DEVELOPMENT 2 1 



their appearance in pairs at the hind end, and gradually travel for- 

 ward 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. 

 13, D). Meanwhile the narrow middle part of the blastopore has 

 closed by a fusion of i.ts 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. 13, D). At 

 this stage the hind end of the body becomes curved ventrally 

 into a spiral (Fig. 13, 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. jaw, 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 ectoderm passing in front of the mouth and behind the anus, 

 and lying on each side of the blastopore along the lines of the 

 somites. The praeoral part of this thickening, which gives rise to 

 the cerebral ganglia, becomes pitted inwards on each side (Fig. 13, 

 F, eg). These pits are eventually closed, and form the hollow 

 ventral appendages of the supra-pharyngeal ganglia of the adult 

 (Fig. 9, d). The lips are formed as folds of the side wall of the 

 body, extending from the praeoral lobes to just behind the jaw 

 (Fig. 13, F, Z). They enclose the jaws {j), mouth (J/), and 

 opening of the salivary glands {o.s), and so give rise to the buccal 

 cavity. The embryo has now lost its spiral curvature, and 

 becomes completely doubled upon itself, the hind end being in 

 contact with the mouth (Fig. 13, G). It remains in this position 

 until birth. The just-born young are from 10-15 mm. in length 

 and have green antennae, but the rest of the body is either quite 

 white or of a reddish colour. This red colour differs from the 

 colour of the adult in being soluble in spirit. 



The mesoblastic somites are paired sacs formed from the 

 anterior lateral portions of the primitive streak (Fig. 13, C). 

 As they are formed they become placed in pairs on each side of 

 the blastopore. The somites of the first pair eventually obtain a 

 position entirely in front of the blastopore (Fig. 13, D). They 

 form the somites of the praeoral lobes. The full complement of 

 somites is acquired at about the stage of Fig. 13, E. The relations 



