27 



After the spindles become completely formed, the cells begin to 

 elongate in the direction of the spindles. A slightly later stage than 

 that just described is shown in Plate 4, Figure 26. Comparing this 

 with Plate 3, Figure 22, it is evident that the cell a*-^ has stretched in 

 the direction of the spindle to such an extent that the difference between 

 the two axes of tlie cells is much diminished. iSTevertheless, in both this 

 cell and a**^ the axes in which the spindles lie are distinctly the shorter. 

 This is still true at the time of the division of the cells. Figure 27 

 (Plate 4) shows the right side of the egg last considered ; in the quad- 

 rant C the processes are much more advanced than in A. The nuclei 

 have separated and tlie cytoplasm is dividing, yet exact measurements 

 both of surface views and optical sections show that the greater diam- 

 eter is still at right angles to the line joining the two nuclei. A frontal 

 section, showing the greatest dorso-ventral extent of the cells of the 

 quadrants A and Cof this egg, is given in Figure 28. 



The two figures last mentioned show another fact of importance. The 

 divisions do not separate the blastomeres into cells of equal size in the 

 quadrants A, B, and G. The completed cleavage is shown in Plate 4, 

 Fig. 30 (anterior view). This, with the figures just cited, shows that the 

 cells a*'^-c*'^ divide very unequally, the dorsal derivatives, a^''*-c^-*, being 

 very much larger than the ventral ones, a'-^ -c"-^. The inequality is less 

 in the division of the ventral cells a*'^ - c*-^ Although the ventral de- 

 rivatives, a^-^-c^-\ occupy a larger area on the surface of the Qgg, there 

 is little difference in actual volume, and such as occurs is in favor of the 

 more dorsal cells a^-^-c^-^. 



The order of division is the same as in the last cleavage ; first, the 

 quadrant D, then in order C, B, A. In quadrant D the larger cell 

 d}-"^ divides first; in the other quadrants the cells are of equal size and 

 divide at the same time. 



The important facts in this fourth cleavage, from a cyto-mechanical 

 standpoint,- may be summarized as follows. 



1 In every case the first measurement was taken througli the two asters ; in the 

 case of rt*-i the real dorso-ventral extent of the cell, into which the spindle later 

 moves, is but 20m, — so that the ratio is as two to five. 



