No. 4.] CELL LINEAGE OF POD ARK E OBSCURA. 



197 



Third. The 64-cell stage is completed by the division of 

 a*' 1 , a 2 ' 2 , b 2 '\ b 2 - 2 , c 2 '\ c 2 ' 2 , d 2 ' 1 , d 2 ' 2 ; and here arises the first 

 distinction which I have discovered between the quadrants. 

 While in three quadrants the division is such that the upper 

 left-hand .cell (a 2 ' 2 ' 1 , etc.) is smaller than the one below it 

 (a 2 ' 2 ' 2 , etc.), in one quadrant the lower cell is much the smaller, 

 has a peculiar deeply staining nucleus, and is easily distin- 

 guished from the corresponding cells in the other quadrants. 

 (See Figs, i and 2, as also Fig. 5, where the size of this 

 cell as compared with the corresponding one in the other quad- 

 rants is shown. It should be remembered that the " left- 



FIG. i. Second group of micromeres in D quadrant after their second division (64-cell stage). 

 FIG. 2. Second group of micromeres of A quadrant at 64-cell stage. Compare relative sizes of 

 ^2.2.2 an d ,,2.2.2. 



hand " cell is the one which is on the left when looked at from 

 the animal pole.) This lies immediately over one of the fourth 

 group of micromeres, and a little to one side of the second 

 cleavage line. For reasons which will appear more fully later 

 on, I believe that this corresponds to the cell x 1 '*, described in 

 Amphitrite, Nereis, Arenicola, Crepidula, and Unio. 1 



The next divisions are those of cells a 1 ' 3 , & 1 ' 3 , c 1 ' 3 , d l '\ at the 

 upper pole, leading to the formation of the apical cross. Two 

 of these cells divide equally or nearly so, while the other two 

 divide very unequally. (See Fig. 4.) Here, although the 

 divisions are still of the spiral type, a bilateral arrangement of 

 cells results. The second line of cleavage passes in the direc- 

 tion indicated by the numerals. It is interesting to note that 



1 Lillie, " Embryology of the Unionidae," ,/0wm. of Morph., vol. x. 



