CLEAVAGE AND DIFFERENTIATION. 25 



ality of the cleavage is directly and causally related to the 

 bilaterality of the larva and the adult, though in some cases 

 extensive rotations of cells and even of entire layers are neces- 

 sary in order to bring blastomeres and planes of symmetry into 

 their proper positions. 



Apart from qualitative cell divisions, which are undoubtedly 

 an important factor in differentiation, differential cleavages are 

 the result of differences in the time and direction of division 

 and in the size of the daughter cells. If divisions were always 

 synchronous, alternating, and equal almost all the visible features 

 of differential cleavage would disappear; it is in the constancy 

 of certain peculiarities in the rate and direction of division and 

 in the size of resulting cells that determinate cleavage is chiefly 

 manifest. 



Among the gasteropods mentioned above, the rate of growth 

 and division of certain cells is highly peculiar, and in general 

 this cannot be explained by the presence of yolk or by other ex- 

 trinsic (that is, non-protoplasmic) causes. Adjacent and appar- 

 ently homogeneous cells may behave in the most remarkably 

 unlike ways in this regard. For example, the trochoblasts are at 

 the time of their formation the smallest cells in the entire egg 

 (Fig. 4) ; they grow rapidly, but divide rarely, and are character- 

 ized by having clear, non-granular protoplasm. On the other 

 hand, the apical cells which gave rise to the trochoblasts are 

 composed of granular protoplasm, and, although they grow 

 scarcely more than the trochoblasts, they divide repeatedly, 

 each of them giving rise at the stage shown in Fig. 10 to twelve 

 cells, the total volume of which scarcely exceeds that of a 

 single trochoblast. Many other illustrations of this same fact 

 might be given. 



In the departure of certain cells from the rule of alternating 

 cleavage, or Sachs' law of rectangular intersection, we have 

 another factor of differentiation and a marked feature of deter- 

 minate cleavage. This is beautifully shown among the gastero- 

 pods named in the transition from radial to bilateral cleavages; 

 in such cases the direction of division is reversed usually in one 

 cell of a quartette (Fig 6). It is also shown in all cases of telo- 

 blastic growth, of which there are many at the posterior pole 



