176 



ONYCHOPHORA. 



K. 



B. 



M. 



ventrally, thus covering the posterior aperture derived from the 

 constriction of the blastopore (Figs. 84 and 85). 



Before describing the further development of the embryo, we 

 must glance at the corresponding processes in other species of 

 Peripatus. The observations recorded above on the development 

 of the external form have dealt chiefly with the shaping of the 

 ventral surface, this being first developed as two symmetrical halves. 

 We are here reminded of the development of the eggs of the 



Oligochaeta and Hiru- 

 dinea that are rich in 

 yolk, and still more of 

 that of the eggs of the 

 Myriopoda, Insecta, and 

 Arachnida. In Peripatus, 

 as in these, a germ-band 

 forms. Its composition out 

 of two halves is still more 

 distinct in P. novae-zea- 

 landiae. In this form, in 

 consequence of the large 

 size of the egg, caused by 

 the abundance of its yolk, 

 the two halves of the 

 germ-band lie somewhat 

 far apart, separated by a 

 ventral protrusion of the 

 yolk-mass covered with 

 ectoderm and entoderm 

 (Fig. 86 A and B), so that 

 a kind of ventral yolk-sac 

 arises resembling the one 

 met with in the Araneae 

 (p. 54). While already 

 well developed at the 

 anterior end, the two 

 halves of the germ-band 

 become less and less 

 differentiated posteriorly, and end near the blastopore in the as 

 yet undifferentiated cell-mass (primitive streak of English authors). 



At first sight there appears to he a fundamental difference between P. capcnsis 

 with P. Edwardsii and P. novcte-zealandiae regarding the position, relative to 



us. 



- ra- 



il. 



m.- 



Fio 



-Embryos of P. capensis to illustrate the 

 closing of the blastopore, the segmentation of the 

 mesoderm, and the flexure of the embryo (after 

 Balfour and Schimkewitsch). a, anus ; U, blasto- 

 pore ; m, mouth ; us, primitive segments ; w, zone of 

 growth. 



