JENNINGS: DEVELOPMENT OF ASPLANCHNA HEEEICKII. 63 



enon of rotatiou of the spindle into the shorter axis of the cell (Plate 5, 

 Fig. 37, Plate 6, Fig. -iG, 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, 

 0?®-*, the companion cell of d^'^, whose division with spindle in the short 

 axis has just heen cited, cleaves with the spindle parallel with that in 

 d^-^, but owing to the form of the cell the spindle in d^-* occupies the 

 long axis (Plate 7, Fig. 57, cleavage finished). TliC cells of the ventral 

 layer (Plate 6, Fig. 47) divide with spindles in the 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 

 pi'ccisely the same form and size, the doi'sal row composed of the blasto- 

 meres a'-^ — d''^ and a'-^ — d".^, the ventral row of a'-^ -d'''^ and a~ •'' —d' •'' . 

 These two rows are shown in Figure 61 (Plate 7), for the quadrants A, 

 B, and C, and in Figure 57 for the quadrant D. In both rows, every 

 cell but one in each row (d''-'' 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 d'-^, the spindle lies in the long 

 axis of the cell ; in every cell of the ventral row, except d'-', 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 direction of cleavage. The fact that the cell 

 d''-^ divides with its spindle in the same direction as the spiudles 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 its 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 i-elative dimensions of the differ- 

 ent axes of the cells does not determine the direction of the spindles in 

 either one way or the other. (The cleavage of these two series of cells 



