396 



EMBRYOLOGT 



is ordinarily flattened somewhat, whereas the dorsal side is 

 more convex. 



Before concluding the consideration of the first develop- 

 mental processes of Synapta, we must refer to a process 

 which takes place before the change in position of the arch- 

 enteron already described ; this is the formation of the mesen- 

 clnjma. At the time when the blind end of the archenteron 

 begins to bend toward the dorsal surface, there appear at its 

 apex two cells which project out beyond the other cells (Fig. 

 175), and are called by Selenka the two primitive cells of the 



P 



Fig. 179. 



Figs. 178 and 179.— Larvae of Synapta digitata, showing the formation of the 

 intestine and vaso-peritoneal vesicle (after Selenka). Bl, blastopore; M, mouth; 

 P, dorsal pore. 



mesenchyma. These cells then separate from their connec- 

 tion with the archenteron, migrate into the blastocoele, and 

 apply themselves to the ectoderm, but not at predetermined 

 points. Subsequently a large number of such mesenchyma- 

 tous or migratory cells are found in the blastoccele (Figs. 

 176 to 179). According to Selenka's description, they arise by 

 the division of the two primitive mesenchyma cells ; but 

 the process of mesenchyma formation in other Holothurioidea 



