No. 3-] EMBRYOLOGY OF UNIO COM PLAN ATA. 571 



the three other cells were purely ectodermic, while, in fact, all 

 contain entoderm. Rabl was right so far that all four cells 

 contain ectoderm, but so do all contain entoderm; while 

 D contains the posterior mesoblast, and A the larval meso- 

 blast. 



The next stage to which I will direct special attention is the 

 eight-cell stage (Fig. 2), which has the typical form of four 

 apical micromeres lying on four macromeres, and alternating 

 with them (being given off in a right-handed spiral). Here I 

 must differ from Rabl's account. The four micromeres do not, 

 indeed, arise simultaneously, but one after the other, and 

 generally in the following order, d,^ c,^ a,^ b,^ from D, C, A, 

 and B, respectively. The last three arise some time after d,^ 

 and very nearly together. These four cells form the first 

 generation of micromeres, and are purely ectodermic. Accord- 

 ing to Rabl d^ (5 of Rabl) divides at the same time as C (3 of 

 Rabl). Whether the difference observed is due to the difference 

 of species or not, I am unable to say. 



The sixteen-cell stage does not actually occur as such, but 

 the stage with seventeen cells corresponds to the sixteen-cell 

 stage of Annelids, with one further division, which, in the 

 Annelid ovum, occurs later. This stage does not arise at once 

 from the eight-cell stage, but almost all the cells of the eight- 

 cell stage divide at different times. The four macromeres are 

 the first to divide, and of these D takes the lead, dividing by 

 an equatorial plane into two unequal parts, of which the 

 smaller D lies on the vegetative pole, and the larger d- 

 between D and c^ and d^ of the apical pole. C, A, and B 

 divide next in the same plane as D, and in this general order. 

 In each case the smaller product lies on the vegetative pole. 

 I have, therefore, reserved the term macromere, and the desig- 

 nation by capital letters for these cells. The three larger cells 

 are c,^ a,~ and d^; together with d~ these cells form the 

 second generation of micromeres. It is interesting to note 

 that the second generation of micromeres is given off from 

 the macromeres in a left-handed spiral ; thus the reverse of the 

 first generation ; and that the third generation is given off in a 

 right-handed spiral again (Figs. 3 and 4). The spiral arrange- 



