DEVELOPMENT. 571 



derm 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. 344 

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

 ventral appendages of the supra-pharyngeal ganglia of the adult 

 (Fig. 339, 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. 344 F, L). They enclose the jaws (j), mouth (M), 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 head (Fig. 344 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 colouring matter 

 differs from that of the adult in being more soluble in spirit. 



The mesoderm is entirely formed from the proliferated 

 ectoderm of the primitive streak, which closely resembles that 

 of the vertebrata. 



The mesoblastic somites are paired sacs formed from the 

 anterior lateral portions of the primitive streak (Fig. 344 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. 344 -D). They 

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

 somites is acquired at about the stage of Fig. 344 E. The 

 relations of the somites are shown in Fig. 345 A , which represents 

 a transverse section taken between the mouth and anus of an 

 embryo of the stage of Fig. 344 D. The history of these 

 somites is an exceedingly* interesting one, and may be described 

 shortly as follows : They divide into two parts a ventral part, 

 which extends into the appendage, and a dorsal part (Fig. 345 

 B). The ventral part acquires an opening to the exterior just 

 outside the nerve-cord, and becomes entirely transformed into 

 a nephridium (Fig. 345 D, 2'). The dorsal part shifts dorsal- 

 wards and diminishes relatively in size (Fig. 345 C). Its fate 

 differs in the different parts of the body. In the anterior somites 

 it dwindles and disappears, but in the posterior part it unites 

 with the dorsal divisions of contiguous somites of the same 



