38 BULLETIN: MUSEUM OF COMPARATIVE ZOÖLOGY. 
completely formed. The dorso-ventral axis of the cell has greatly in- 
creased, but is still distinetly 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 (d™ 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 egg, 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, de- d, and 
OP Deb 
The cleavage of 49^ has been described. The small vesicle d does 
not divide farther. The other 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 a1 - di, a7. d, a8- l., 
and d . 
Second Layer, containing the cells aë? — 499 and a't- q94, 
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 G), 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*4 is shown 
in Figure 57 (the products being d^" and dis). 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™'—d7™5, ar. d, a1 = d'1, and 
ST, 
Third Layer, containing the eight cells a95— de., and e - d°., 
In all these cells the division is meridional, not equatorinl, as in the 
cells of the first and second layers. One of the spindles (059) is shown 
in Figure 55 (Plate 7). The cleavage in d^? and des is shown in Fig- 
