No. 3.] THE CELL-LINEAGE OF NEREIS. 419 



tJie viedian ventral Ihie of the adult body does not yet exist. 

 Month and anal area arise side by side in the region where the 

 blastopore closes, though the anus does not break through until 

 they have been widely separated. 



In the meantime the remaining descendants of d^, b^ extend 

 themselves over the anterior and lateral regions, between the 

 stomodseum and the prototroch. The smaller cells budded off 

 from the median stomatoblast, and also the descendants of 

 b"^-^ become indistinguishably intermingled with them. From 

 this group of cells arises the ectoblast surrounding the stomo- 

 daeum and forming the superficial part of the body-wall of the 

 antero-lateral region. The postero-lateral region on each side 

 (between the margins of the ventral plate and the prototroch) 

 is occupied by the descendants of the post-trochal cells (off- 

 spring of a^-^ on the left side and of c"^-^ on the right) and of c^ 

 and d^. These cells are continuous behind with the small cells 

 that separate the posterior teloblasts from the prototroch ; i.e. 

 the descendants of ;t^ and (?) of x^, x^ (see p. 410). From the 

 latter, as I believe, arise the cells that occupy the triangular area 

 between the tivo residual teloblasts after their divergence. This 

 area afterivards forms the middle dorsal region of the trunk. 



The line of parapodia may be taken to represent the lateral 

 region of the adult body on each side. Now, the seta-sacs 

 (which mark the position of the parapodia) are developed from 

 the ventral plate. Hence the cells lying above the ventral plate 

 on each side (i.e. the descendants of the post-trochal cells) form 

 the lateral portion of the dorsal region of the trunk. (This con- 

 clusion v^^ill be rendered clear by referring to the discussion of 

 the axial relations, with the accompanying diagrams, at p. 426.) 



With this my account of the cleavage closes. The general 

 relation of the primary blastomeres to the larval and adult 

 bodies are shown in the following: table : — 



