PHYLUM ARTHROPODA 



611 



complicated cases which enclose masses of sperms to form 

 spermatophores. 



Development. The differences between the species of Peripatus 

 as regards the segmentation of the egg and the formation of the 

 germinal layers as described by various observers are very con- 

 siderable. Nearly all the species are viviparous, but in some the 

 egg, before the completion of embryonic development, is enclosed 

 in a well-formed shell, and in certain species the eggs pass out to 

 the exterior before the emergence of the embryo. In some species 

 the egg encloses a considerable amount of food-yolk, in others the 

 quantity of food-yolk is small, and nutriment is obtained from the 

 parent. 



In P. novce-zealanduc there is a superficial segmentation. The 

 first segmentation-nucleus is itself superficial, and segmentation 



FIG. 4^4. Two early stages in the development of Peripatus novae- zealandi ae. A, transverse 

 section of an ovum in which the yolk is nearly covered by the blastoderm (M) ; B, transverse 

 section of an ovum in which the blastopore (W.P-) is formed. (After Sheldon.) 



results in the development of a number of nuclei, each with its 

 island of protoplasm, which arrange themselves on what is destined 

 to become the dorsal side (Fig. 484 A\ opposite the site of the 

 future blastopore, while some pass inwards to the central part of 

 the ovum. The peripheral nuclei multiply rapidly and grow round 

 the yolk so as completely to enclose it except on a small space 

 (blastopore) in the middle of the ventral side (B). There a 

 thickening takes place, and an involution of the lips of the 

 blastopore results in a sort of invagination, the floor of the 

 in\ agination-cavity being formed of yolk with scattered nuclei. 



In another species P. capensu the segmentation has the 

 appearance of being total; but the cells, though separated by 

 fissures externally, are fused internally. A peripheral ectodermal 

 layer becomes formed, enclosing a central, nucleated, vacuolated 



