24 bulletin: museum of comparative zoology. 



Views of the eight-cell stage are shown in Figures 15 to 18. 



From a cyto-mechauical standpoint, the third cleavage may be char- 

 acterized as follows. The iirst division of the asters is along a line at 

 rio-ht angles to the axis of the previous spindles, and indicates the posi- 

 tion of the spindles for the next cleavage. These lie in the long axes 

 of the cells, and the cell walls are formed in the position demanded by 

 the principle of least surfaces. 



Fourth Cleavage. 



Immediately after the division of D^ (Plate 2, Fig. 12), the asters in 

 d*-^ and d*- begin to extend laterally, at right angles to the axis of the 

 preceding spindles, and each becomes divided into the two asters for the 

 following spindle. In c?*'^, Figure 11, the two asters for the succeed- 

 ing cleavage are still connected by a striate band. Figui-e 11 shows a 

 ventral view of the same egg as Figure 12, the five-cell stage. The 

 corresponding dorsal view of a slightly later stage is shown in Figure 

 14. The asters in d*-' are moving apart in the same manner as in d*-^, 

 save that the line of separation is slightly oblique, the left aster being 

 higher. 



In the same way the asters in the cells d^-^-c*-^ and a^-'-c*'"^ become 

 constricted, and divide at right angles to the axis of the preceding 

 spindles. The dividing asters in c*- are shown in Figures 14 (Plate 2) 

 and 17 (Plate 3), and those of c*-^ in the latter figure. Views of the 

 other four cells would show similar conditions. 



From the manner in which the asters separate in all of the eight 

 cells, cue would be led to expect that the next cleavage would be merid- 

 ional, at right angles to the third cleavage. This expectation is 

 strengthened by the fact that the lateral dimensions of the cells in 

 which the asters lie are considerably greater (in the quadrants A, B, and 

 C, at least) than the opposite measurements (Fig. 17). 



But in a slightly later stage it is observed that the line joining the 

 asters in d*-^ has become oblique, like that joining those of d*-^ (as 

 mentioned above). This oblique position of the asters in c?*-^ is shown 

 in the ventral view (Fig. 1.5). The left aster (right side of the figure) 

 has become ventral, the right one dorsal. The sagittal section (Plate 1, 

 Fig. 7) of a slightly later stage shows the completion of the rotation 

 thus begun ; the line connecting the two astei's and passing through the 

 nucleus is now approximately dorso-ventral in direction. 



At the same time a similar rotation has taken place in the cell d*-^, 

 but the position taken by the two asters is not the same as in d^'^. One 



