246 MEAD. [Vol. XIII. 



(Fig. 57). (3) In the late stages of cleavage, as in the early ones, 

 all individuals of the species are alike. (4) The upper product 

 of a°■^ which forms the larval mesoblast in Unio, is in Amphi- 

 trite minute (Figs. 41-44), but its exact fate is unknown. The 

 lower products of a^■^ U"'^, c""'^ come to lie in about the same 

 position as the " stomatoblasts " in Nereis, but I do not know 

 their ultimate destiny. In each of the three groups, even 

 at a late stage, one or two of the cells are of comparatively 

 large size, and the rest very small; the former correspond in 

 position to Wilson's " stomatoblasts " (Wilson,^ p. 414). 



In reference to the products of a^, b^, c^, a^, it need only be 

 said that they behave in the same manner in all individuals, 

 through several generations, and form important landmarks 

 (cf. Figs. 41-62). 



Entoderm. — At the 64-cell stage the entoderm consists of 

 seven cells, which occupy a large part of the surface on the 

 posterior hemisphere. They are A^, B^ C^, D^, and «■*, b , c", 

 {df^ being the mesoderm cell). The four cells first mentioned 

 all divide again, and except for D^, which divides obliquely to 

 the right, and in advance of the others, they all cleave practi- 

 cally in the same direction as did the parent cells. The 

 relative size and position of the resulting cells is remarkably 

 constant in every egg. In consequence of these divisions the 

 entoderm plate assumes the form of a cross, consisting of eleven 

 cells arranged in the following manner : 



d'' = M—M=d'' 

 d^ 



'>^<' 



The surface area of the cross gradually diminishes as the 

 cells elongate at right angles to the surface. 



None of these cells divide again until after they have 

 migrated into the segmentation-cavity, where they give rise to 

 the entoderm of the larva (Figs. 43-56, text Fig. VI). 



Mesoderm. — The mesoderm cell d* = M, of the 64-cell stage, 

 divides very early into two equal cells bilaterally placed upon 



