444 TREADWELL. [Vol. XVII. 



size of the macromeres in, e.g., Crepidula is to a certain extent 

 due to the yolk which they contain. But even in these cases 

 size differences, plainly not referable to yolk, appear among 

 the blastomeres, and when we attempt any general survey of 

 the cleavage phenomena the principle wholly fails. 



What seems to me the most probable explanation was first 

 advanced by Lillie (No. 21). In a discussion of this subject he 

 says (p. 45) : " Unequal cleavage is conditioned by the constitu- 

 tion of the segmenting ovum, and always means the precocious 

 localization of an organ or set of organs in the larger cell." 

 This principle was subsequently elaborated by Conklin (Nos. 

 5, a and 5, b), who added the qualification that the size of a 

 blastomere stands in direct relation to the size and the time of 

 formation of the part to which it gives rise, and he explained 

 many of the phenomena in the development of Crepidula by 

 the aid of this principle. 



Turning now to equal cleavage, it might be supposed that 

 this form of division was due to a lack of differentiation in the 

 early stages, and, so far as I know, this has been the usual 

 explanation given for it. Rabl (No. 27, a) explained the differ- 

 ence between the unequal cleavage of a mollusk and the equal 

 cleavage of an ascidian by supposing that in the former there is 

 a greater differentiation of parts, manifesting itself in size dif- 

 ferences among the blastomeres, which does not appear in the 

 latter. In the ascidian this differentiation begins much later, 

 and only then do size differences make their appearance. Rabl 

 believed that an early differentiation was of distinct advantage 

 in the struggle for existence, and that it probably had appeared 

 in the mollusks on that account, while for some reason it was 

 delayed in the ascidian. 



From the facts given in the earlier part of this paper, there 

 can be, I think, no doubt that in the embryo of Podarke there 

 is as great a differentiation, appearing as early as in any form 

 with the equal type. The polar differentiation is obviously as 

 well marked. A bilateral arrangement of parts, foreshadowed 

 by the constant relation of the polar furrow to the body axes, 

 is evidently present, and the only doubt would be whether in 

 the 4-cell stage there may not be two possibilities of orientation, 



