142 



INVEETEBRATA 



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 5c ]2 , 5d u 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 

 flattened shape. The 

 cells which form the 

 dorsal wall, from being 

 cubical become flattened 

 and converted into thin 

 arched shells of cyto- 

 plasm. The front wall 

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



FIG. 105. Four diagrammatic transverse sections of lower 

 part of young Trochophore larva to show mode of 

 closure of blastopore. (After Woltereck. ) 



A, gastrulation beginning ; B, constriction of blastopore, the 

 cells of fouith quartette approach one another ; C, inner lips of 

 blastopore closed ; D, outer lips of blastopore closed. The 

 figures refer to the quartettes to which the cells thus designated 

 belong. M, residual macromeres. 



