26 



BIOLOGICAL LECTURES. 



of the egg, where repeated divisions are in the same direction, 

 and apparently in the shortest diameter of the protoplasm and 

 in the line of greatest resistance. It appears also in the for- 

 mation of certain definite structures, such as the ectoblastic 

 cross, where the direction of a certain division is reversed in 

 each arm. Upon this reversal depends the existence of the 

 cross as such, and presumably of certain structures to which it 

 gives rise. 



Another remarkable instance of determinate cleavage is 

 found in the unequal division of cells. Such unequal division 

 constantly occurs in the formation of certain cells and is one 

 of the most striking features of determinate cleavage. As has 

 been said, the first cleavage may be unequal, though in most 

 species the first and second cleavages divide the egg into nearly 

 equal cells. In the formation of the three quartettes of ecto- 

 meres, however, the divisions are usually very unequal (Figs. 2, 

 3, 4), while in the formation of the fourth and fifth quartettes 

 divisions are again more nearly equal. I have already called 

 attention to the very small size of the trochoblasts when first 



FIG. 5. 



FIG. 6. 



FIG. 5. Crepidula, 42-cell stage. Shading as in Figs. 3, 4. The cross (shown in strong 

 outline) lies in the position in which it was first formed. The heavy, radiating lines 

 separate the cells of the different quadrants. 



FIG. 6. Crepidula, 6o-cell stage. The whole of the ectoblast has rotated to the left, due 

 to the rotation of the fourth-quartette cells. The middle cells in three arms of the 

 cross have divided transversely. The third-quartette cells of the posterior quadrants 

 have divided bilaterally. 



formed; another illustration is found in the tip cells of the 

 cross (Fig. 5). In fact, no phenomenon is more common in 

 determinate cleavage than the unequal division of apparently 



