lOS BIOLOGICAL LECTURES. 



for that reason, while in the ascidian the equal cleavage indicates 

 that differentiation begins much later. 



From the facts which I have given above there can be no 

 question that in Podarke there is as great a differentiation as 

 in any annelid of the unequal type. The position of the " polar 

 furrow," the appearance of the cell Xi, 2, and the perfectly 

 regular formation of definite cells at definite times, all indicate 

 this beyond the shadow of a doubt. 



In a note published about a year ago ^ I ventured a suggestion 

 concerning the meaning of this equal cleavage, and what I have 

 seen since then confirms me in my original opinion. Accepting 

 the principle laid down by Lillie, and further developed by 

 Conklin {Biol. LectJires, 1896, 1897), that the initial size of a 

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

 formation of the part to which it gives rise, I pointed out that 

 in all probability the small size of 2d and 4d is connected with 

 the slow development of the parts of the embryo arising from 

 these cells. The facts upon which this conclusion is based will 

 be evident from the comparison which I have made between 

 Podarke and AmpJiitrite, where the slow development of ecto- 

 derm and mesoderm in the former (the descendants of 2d and 

 /}.d), and the rapid development of the corresponding portions 

 of the embryo in the latter, are noticeable features of the ontog- 

 eny. In Podarke, also, a large part of the embryo, which is 

 composed of definitive mesoderm in other annelids, is made up 

 of the larval mesoderm mentioned above, — an additional reason 

 for the slow development of /{.d. I believe further that absence 

 of yolk is not alone responsible for the equality of the first 

 three cleavages ; but that the fact that the third division (form- 

 ing the eight-cell stage) is equal is connected with the large 

 size of the umbrella, relative to the subumbrella of Podarke, 

 when compared with other annelids, and with the fact that a 

 large part of the subumbrella ectoderm really comes from the 

 umbrella, having migrated through the break in the proto- 

 troch. 



We have already seen that at the sixteen-cell stage, and later, 

 the largest cells in the embryo are at the upper pole. 



1 Zool. Bulletin, No. 4. 



