142 



INVEETEBEATA 



CHAP. 



B 



trunk blastema, are 



3c 2 P aa , 3d 2 P aa , and 2d 222 . 

 Outside and behind 

 these are thin plate-like 

 cells, belonging to the 

 quadrant D of the 

 second quartette, but 

 immediately behind 

 them lie the cells 2d 212 

 and 2d 221 , which mark 

 the point at which, 

 about this time, the 

 anus is re-formed. 



Proceeding now to 

 examine the cells which 

 enter into the gut-wall, 

 we find that the ven- 

 tral surface of the 

 gut is formed (1) by 

 the union of the pair 

 4a a and 4c a which, as 

 we have seen, by join- 

 ing with one another, 

 cut the original blasto- 

 pore into two openings ; 

 and (2) by the union of 

 the following pairs of 

 cells belonging to the 

 fifth quartette, viz. 5d 12 

 and oc 12 , 5d n and 5c n , 

 and at a slightly later 

 period, the pair 5d 21 and 

 5c 21 , which also unite 

 with one another. 



As the dorsal wall 

 of the gut is invaginated 

 the Trochophore swells 

 out and becomes arched 

 dorsally, recovering in 

 this way from its 



FIG. 105. Four diagrammatic transverse sections of lower flattened slianp Thp 

 part of young Trochophore larva to show mode of ,. . 



closure of blastopore. (After Woltereck.) cells which form the 



A, gastrulation beginning ; B, constriction of blastopore, the dorsal Wall, irOlll being 



cells of fourth quartette approach one another ; C, inner lips of Cubical become flattened 



blastopore closed ; D, outer lips of blastopore closed. The Qj}(J COllVCl'ted illto thill 

 ligures refer to the quartettes to which the cells thus designated , 



belong. M, residual macromeres. ai'Clied Shells OI CytO- 



plasm. The front wall 

 of the mid -gut where it joins the stomodaeum is formed by the 



