44 BIOLOGICAL LECTURES. 



This principle is meant to apply only to the " determinate type 

 of cleavage " (Conklin), in which larval, and hence adult struc- 

 tures, can be shown to have a definite cell-lineage extending 

 back to the unsegmented ovum. The illustrations and argu- 

 ments will be drawn, first, from the cleavage of the egg oj: a 

 fresh-water lamellibranch, Unio, which I have specially studied, 

 and then, to secure a broader foundation in fact, some signifi- 

 cant variations in the cleavage of the eggs of annelids will 

 be considered. This theory was first used, I believe, for the 

 explanation of determinate cleavage in my papers on the 

 "Embryology of the Unionidce" (6 and 7), where I pointed 

 out that the size of the cells and the rate and direction of 

 their cleavages "are ruled by the needs of the embryo," and 

 concluded : " The peculiarities of the cleavage in Unio are but 

 a reflection of the structure of the glochidium, the organiza- 

 tion of which controls and moulds the nascent material." In 

 his lecture on " Cleavage and Differentiation " (Biological Lec- 

 tures, 1896-97), Conklin has maintained a similar view, viz., 

 "that all differential cleavages . . . are directly and causally 

 related to the uses to which these cells are put." In the 

 second part of this paper the retrospective significance of 

 this principle will be considered in the light of some observa- 

 tions on the maturation, fertilization, and first two cleavages 

 of the egg of Unio. 



If we are to understand the special features of the cleavage 

 in Unio, which are, I believe, purely adaptive, it will be neces- 

 sary, first of all, to describe the peculiar larva. The glochidium 

 (Fig. i) possesses two shell-valves, shaped something like the 

 bowls of two spoons cut across near their bases, and united by 

 a central hinge. The curvature of the anterior margin of each 

 valve is somewhat greater than that of the posterior margin, 

 and each lateral angle is provided with a strong hinged hook, 

 which is secondarily toothed in Anodonta on its outer face. 

 The valves may be closed with great force by a single strong 

 adductor muscle, the main bulk of which is somewhat anterior 

 to the middle of the body. The interior of the spoon-shaped 

 valves is lined by the larval mantle, composed of very large five 

 or six sided cells, in the exact centre of which is the opening 



