174 EDWIN G. CONKLIN 



large macromeres which contain httle cytoplasm and almost all 

 the yolk. The early subdivisions of the ectomeres take place in 

 exactly the same way in the largest as in the smallest eggs, 

 though the individual cells are larger in the former than in the 

 latter. When the third and last quartet of ectomeres is separated 

 from the macromeres, the first quartet has divided, and, in the 

 smaller eggs of C. plana, the second quartet also, so that the 

 completely segregated ectoderm consists of a plate of sixteen, or 

 twenty, protoplasmic cells resting upon the great yolk cells, or 

 macromeres (figs. 6, 7). Since this ectodermal plate contains 

 most of the cytoplasm of the egg, its dimensions in the different 

 species give a fair idea of the relative amounts of cytoplasm in 

 these eggs. This plate is larger in the large eggs than in the small 

 ones, as table 5 shows, and of course the individual cells of which 

 it is composed are larger in the former than in the latter. It 

 will be seen by consulting table 5 that the diameter of the ecto- 

 dermal plate is considerably greater than the diameter of the cyto- 

 plasmic area of the unsegmented egg; this is due in large part to 

 the more complete segregation of the cytoplasm in the later stage 

 than in the earlier one, though in part it is due to the growth 

 of cytoplasm at the expense of yolk, as I have shown elsewhere 

 ('12.) 



It is a matter of capital importance that all differential cleav- 

 ages of the egg are precisely similar in number and character in 

 all these species of Crepidula, whatever the size of the egg may 

 be. Not only are all the cleavages which give rise to ectomeres, 

 mesomeres and entomeres the same in all species of the genus, 

 but all subdivisions of these cells, which are differential in char- 

 acter are the same in all these species, so far as I have been able to 

 determine. It is only in the case of non-differential cleavages 

 that differences in the number of cells appear in the different 

 species. But while the differential cell divisions do not vary 

 in number under normal conditions, this number is not to be re- 

 garded as irrevocably fixed, for it may be experimentally altered, 

 as I shall show in another paper. 



Although the number of cleavage cells is the same in all species 

 during the early cleavage stages, it comes to differ greatly in the 



