38 LILLIE. [Vol. X. 



The cell {a''^) which in Unio supplies the larval mesoblast, 

 is in Nereis the left "somatoblast." The larval mesoblast 

 forms, as the name implies, only transitory organs, viz., the 

 isolated muscle-cells, which span the primary body cavity in 

 various directions (V, part III), and the adductor muscle of the 

 larva, which is nothing but a bunch of these myocytes,* 

 having nothing to do with the adult adductors (Schmidt, No. 31 ; 

 Braun, Nos. 6-9). The myocytes are widely spread larval organs 

 in annelids and molluscs ; in Unio they have increased greatly 

 in number and importance, and have come to be represented 

 in cleavage by a single cell, which thus must differ in fate from 

 the cell of the same lineage in other forms. 



If we can speak of an homology of cells of the same lineage 

 in so many annelids and molluscs, it does not, of necessity, 

 follow that the homology must be extended to other forms of 

 the same type of cleavage. I agree with Wilson (No. 64, 

 p. 455) that "the fundamental forms of cleavage are primarily 

 due to mechanical conditions, and are only significant morpho- 

 logically in so far as they have been secondarily remodeled 

 by processes of precocious segregation." But the fact that 

 the segregation has taken place in the same way in such widely 

 separated instances as have been quoted above, is surely of the 

 widest significance, whether we take it to mean that the ulti- 

 mate fate of a cell is a function of its position in the cell- 

 complex or not. It is parallel precocious segregation in different 

 cases that conditions cell homologies. 



Almost every detail of the cleavage ^ of the ovum of Unio 

 can be shown to possess some differential significance. The 

 first division is unequal. Why } Because the anlage of the 

 immense shell-gland is found in one of the cells. The apical- 

 pole cells divide very slowly and irregularly, lagging behind 

 the other cells. Why } Because the formation of apical 

 organs is delayed to a late stage of development. The second 



^ I propose this term " myocyte " for the unicellular muscles spanning the primary 

 body cavity in so many larvae ; and functioning as retractors or protractors of the 

 velum or prototroch, etc. We have no convenient name in English for these cells, 

 which are known to the Germans as " Strangzellen." 



2 I do not include in this the oblique character of the cleavage or its general 

 form. 



