CELL-LTNEAGE. 



thus formed some of the small cells usually extend, appearing 

 in sections in the sharpest contrast both to the large rounded 

 mesoblast-cells and to those of the lateral ectoblast (Fig. 3, C\ 

 From this point the mesoblast-bands extend towards the sides 

 and ultimately curve upwards (forwards with respect to the 

 adult long axis) at the sides of the embryo.^ 



A 



B 



Fig. 2. Corresponding surface views, from the lower pole, of early embryos oi Avici a 

 (A) and A'ereis {B) ; the limit of the ectoblast, i. c, the lip of the blastospore, 

 is shown by the heavy line. A shows the single pair of vestigial entoblasts (e, 

 e) of Aricia lying in front of the primary mesoblasts which are dividing to fonn 

 the mesoblast-bands (cf. Fig. i, C, which shows the same specimen in sagittal 

 section). B shows two pairs of superficial entoblasts, lying behind the macro- 

 mere D, and the spindles of a deeper budding of the " primary mesoblasts" (cf. 

 Fig. 3, A, for section of this stage). 



A, B, C, D, the four basal entoblasts or macromeres ; a^-c^, the fourth quartet of 

 " micromeres " (entomeres); a^-d^, the fifth quartet (entomeres) ; c'^-d^, deriva- 

 tives of the third quartet (ectomeres) ; M, J/, the primary mesoblasts (shaded in 

 B). 



Up to this point the account here given is substantially the 

 same as that contained in my earlier paper on Nereis. Regard- 



iJn Aricia the mesoblast-bands are foraied much earlier, while the primaiy meso- 

 blasts still lie at the surface (Fig. i, C) ; and they lie at first side by side, nearly 

 parallel to each other, extending upwards behind the entoblast-plate (Fig. 7). In 

 both these respects Aricia is somewhat similar to Lunibriciis (Cf. \Vilson, Embry- 

 ology of the Earthworm, Fig. 30 : Joiirn. Morph., 1 889). 



