52 BIOLOGICAL LECTURES. 



great bulk of the material passing into the two somatoblasts. 

 In the thirty-two-celled stage (Fig. 5) the largest cells are 

 those of the second generation of ectomeres, and this finds a 

 sufficient explanation in the fact that the material of the first 

 somatoblast, of the larval mesoblast, and of the larval mantle, is 

 found in these cells. The relatively small size of the first and 

 third generations of ectomeres and of the endoderm-cells is 

 correlated with the relatively small part that they play in the 

 upbuilding of the larva. The mesoderm proteloblast d^ is 

 actually smaller than the larval mesoblast Y (Fig. 5) because 

 of its deferred functional activity. 



If there were any other explanation of these differences in 

 size of the cleavage products than this seemingly teleological 

 one, it might seem to simplify the problem of development. 

 However, all theories that seek to explain inequalities of cleav- 

 age by the presence of yolk or the action of any known external 

 force seem inadequate to explain the unequal cleavages in the 

 egg of Unio. 



c. Variations in the Direction of the Cleavage-Planes. The 

 early divisions follow the law of alternating spiral cleavage 

 (Kofoid) for some time ; and I shall not stop to consider these, 

 although we are far from an adequate mechanical explanation 

 of this law. I shall take up, instead, certain special divisions, 

 which illustrate in a more striking manner the adaptiveness of 

 variations in the direction of cleavage. The first somatoblast 

 is established in the nine-celled stage by the second division of 

 D ; it is much the largest cell in the embryo, as we have seen, 

 and it goes through with an exceedingly characteristic series 

 of cleavages. It first buds off a small cell, x l , on the right 

 side of the vegetative pole, then another small cell, x 2 , symmet- 

 rically placed on the left side, then a third small cell, x\ in the 

 middle line, but towards the animal pole, then a fourth small cell, 

 x* y in the middle line towards the vegetative pole (see Fig. 5); it 

 then divides equally and bilaterally, and each half buds forth 

 another small cell, x$ t behind x^ on the vegetative pole. I do 

 not know the fate of the cell *3, budded forth towards the ani- 

 mal pole. However, the other small products form the rudi- 

 ment of the foot, which is an exceedingly small organ in the 



