458 W/LSOJV. [Vol. VI. 



surface over only a small area. The primary mesoblasts pass 

 into the cleavage-cavity before the formation of the blastopore ; 

 i.e. at a much earlier period than in the other forms, so as to 

 approach the condition found in Lumbricus. 



(5) In Spio and Aricia fcetida the primary mesoblasts first 

 bud forth small superficial cells, from their anterior margin, at 

 the posterior lip of the blastopore ("secondary mesoblast " in 

 Nereis), a process which takes place at a relatively earlier period 

 than in Nej^eis ; i.e. before the fission of X. In Aricia fcetida 

 (probably also in Spio) only a single pair of such cells are found, 

 which are very minute and appear to be rudimentary. After 

 their formation the mesoblasts bud forth into the cleavage-cavity 

 (as in the "blastula" of Lumbricns) two rows of cells that form 

 the mesoblast-bands. 



In Polymnia the preliminary superficial budding seems not to 

 take place, and the mesoblasts only divide after their complete 

 enclosure (as in EiLpomahis, t. Hatschek). 



(6) In Polymnia and Aricia foetida, (probably also in Spio) 

 the macromeres undergo two additional spiral divisions after the 

 separation of the twelve ectomeres. The first of these (fourth 

 spiral division) is left-handed, and of the four resulting cells one 

 {d^ = M), formed somewhat earlier than the others, becomes 

 the second somatoblast, while the other three (a^, b^, c^) form 

 part of the entoblast-plate. The second (fifth spiral division) 

 gives rise to four entomeres {a^, b^, c^, d^), formed in a right- 

 handed spiral, after which no further divisions take place in the 

 entoblast-plate until after the completion of gastrulation. 



The entoblast-plate now consists of eleven cells, which (in 

 Aricia fcetida) assume the bilateral arrangement shown in the 

 following diagram : — 



35 b^ d^ 



\ I I 

 c^—C B A — d* 



I 

 c^ D — d^ 



M—M. 



(7) In Polymnia and Aricia fcetida {Spio not observed) an 

 embolic invagination now rapidly takes place. The blastopore 

 is at first elongated, its floor being formed (in Aricia fcetida) 

 by D, B, and b^, which lie in the median line, and its posterior 

 limit by the primary mesoblasts, M, M. Thus (in Aricia 



