514 CELL DIVISION IN EGGS OF CREPIDULA. 



macromeres of the second set are larger than normal, and 2a and 2c contain a 

 quantity of yolk. In figs. 58 and 59 pressure in the direction of the chief axis 

 of the egg has led to the formation of the first set of micromeres between the 

 macromeres and in the same plane with them. In fig. 59 each of the eight cells 

 has divided or is dividing in a dexiotropic direction (with the possible exception 

 of the cells in quadrant D). The direction of this cleavage is therefore like that 

 by which the first set of micromeres are formed, and there are thus formed 

 almost simultaneously eight protoplasmic cells at the animal pole which may be 

 regarded as micromeres of the first set. In fig. 58 the 16-cell stage is completed; 

 it consists of four "micromeres" of the first set, each of which by Inotropic 

 division has given off peripherally a micromere, two of which (16 2 and Id 2 ) are 

 much larger than normal, while the other two (la 2 and lc 2 ) have been given off on 

 the vegetal side; the macromeres have also given rise to "micromeres" of the 

 second set, which contain yolk and are much larger than usual; this division was 

 Isotropic in A, B, C, but dexiotropic in D. Fig. 60 represents an egg which was 

 pressed 4 hrs. and freed 6 hrs. Quadrants A, B, D are normal except that the 

 "turret" cells in quadrants B and D (1& 2 , Id 2 ) are larger than normal; in quadrant 

 C the turret cell (lc 2 ) is also larger than normal and the micromere of the second 

 set (2C 1 ) is as large as a macromere. In all cases the direction of division is 

 normal. Fig. 61 is normal except in quadrant A; evidently the first set of 

 micromeres was formed in typical fashion but the second division of macromere 

 A was nearly equal, giving rise to 2 A 1 and 2 A 2 , the former of which has again 

 divided into 2A 1 and 2a 1 , but the latter (2.4 2 ) gives rise to no micromere of the 

 second set, all of its second micromere material having gone into its sister cell 

 2A 1 . Fig. 62 represents an egg like the preceding one in which the first division 

 of macromere C was nearly equal; the upper one of the macromeres thus formed 

 (1C) has divided, giving off an apical cell lc 1 ; the elongation of this cell and the 

 spireme in its nucleus indicate that it will soon cut off a turret cell at its periph- 

 eral pole; the position of cytoplasm, nucleus and centrosome in 1C indicates that 

 this cell is about to give off a micromere of the second set (2c 1 ), whereas the same 

 indications show that the macromere 2C has given off a micromere of the second 

 set (2c), but none of the first set; in other respects this egg is normal. In fig. 63 

 there are six macromeres, owing to the fact that the first division of A and D was 

 nearly equal; each of these six macromeres has produced a micromere of the first 

 set, while one (C) has produced a micromere of the second set. The positions of 

 the nuclei and spheres in IB, 14, ID, ID 1 indicate that the next division in these 

 cells will be laeo tropic, and will give rise to micromeres of the second set; on the 

 other hand in L4 1 it will be dexiotropic. In fig. 64 the conditions are much like 

 those shown in fig. 58, viz., the first micromeres have given off, or are just forming 

 the "turret" cells; two of these, lo 2 , lc 2 , lie at the lower pole; the second micro- 

 meres are larger than usual and contain yolk; one of these, 2a, was formed by a 

 dexiotropic division, the others by Isotropic division as in normal eggs. In fig. 65 

 quadrants C and D are approximately normal, 2B has been crowded under the 



