CLEAVAGE IN ANNELIDS. Ill 



other words, the greatest amount of yolk accumulates in the 

 largest cell, and is not in itself the cause of that larger size. 



To summarize : equality of cleavage is not an indication of 

 lack of differentiation in the ovum, for definite cells appear 

 at definite places and at definite times, just as accurately as in 

 unequal cleavages. 



Inequality of cleavage may be due to a limited extent to the 

 accumulation of yolk ; but a more important factor is the method 

 of arrangement of the formative material. If an organ is to 

 appear early in the ontogeny, or is very large, it will be repre- 

 sented in the cleavage by a larger amount of material than if it 

 appears late, or is small when formed ; and it is this accumula- 

 tion of material within what we call a "cell" that leads to the 

 "unequal," as opposed to the "equal" type. It is not merely 

 the segregation of material in a single cell, but the segregation 

 of a large amount of material, which produces the differences 

 between the two types ; i.e., this difference is not qualitative, 

 but quantitative. 



