38 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



completely formed. The dorso-ventral axis of the cell has greatly in- 

 creased, but is still distinctly less than the width of the cell at right 

 angles to the spindle. The cytoplasm has become grouped symmetri- 

 cally about the spindle, with the latter in its short axis. A longitudinal 

 section of the same egg is shown in Plate 6, Fig. 48 ; the completed 

 cleavage {(T-^ and d'-^) is shown in section in Figure 50, and from the 

 surface in Plate 7, Fig. 57. The cleavage is unequal, the ventral cell 

 being much the smaller. 



Cleavage in the other cells of this generation takes place in a sequence 

 that is complicated by various factors, so that the account will be 

 clearer if the divisions of the cells are described in connection with their 

 relative positions in the e^'g, reserving a discussion of the order of 

 cleavage till the end. The divisions will be taken up according to the 

 layers of cells, beginning with the ventral layer. 



First or Ventral Layer, consisting of the eight cells, a^-'^-d^-^ and 



The cleavage of d^'^ has been described. The small vesicle c?®"^ does 

 not divide farther. The otlier cells of this layer divide equatorially 

 into cells of equal size. Two of the spindles leading to this cleavage 

 are shown in Figure 56 (Plate 7), and the completed cleavage in Figure 

 61. The resulting fourteen cells are d'-'^-d'-^, d'-'-d''^, d'-^-c^-^, 

 and d'-^ — c'-*. 



Second Layer, containing the cells a^-^-d^-^ and a^-^-d^*. 



The cleavage of d^-^ has been described ; it is equatorial and unequal. 

 The remainder of the cells also divide by equatorial furrows, but the 

 products are equal in size. One of the spindles is shown in c®-*, in Fig- 

 ure 47 (Plate 6), and in Figure 55 (Plate 7) the nearly completed 

 cleavage; the nuclei in all but the products of c^-^ are still connected 

 by interzonal filaments. The same condition of the cell d^-'^ is shown 

 in Figure 57 (the products being d'^-'' and rf'**). In all of these cells, 

 except d^*, the spindles lie at first in the shorter axes of the cells, as 

 indicated in Figure 47 (Plate 6) ; but as the karyokinetic processes 

 progress, the cells elongate in the direction of the spindles until the axes 

 in which the spindles lie are the longer. 



The products of this division are a'-°-d''-^, d'-^-d'-^, d-"^ -d''-\ and 

 d-^-d''-\ 



Third iMyer, containing the eight cells a^-^ - d^-^ and a®-^- d^-^. 



In all these cells the division is meridional, not equatorial, as in the 

 cells of the first and second layers. One of the spindles (i®-^) is shown 

 in Figure 55 (Plate 7). The cleavage in d'^° and d^'^ is shown in Fig- 



