WILSON. 



Fig. I.i Early embryos oi Aricia [A, C) and Nereis (JB, J>) in sagittal section 

 (A, ^, C, optical, J), actual). Showing the formation of small posterior 

 entoblasts (^) between M and D. 



A, B, D, b*, cells of the entoblast-plate (cf. Fig. 2); M, the ** primary' mesoblast ;" 

 m, mesoblast-band ; X, the first somatoblast or its derivatives, forming the soma- 

 tic plate. 



mesoblasts. Thus are formed the mesoblast-bands which form 

 together a V-shaped mass of cells lying between the macromeres 

 and the overlying ectoblast. Near the middle line the two 

 halves of the V are often slightly separated ; and into the space 



' All the figures are from camera drawings, made from preparations unless other- 

 wise stated. Optical sections have been fully confirmed by actual. 



