ADAPTATION IN CLEAVAGE. 



47 



The first cleavage is meridional and very unequal, although the 

 distribution of the yolk throughout the cytoplasm is perfectly 

 uniform in the form of small granules (Fig. 20). The larger 

 cell marks the position of the posterior end of the enjbryo. This 

 cell divides before the smaller one, and unequally, the larger 

 product again being posterior in position. The smaller cell of 

 the two-celled stage then divides somewhat unequally, so that its 

 larger product lies on the left side of the embryo. Thus we reach 

 a four-celled stage (Fig. 2) 

 composed of one very large 

 cell, D, posterior in position, 

 and three smaller cells, of 

 which the left one, A, is the 

 largest. The anterior and 

 posterior cells, B and D, meet 

 in a broad cross-furrow at the 

 animal pole (Fig. 3), the cross- 

 furrow at the vegetative pole 

 being much smaller, contrary 

 to the general rule in eggs of 

 this type of cleavage. Thus 

 the first two cleavages illustrate the three possible modes of 

 variation, viz., inequality of the cleavage products, difference in 

 the rate of cleavage, apd difference in the position of the cells, 

 indicated by the broad cross-furrow at the animal pole. What 

 is the meaning of these differences.'* 



a. Relative Rates of Cleavage of tJic Cojistitrient Cells. — Let 

 us place side by side a table of the constitution of the thirty- 

 two-celled stage of the ^gg of Unio and of an ideal ovum fol- 

 lowing, in direction of cleavage, the ordinary spiral type, with 

 equal cleavage throughout, and the same rate of division in all 

 the cells. 



First generation of ectomeres 

 Second generation of ectomeres 

 Third generation of ectomeres 

 Fourth division of D (mesoblast cell) 

 Entomeres 



Fig. 2. — Four-celled stage of Unio coviplanata 

 seen obliquely from the left side and above. 



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