26 BULLETIN: MUSEUM OF COMPAEATR^E ZOOLOGY. 



taiuing the larger yolk granules shown in Figure 7. The larger deriva- 

 tive retains its position at the macromere end of the egg (Fig. 19). 

 The smaller cell d^-^ is therefore pushed dorsad, and this, together with 

 the extension of d*'^ at the time of the formation of its spindle, dis- 

 places the animal pole, marked by the polar cell, still fixrther toward 

 the micromere end of the egg. As a further result, the cells of the 

 quadrants A, B, and C are still more compressed doi'so-ventrally, so 

 that, especially in a**^-c'*-\ the lateral extent is much gi-eater than the 

 dorso-ventral (Fig. 19). 



Nevertheless, as Figure 19 shows, the rotation of the future spindle 

 axis still continues. The line joining the asters becomes dorso-ventral 

 first in the dorsal cells a*-^-f*--, while in a^-^-c*-Hhe asters are still 

 oblique, as shown in c^-^, Figure 19. In quadrant B of this same figure, 

 the axis has become dorso-ventral in both cells. 



Now occurs the cleavage of c?**^ with still further elongation, shifting 

 of the animal pole toward the micromere end of the egg, and resulting 

 greater compression of the cells of the quadrants A, B, and C (Figs. 

 20-2-1). "Without regard to this, the asters in the cells of these quad- 

 rants continue their movements until the future spindle axes are in 

 every case dorso-ventral. Spindles ai'e now formed in all of the six 

 cells, the spindle being in every case in the shortest axis of the cell 

 (Figs. 20-24). 



The conditions at this stage are so significant from a cy to-mechanical 

 standpoint, that I have thought it best to analyze and illustrate with 

 esnecial fulness a typical egg at this stage. Figures 20-25 are views 

 of a single egg. Figure 20 shows the right side (quadrant C), Figure 

 22 the left side (quadrant A), Figure 21 an anterior view (quadrant B), 

 and Figure 25 a posterior view (quadrant D). In Figures 23 and 24 are 

 given respectively sagittal and frontal optical sections, for comparison 

 with the surface views. 



In order to exclude possibility of error, the egg from which the above 

 figures were taken was moved about so that the six cells belonging to 

 the quadrants A, B, and C occupied successively the middle of the 

 upper surface of the egg ; careful camera figures of each cell were made 

 in this position. Then optical sections were taken in the same way, 

 both along the axis in which the spindles lie, and at right angles to 

 these. Accurate measurements of the dimensions of the cells could 

 thus be made ; the results are as follows. 



