570 



CLASS II. ONYCHOPHORA. 



hind end of the blastopore (Fig. 344 B). This elongation of 

 the embryo is accompanied by an elongation of the blastopore, 

 which soon becomes dumb-bell shaped (Fig. 344 C). At the 

 same time the mesoblastic somites (embryonic segments of 

 mesoderm) have made their appearance in pairs at the hind end, 

 and gradually travel forward on each side of the blastopore 

 to the front end, where the somites of the anterior pair soon 

 meet in front of the blastopore (Fig. 344 D). Meanwhile the 

 narrow middle part of the blastopore has closed by a fusion 



FIG. 344. A series of embryos of P. capensis. The hind end of embryos B, C, D is upper- 

 most in the figures, the primitive streak is the white patch behind 'the blastopore (after 

 Sedgwick). A gastrula stage, \entral view, showing blastopore. B older gastrula stage, 

 ventral view, showing elongated blastopore and primitive streak. C ventral view of 

 embryo with three pairs of fmesoblastic somites, dumb-bell-shaped fclastopore and primitive 

 streak. D ventral view of embryo, in which the blastopore has completely closed in its 

 middle portion, and given rise to two openings, the embryonic mouth and anus. The 

 anterior pair of somites have moved to the front end of the body, and the primitive groove 

 lias appeared on the primitive streak. E side view of embryo, in which the hind end of the 

 body has begun to elongate in a spiral manner, and in which the appendages have begun. 

 At antenna ; d dorsal projection ; p.s preoral somite. F ventral view of head of embryo 

 intermediate between E and G. The cerebral grooves are wide and shallow. The lips 

 have appeared, and have extended behind the openings of the salivary glands, but have 

 not yet joined in the middle line. At antennae : c.r/ cerebral groove : 7" jaws ; i.s swelling 

 at base of jaws : L lips : M mouth ; or.p oral papillae ; o.s opening of salivary gland. 

 G side view of older embryo with the full number of appendages, to show the position in 

 which the embryos lie in the uterus. 



of its lips, so that the blastopore is represented by two 

 openings, the future mouth and anus. A primitive groove 

 makes its appearance behind the blastopore (Fig. 344 D). At 

 this stage the hind end of the body becomes curved ventrally 

 into a spiral (Fig. 344 E) , and at the same time the appendages 

 appear as hollow processes of the body wall, a mesoblastic somite 

 being prolonged into each of them. The first to appear are the 

 antennae, into which the praeoral somites are prolonged. The 

 remainder appear from before backwards in regular order, viz. 

 jaws, oral papillae, legs 1-17. The full number of somites and 

 their appendages is not, however, completed until a later stage. 

 The nervous system is formed as an annular thickening of ecto- 



