the early embryo in this case 

 being a sac not only containing yolk, but also surrounded by it. The 

 outer yolk is later absorbed. In this species the manner of germ- 

 layer formation has not been definitely determined, but the endoderm 

 cells appear within the yolk, and the mesoderm takes the form of 

 two widely separated bands along the sides of the embryo, in which 

 the coelomic sacs are formed. 



%r H 



Fig. 22. — ^Early developmental stages of Onychophora. 



A-D, successive embryonic stages of Peripatopsis capcusis Grube, showing 

 elongation and closure of the blastopore except at oral and anal extremities, 

 and forward growth and segmentation of mesoderm bands (from Balfour, 

 1883). E, young embryo of Eoperipattis iveldoni Evans, ventral view, mouth 

 covered by external yolk (from Evans, 1902). F, young embryo of Peri- 

 patopsis mosclcyi Wood-Mason with open blastopore (from Bouvier, 1905). 

 G, cross-section of embyro of Peripatopsis capensis through open blastopore 

 (from Balfour, 1883). H, cross-section of embryo of Eoperipafiis zwldoni, 

 blastopore covered with yolk (from Evans, 1902). 



An, anus ; Bpr, blastopore ; Coel, coelomic cavity ; 2Cocl, sCocl, second and 

 third coelomic cavities ; Ecd, ectoderm ; E}id, endoderm ; Gc, gastrocoele ; He, 

 haemocoele; Msd, mesoderm; MsT, mesodermal teloblast ("primitive streak") ; 

 Mth, mouth ; Y, internal yolk ; y, external yolk. 



In Peripatopsis capensis, Sedgwick (1885) says, cleavage is complete 

 but unequal, the blastomeres being differentiated into four small, dark 

 ectodermal cells at the animal pole of the egg, and four large, clear 

 endodermal cells at the vegetative pole. Subsequent divisions proceed 

 in each group separately. The endoderm cells soon draw together 

 into the center of the egg, and are here overgrown by the ectoderm 

 until completely enclosed by the latter, except at one point where 



