JEXXIXGS : DEYELOPMEXT OF ASPLAXCHXA HEEEICKII. 31 



demanded by the principle of least surfaces. Flat plates, such as we see 

 in d^-' and d^-^, and in all the cells of the third layei", retain their form 

 in virtue of some force working strongly against surface tension. 



After the fourth cleavage, the astei-s in all the cells at first separate 

 at right angles to the axes of the preceding spindles, as happened after 

 the third cleavage. The later changes are essentially the same in all 

 the quadrants, so far as the asters are concerned, so that quadrant D 

 may be described as a type. 



Figure 31 (Plate 4) shows the conditions in the four cells of quadrant 

 D, after the asters have divided. The two asters of each cell lie upon 

 opposite sides of their nuclei in such a position that, if no change 

 occurs, the ensuing division will be meridional. 



In the cells of the first three layers the astei's retain their original 

 positions. But in d^-^ a rotation takes place, such as occurred in all the 

 cells in preparation for the preceding (fourth) cleavage, so that the axis 

 of the spindle in d^-* is at right angles to the axes of the spindles in the 

 other three cells of the quadrant. This condition is shown in Figure 

 33, and the completion of the division is shown in Plate 5, Fig. 37. 



The same processes take place in the other quadrants, so that all the 

 cells of the first three layers have spindles extending laterally, while in 

 the fourth or dorsal layer the spindles are directed doi-so-ventrally 

 (Fig. 40). 



We must now consider the cleavage in the several cells more in 

 detail. 



As in previous cleavages, division takes place first in the cells of 

 quadrant D. The nucleus of the large ventral cell, rf^'\ is earliest to 

 enter upon the karyokinetic process, followed immediately by d^--, and 

 a little later by d^ ^ and d^-'^. Figure 33 (Plate 4) gives a view of the 

 posterior surface of the egg at this stage, showing the spindles in all the 

 cells. As this figure shows, the spindles in the three cells rf^-\ c?^-*, 

 and d°-^ do not lie in the middle of the cells, but nearer the right ends. 

 (See definition of right and left, page 17.) In the dorsal cell, cf^•^ the 

 spindle is at right angles to those in the other cells. The plane of 

 cleavage indicated in the three ventral cells is meridional; in the dorsal 

 cell it is equatorial, like that of the two preceding cleavages. The division 

 of each of the four cells must be considered separately. 



To understand the cleavage of the large ventral cell, d^-^, it is neces- 

 sary to observe accurately its position and relation to the other cells. 

 A longitudinal section of about the same stage as that shown in Figure 

 33 is given in Figure 34. Comparing this with the earlier correspond- 



