No. 3.] FO DARKE OBSCURA VERRILL. 443 



his work on Planorbis, describes a series of mollusks in which 

 at the 4-cell stage are found all gradations from four equal 

 cells through others with an accumulation of yolk in three, 

 then in two, and finally in one cell, the yolk-laden cells being 

 larger than the others. From these observations Rabl argues 

 that the primary cause of a difference in size among cleavage 

 blastomeres is an accumulation of yolk in one cell, or a group 

 of cells ; this mass of yolk causing the cell which contains it 

 to be larger than the others. If a difference in size appears 

 between micromeres above and macromeres below, it indicates 

 that there is a good deal of yolk in the ^gg. If, on the other 

 hand, this size difference appears in the 4-cell stage, there was 

 not necessarily either more or less yolk involved, but its dis- 

 tribution is different. As far as I can discover, however, Rabl 

 makes no attempt at explaining why yolk should have this 

 unequal distribution. 



If the problems of equal and unequal cleavage concerned 

 only the first three divisions of the ovum, an explanation of 

 this sort might sufifice ; but the most superficial observation 

 will show that it does not apply at all in later stages. Here 

 cells of the same size develop equally or unequally, apparently 

 without any reference whatever to the presence of yolk, and 

 solely with reference to the needs of the developing embryo. 

 Examples of this sort are numerous in the development of 

 Podarke. At the 32-cell stage the cells at the upper pole are 

 a very little larger than those at the lower, but at the next 

 division the very small apical rosette cells appear at the upper 

 pole, while at the lower the division is nearly equal. It cannot 

 be maintained that there is more yolk in the cross cells than in 

 the cells which later become entoderm. Again, the cell 2^2.1 

 certainly contains no more yolk than the corresponding cells in 

 the other quadrants, but its division, as we have seen, is much 

 more unequal than theirs. Examples of this sort might be 

 multiplied indefinitely. 



Within certain limits yolk does undoubtedly modify the form 

 of cleavage. No one would dispute, I think, that the mero- 

 blastic cleavage of the zgg of the teleost or the bird is caused 

 by an accumulation of yolk at the lower pole, and the large 



