202 TREADWELL. [VOL. I. 



of the vegetative, but are hard to distinguish from them except 

 for the presence of tJie polar globules. The next division, how- 

 ever, leads to the formation of the rosette cells at the top, and 

 the fourth group of micromeres at the bottom, the latter divi- 

 sion being nearly an equal one, the former very unequal. 1 It is 

 difficult to explain cases of this kind except on the assumption 

 of a complex differentiation in the protoplasm of the egg. 



That mechanical conditions can play but a small part in the 

 regulation of cell divisions is shown especially well by the small 

 cell x 1 ' 2 . This cell arises in exactly the same manner in 

 Amphitrite, Clymenella, Arenicola, Unio, and Crepidula (Con- 

 klin's 2d 2 ' 2 } of the unequal, and in Lepidonotus, Sthenolais, 

 Podarke, and Hydroides of the equal type. Comparison of the 

 figures of the first forms with those of the second will show 

 that mechanical conditions must be very different in the two 

 cases. On the one hand, we have large mesoderm and entoderm 

 cells which must exert considerable pressure on the cells sur- 

 rounding them; on the other, the largest cells in the embryo 

 are at the animal pole. These facts point to some definite 

 complex organization of the egg protoplasm. 



The problem to be settled by a study of equal cleavage has 

 been stated by Mead thus: 2 "Whether one of the two cells in 

 equal cleavage is homologous with the larger cell in unequal 

 cleavage." I believe that one of these cells in the one case is 

 homologous with one in the other, and that the second of 

 Mead's alternatives is correct, that the " peculiar destiny of the 

 cell' is "the cause of its larger size," and I would suggest 

 that the cell D in the unequal type is larger, not simply because 

 it contains somatic and mesodermic material, as Wilson sup- 

 posed, but because it contains an extra supply of this material. 

 Sufficient data for wide generalizations are not at hand, but 

 such as we have bear out this supposition. 



The trochophore of Podarke is small, with very thin walls 

 and a feeble development of mesodermal tissue. It grows very 

 slowly, so that scarcely any change except a slight increase in 

 length is perceptible from twenty-four to seventy-two hours. 

 Dr. Mead informs me that the same is true of the trochophore 



1 Cf. Mead, /. c., p. 293. 2 L. c., p. 278. 



