(k6) 



At the extreme posterior enci of the eiiteron, however, a trana- 

 verse aection across ti:e larva ( t^'ig. ii2(d) ) shows a mass of 

 cells which mig-ht be taken for proliferating mesoderm cells. 

 Traced farther back, this mass of cells is found to be part of 

 the wall of the "posterior pit" or as Ikeda (9) has called it 

 "the nephridial pit". C^igs. iiii(e) - ( f )- (g) ) I siiall speak of 

 the fate of the cells of the nephridial pit in the description 

 of the larva with two tentacles. 



Larvae like the one just described do not show thie least 

 trace o -^ a mesentery between the collar and trunk. In fact, 

 one could hardly say tiiat a trunk existed at this time. The 

 oblique strongly ciliated tract of ectoderm v/hich indica'oes the 

 line of origin of the larval tentacles has not appeared. 

 T^jrther Growth of t he Young Larva. 



The flexure of the preoral lobe continues as the larva 

 grows older. (Fig. 24) In this way a vestibule is fb rmed and 

 the original blastopore becomes the' part which connects the ves- 

 tibule and archenteron. (Fig. £4) This relation between the 

 vestibule and blastopore has been recognised by Masterman (16), 

 Roule (20), Ikeda (9) aiid Longchamps (12). Longchamps speaks 

 of it as a "stomodaeum" and MastenTiari ao ea also but the latter 

 adds "oesophagus" after it. (If my idea that a stomodaeujri 

 is a pitting in of the ectoderm wliich finally breaks through 



