34 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



cells of quadrants A, B, and C the spindles are not in the short axes, as 

 ill d'"'*, since these cells are not nearly so broad as that one, while the 

 dorso- ventral dimension is about the same or greater (Plate 4, Figs. 30 

 and 32). Figure -il (Plate 5) shows the process of cleavage in these 

 cells, while Figure 45 (Plate 6) shows the cleavage concluded. 



The division of the second layer follows upon that of the fourth, and 

 again in the order c^-, b^-^, a^-^. The division is here meridional, as it is 

 in the corresponding cells of the quadrant D, and equal, as it is not in the 

 corresponding cell of the quadrant D. (See Figs. 39, 40, 43, and 44.) 



Next follow the divisions of the cells of the first layer, in the same 

 order as in the previous cleavages, and, slightly later, the divisions in 

 the third layer, also in the same sequence. The cleavages here also are 

 meridional and equal. 



The nuclear conditions leading to these cleavages are shown in Figures 

 39, 40, 42 (Plate 5), all from the same egg. A somewhat later stage 

 is shown in Figure 43, which exhibits the conditions in the quadrants 

 A, B, and C at a time when the divisions just described are completed 

 in most of the cells. All the divisions are nearly finished except in the 

 cells of the third layer (a^-*-c^-^), which still contain spindles. This 

 view shows also another peculiar fact. During the cleavages the cells 

 about the dorsal pole of the egg have shifted, and the cells a®'^ and c^-"^ 

 have pushed ventrad to such an extent that on the right side the cells 

 e'-^ and If^ have become completely separated, a part of the cell c^-"^ 

 lying between them. This condition is only transitory, however; the 

 cell c^'^ is very soon pushed dorsad again, and the cells of the third layer 

 again form a continuous row. (Compare Figure 47, Plate 6.) Figure 46 

 shows the cells of the quadrant D at the close of this division, while 

 Figure 45 is a view of the animal pole at the same stage. 



The features of the fifth cleavage may be summarized as follows. In 

 the twelve cells of the three ventral layers, the asters separate after the 

 fourth cleavage at right angles to the position of the preceding spindles 

 and retain the position first taken ; the cleavage is therefore meridional. 

 In all these cells the spindles are in the long axes of the cells. In the 

 dorsal laj-er the asters at first assume the same position as in the other 

 cells, but later a rotation takes place, and the spindles when formed 

 have a dorso-ventral direction ; the resulting division is equatorial. The 

 spindle is in the longer axis of the cells o^''*, 5^'*, and c"^, in the shorter 

 axis in <f^'*. 



The division is unequal in the four dorsal cells of all the quadrants, 

 and in all the cells of the quadrant D. In the other cells it is equal. 



