TEACH EAT A. 



37o 



mp.s 



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 become some- 

 what smaller as development pro- 

 ceeds ; but the main products of 

 the division of the yolk nuclei and 

 the protoplasm around them are 

 undoubtedly cells which join the 

 mesoblast (fig. 203 A). The per- 

 manent alimentary tract is formed 

 of three sections, viz. stomodasum, 

 proctodasum, and mesenteron. The 

 stomodaeum and proctodaaim are 

 both formed before the mesenteron. 

 The stomodgeum is formed as an 

 epiblastic pit between the two pro- 

 cephalic lobes (fig. 200 and 205, st}. 

 It becomes deeper, and by the 

 latest stage figured is a deep pit 

 lined by a cuticle and ending blind- 

 ly. To its hinder section, which 

 forms the suctorial apparatus of 

 the adult, three powerful muscles 

 (a dorsal and two lateral) are at- 

 tached. 



The proctodceum is formed con- 

 siderably later than the stomo- 

 dgeum. It is a comparatively 

 shallow involution, which forms 

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

 Malpighian vessels early grow out from it. 



The mesenteron is formed in the 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 proctodseum, and open 

 to the yolk in front. The later history of the mesenteron has not been 

 followed, but it undoubtedly includes the whole of the abdominal 

 section of the alimentary canal of the adult, except the rectum, and 

 probably also the thoracic section. The later history of the yolk 

 which encloses the mesenteron has not been satisfactorily studied, 

 though it no doubt gives rise to the hepatic tubes, and probably 

 also to the thoracic diverticula of the alimentary tract. 



The general history of the alimentary tract in Scorpio is much the same 

 as in Spiders. The liypoblast, the origin of which as mentioned above is 

 somewhat uncertain, first appears on the ventral side and gradually spreads 

 so as to envelop the yolk, and form the wall of the mesenteron, from 

 which the liver is formed as a pair of lateral outgrowths. The procto- 

 clseum and stoniodseum are both short, especially the former (vide fig. 207). 



FIG. 207. TAIL OF AN ADVANCED EM- 

 BRYO OF THE ScOKPION TO ILLUSTRATE 

 THE STRUCTURE OF THE MESOBLASTIC SO- 

 MITES. (After Metschnikoff.) 



aJ. alimentary tract; an.i. anal in- 

 vaginatiou; ep. epiblast; me.s. rneso- 

 blastic somite. 



