CLEAVAGE IN ANNELIDS. lO'J 



at the four-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 he concludes that unequal 

 cleavage is caused first by an accumulation of yolk, and second 

 by an unequal distribution of this yolk ; e.g., if there is a 

 difference between " micromeres " above and " macromeres " 

 below, it is because the yolk is collected at the lower pole of 

 the G:gg ; if a difference in size appears at the four-cell stage, 

 it is because of an unequal distribution of the yolk among the 

 blastomeres, etc. As far as I can discover, however, Rabl 

 makes no attempt at explaining why the yolk should have this 

 unequal distribution. While, as far as the first three cleavages 

 are concerned, this explanation might suffice, the most super- 

 ficial observation will show that it does not apply at all in later 

 stages. There cells of the same size divide equally or unequally, 

 and that, whether there is any yolk present or not. A glance 

 at any plate of illustrations of a cell-lineage paper would show 

 plenty of examples of this. In a discussion of this subject, Lillie 

 {loc. cit., p. 45) comes to the following conclusion : "Unequal 

 cleavage is conditioned by the constitution of the segmenting 

 ovum, and always means the precocious localization of an organ 

 or set of organs in the larger cell. This organ may be the 

 entoderm, in which case it is usually accompanied by yolk ; but 

 the inequality of the first two cells in the annelids and molluscs 

 is the earliest visible indication of another differentiation, the 

 larger cell containing the two somatoblasts." This explanation 

 seems to me in perfect accord with the facts. 



Turning now to equal cleavage, we might suppose it to be 

 due to a lack of differentiation in the early stages, and, in fact, 

 this explanation has been offered. Rabl ^ ('76) tried to explain 

 the difference between the unequal cleavage of a mollusc and 

 the equal cleavage of such a form as an ascidian by the assump- 

 tion that in the former there is an early differentiation of parts 

 manifesting itself in cells of different sizes, which does not 

 appear in the latter. This early differentiation, being of distinct 

 advantage in the struggle for existence, probably has developed 



^ " Entwick. der 'MzXermus.ch&X," Jeiiaische Zcitschrift, 1876. 



