472 TREADWELL. [Vol. XVII. 



This position is not, I think, inconsistent with the explana- 

 tion given before for the distinction between equal and unequal 

 cleavage. I do not believe that the accidental accumulation of 

 a certain amount of material in any particular cell necessarily 

 implies that that material is, at the time of localization, dif- 

 ferentiated. As a physiological rather than a morphological 

 process, the egg has cleaved into a number of cells, and the 

 organism, making use of these cleavage products, has stored in 

 each variable amounts of material according to the needs of 

 that particular region of the body. If this be true, there would 

 follow a different explanation for the small size of some cells 

 than the one given by Professor Wilson. He suggests that 

 the small size of many cells in the cleavage stages indicates, 

 like the degenerate organs of adults, degenerated structures 

 which were once functional, but have in the phylogeny of the 

 group become so functionless that they are now represented 

 in the ontogeny merely by rudiments. (While this caution 

 would possibly not apply to the small " mesentoblasts " of 

 Nereis or Podarke, yet it should be remembered that mere 

 size is in itself no indication of a degenerate condition. Cells 

 which are very small when first formed may increase in size 

 later and make up an appreciable portion of the embryo.) 



If organs which are to appear early, or make up a large 

 portion of the embryo, may be and are represented in the 

 ontogeny by large cells, the converse of this proposition is 

 correct, and the small size of many cells is due to the fact 

 that the portion of the organism which is to arise from them 

 is either small when formed, or appears late. (See above, 

 p. 448.) 



On this supposition portions of the embryo which are small 

 are regarded as having a smaller amount of material set apart 

 for them in the cleavage stages, and would thus be represented 

 by smaller cells. Since these small cells are rarely the same 

 in number {cf. the number of "mesentoderm cells" in the 

 various cases), and their behavior is different in different forms 

 (cf. the anomalous division of the cells corresponding to the 

 prototroch in Chaetopterus, No. 22, Figs. 131 and 132), the 

 rudimentary character of the cells must be regarded as individual 



