TRAi III A I V 



453 



The hypoblait and alimentary tract. It has already 

 been stated that the yolk is to be regarded as corresponding to 

 tin- hypoblast of other types. 



For a considerable period it is composed of the polygonal 

 yolk cells already described and shewn in figs. 203, 204, and 205. 

 The yolk cells divide and be- 

 come somewhat smaller as de- 

 velopment proceeds ; but the 

 main products of the division 

 of the yolk nuclei and the pro- 

 toplasm around them are un- 

 doubtedly cells which join the 

 mesoblast (fig. 203 A). The 

 permanent alimentary tract is 

 formed of three sections, viz. 

 stomodaeum, proctodaeum, and 

 mesenteron. The stomodaeum 

 and proctodaeum are both 

 formed before the mesenteron. 

 The stomodaeum is formed as 

 an epiblastic pit between the 

 two procephalic lobes (figs. 200 

 and 205, st). It becomes 

 deeper, and by the latest stage 

 figured is a deep pit lined by a 

 cuticle and ending blindly. To 

 its hinder section, which forms 

 the suctorial apparatus of the adult, three powerful muscles (a 

 dorsal and two lateral) are attached. 



The proctodapum is formed considerably later than the 

 stomodaeum. It is a comparatively shallow involution, which 

 forms the rectum of the adult. It is dilated at its extremity, and 

 two M.-ilpi^hian vessels early grow out from it. 



The mesenteron is formed in tlte interior of the yolk. Its 

 walls are derived from the cellular elements of the yolk, and the 

 first section to be formed is the hinder extremity, which appears 

 as a short tube ending blindly behind in contact with the procto- 

 d.i-um, and open to the yolk in front. The later history of the 

 UK ->enteron has not been followed, but it undoubtedly includes 



FIG. 307. TAILOFAN ADVANCKPKM 

 BRYO OF THE SCORPION TO ILLUSTRATE 

 THE STRUCTURE OF THE MESOBI.ASTIC 

 SOMITES. (After Metschnikoff.) 



al. alimentary tract ; ati.i. anil in- 

 vagination ; ep. epiblast ; mtj. meso 

 Mastic somite. 



