ADAPTATION IN CLEAVAGE. 53 



larva ; and the large cells, which henceforward divide equally, 

 the immense shell-gland. Can one imagine a more manifest 

 adaptation? The small cells are placed just where they are 

 needed, and are of the proper dimensions. Is there any other 

 reason why the somatoblast should not have divided equally 

 from the first, and the cells lying in one position have gone 

 into the foot, and the rest into the shell-gland.^ If this were to 

 happen, there would be no indication of adaptiveness in the 

 cleavage, for then it might seem possible that, simply because 

 of their different positions and conditions, part of the cells 

 formed the foot and part the shell-gland. 



d. To simimarize. — In the cleavage of the &^g of Unio there 

 are marked variations in the size of the cells and in the rate 

 and direction of their cleavages ; in every case, these possess 

 prospective significance, and by means of them the organism 

 is able, so to speak, to realize, in the most direct manner 

 possible, on its 'available capital, the substance of the Q.^g. To 

 this principle I have given the name of adaptation^ in cleavage. 



III. Adaptation in Cleavage in the Annelids. 



In the annelids, as in the mollusks, the entire ectoblast of 

 the trunk is segregated in a single cell, d- or X, known as the 

 first somatoblast, and the mesoblastic germ-bands in the second 

 somatoblast, c/4 or M. Now the cell-lineage of a great many 

 annelids is known through the studies of Whitman {i i), Wilson 

 (13 and 14), Mead (9), Eisig (5), Treadwell (10), Child (i), and 

 others. It is possible to arrange these annelids in a series 

 according to the relative size of their somatoblasts. First there 

 are forms like Polygordiiis, Lepidonottis, Podarke, Hydroides, and 

 Eiipomatus, with equal cleavage, in which the somatoblasts do 

 not differ in size from the other cells of the same quartets (Fig. 

 6). Then in the order of increasing relative size of the somato- 

 blasts come Amphitrite (Fig. 7) and ChcBtoptenis, Arenicola, 

 Nereis limbata (Fig. 8), Clymenella (Fig. 9), Capitella, Aricia 

 (Fig. 10), Scolecolepis and Spio, Nereis diimerilii, and finally, 

 Clepsine (Fig. 11). Now all of the forms with equal cleavage, 

 although most widely separated in relationships, possess trocho- 



