CELL DIVISION IN EGGS OF CREPIDULA. o09 



and in a slightly Inotropic direction, into two macromeres (fig. 30, A, B) and each 

 of these macromeres then gives rise to a first micromere of normal size and qualify 

 by dexiotropic cleavage (fig. 31, 34), then to a second one by Inotropic cleavage 



(fig. 35), and then to a third one by dexiotropic cleavage (fig. 37), exactly as 



normal eggs. If isolation takes place in the 4-cell stage each J4 blastomere 

 (macromere) gives off a first micromere by dexiotropic, a second by laeotropic, 

 and a third by dexiotropic cleavage (figs 32, 33, 34). If one of the four macro- 



meres is separated from the other three, the % blastomeres, if uninjuied. behave 

 as they would if the egg were still entire, except that they move together to close 

 the gap left by the missing macromere (figs. 39, 41, 43, 44), unless one or more of 

 them are injured, in which case the gap may not be completely closed (figp. 36, 

 40 42). Every x /± macromere whether isolated or joined with one or more 



gives rise to three micromeres of typical size and quality by divisions which are 



typical in direction. 



In all cases the relative sizes of macromeres and micromeres are approxiniai 1\ 

 the same as in normal eggs, and the micromeres are always free from yolk spher- 

 ules. The direction of the spindle axes, the direction and extent of rotation 

 of the cells in telokinesis, and the final positions of nuclei and spheres in the 

 cells are always the same as in normal eggs, with the single exception, mentioned 

 above, that owing to the closing of the gap left by the removal of one or more 

 macromeres the direction of cleavage, with reference to the whole, is slightly 

 altered. This closing of the gap is caused chiefly by the turning in or roi at ion of 

 entire cells and the axis of each of these cells, as marked by its nucleus and 



centrosome, remains unchanged. 



The subdivisions of the micromeres of typical eggs are also highly character- 

 istic; the first division of the first micromeres is laeotropic and very unequal, the 

 small peripheral products being the turret cells (la'-ld 2 ), while the large central 



products are the apical cells (la 1 -!^). The first division of the second micro 



meres (2a-2d) is dexiotropic and approximately equal. The first division of the 

 third micromeres (3a-3d) is Inotropic and slightly unequal, the peripheral 



product being smaller than the central one. In all of these respects the sub 

 divisions of the micromeres of partial eggs are wholly typical; in the first division 

 of the first micromere the spindle is eccentric in position and the division gives 

 rise to a small "turret cell" (la 2 -ld 2 ) whether the partial egg consists of 



two or three quadrants (figs. 33, 35, 37-44) ; in the first division of the second 

 set of micromeres the spindles are not eccentric and the resulting daughter cells 

 are approximately equal (figs. 37, 41, 42, 44); in the first division of the third 

 set of micromeres the spindles are again eccentric in the cell and the peripheral 

 daughter cells are smaller than the central ones (fig. 43). 



Later subdivisions of the micromeres of partial eggs are also entirely typical 

 for each quadrant. The second division of the apicals (la*-l«P) is unequal giving 

 rise to a larger peripheral cell, the "basal cell" (la^-l^ 2 ) and a smaller apical 

 cell" (W* <-!#.!) . at the same time the right half of each micromere of the 



