JENNINGS: DEVELOPMENT OF ASPLANCHNA. HERRICKII. 63 
enon of rotation of the spindle into the shorter axis of the cell (Plate 5, 
Fig. 37, Plate 6, Fig. 46, and Plate 7, Figs. 53 and 54). There is the 
same regularity in the direction of cleavage as noticed in the preceding 
cleavages, although there is variation in the form of the cells. Thus, 
d*^, the companion cell of des, whose division with spindle in the short 
axis has just been cited, cleaves with the spindle parallel with that in 
des, but owing to the form of the cell the spindle in d** occupies the 
long axis (Plate 7, Fig. 57, cleavage finished). Tue cells of the ventral 
layer (Plate 6, Fig. 47) divide with spindles in tho same direction as 
those in the cells of the second layer, although in the one layer the 
spindles must thereby occupy the short axes of the cells, in the other 
the longer axes. 
In the seventh cleavage a still more striking case occurs. The mid- 
dle of the embryo is surrounded by two rows of eight cells each, of 
precisely the same form and size, the dorsal row composed of the blasto- 
meres d. — d'9 and de - d'8, the ventral row of die -d, and at =. 
These two rows are shown in Figure 61 (Plate 7), for the quadrants 4, 
D, and C, and in Figure 57 for the quadrant D. In both rows, every 
cell but one in each row (di and d", Fig. 57) is strongly flattened 
dorso-ventrally, so that the lateral extent of the cell is much greater 
than the dorso-ventral extent, If the form of the cell determines in 
any way the direction of the spindle, it is certainly to be anticipated 
that the direction of cleavage will be the same for the cells of both 
rows, On the contrary, all the cells of the dorsal row divide meridi- 
onally, while all the cells of the ventral row divide equatorially. In every 
cell of the dorsal row, except possibly s, the spindle lies in the long 
axis of the cell; in every cell of the ventral row, except , the spindle 
lies in the short axis of the cell. The exception of a single cell in each 
row gives a finishing touch to the proof that the form of the cells is not 
the factor determining the direetion of cleavage. The fact that the cell 
ds divides with its spindle in the same direction as the spindles of all 
the cells in the same row, but not, as in the other cells, in the long axis 
of the cell, while d*? likewise divides with its spindle in the same 
direction as those of the other cells of dts row, but not, like them, in the 
short axis, demonstrates that the dorsal row is not so constituted that 
all the spindles must take their positions in the long axis, nor the ven- 
tral row so that all must take their positions in the short axis. It 
demonstrates, in other words, that the relative dimensions of tho differ- 
eni axes of the cells does not determino the direction of the spindles in 
either ono way or the other, (Tho cleavage of these two series of cells 
