78 bulletin: museum of compaeative zoology. 



Figures 49, 50, 51, and 64 (Plates 6 and 8), until the cloud of granules 

 is enclosed at the seventh cleavage in the smaller of the two entoderm 

 cells (Fig. 65). This whole process shows clearly that other changes 

 of a striking character are taking place at the same time as the division 

 of the egg into smaller portions ; evidently cleavage is not a mere 

 separation of the egg into smaller masses, each similar to the other and 

 to the original egg. The final destiny of this granular mass is not 

 known, but such a peculiar and well characterized phenomenon as it 

 exhibits cannot be considered meaningless. 



The differentiation in this instance is of the kind admitted by Driesch 

 (see page 9), in that it is cytoplasmic in nature. It is not, however, 

 a direct consequence of the original distribution of materials within the 

 egg; the migrations of the granules show that processes are taking 

 place in the cytoplasm that are only indirectly connected with cell 

 division. 



We have in this case a distinctly visible differentiation accompanying 

 cleavage. Certain other phenomena give evidence that there are like- 

 wise invisible differentiations accompanying the process. 



At the division of the second " layer " of ectodermal cells in the 

 sixth cleavage, shown in Figure 55 (Plate 7), the two rows of cells d!-^- 

 c''^, a'-^-c'-^, <£'■''-€'•'', and d'-^-c'-^ are produced. The cells of these 

 two rows, as shown in Figure 61, are of the same size and the same 

 form, having similar relations to the surrounding cells and to the axis of 

 the embryo. Yet, as has been repeatedly stated, all the cells of one row 

 divide meridionally and equally with spindles in the long axes, the cells 

 of the other row equatorially and unequally with spindles in the short 

 axes. What causes this difference 1 



The difference must, of course, be due either to a different stimu- 

 lus from the outside, or to a different structure of the cells. The 

 problem maj^ be expressed clearly in this way : If one of the cells of the 

 more dorsal row, as a^-'^ (Fig. 61), could be removed and placed in 

 the position now occupied by a'-'', in the more ventral row, would it 

 change its method of division ? That is, would it cleave equatorially 

 and unequally, with its spindle in the short axis, like the other cells of 

 the ventral row, instead of meridionally and equally, with its spindle in 

 the long axis, as it actually does? 



There is, of course, no way of answering this question directly. It 

 scarcely appears probable, however, that there is such a difference in the 

 influences affecting the two cells as to cause so fundamental a difference 

 in the cleavage. And if there is not, the only alternative is, that there 



