JENNINGS: DEVELOPMENT OF ASPLANCHNA HERRICKII. 43 
on all sides by other cells. It thus occupies a position in the egg which 
is fundamentally different from that occupied by any other cell. Cor- 
related with this fundamentally different position, the cell acquires a 
fundamentally different method of division. 
It is impossible to say whether any particular feature of the different 
position of the cell is the essential one in bringing about this altered 
method of cleavage. As the cell moves inward, it very probably 
accomplishes a partial rotation (see below); if the axes of the cell are 
definite, and determined within the cell alone, then this rotation would 
cause a change in the position of the axes of the cell dé in comparison 
with the axes of the other cells, and a different direction of the spindle 
would result, But any such explanation is hypothetical. 
During the later stages of the sixth cleavage, the process of gastrula- 
tion has made much progress. At the end of the fifth cleavage the 
large ventral cell d^t had already moved some distance toward the in- 
terior of the egg (Plate 5, Fig. 38). As the cells des and des now 
withdraw successively their deeper parts and increase their surface 
extension during division, they push ventrad, displaeing the posterior 
part of the ventral cell (now d") (Compare Plate 6, Figs. 48, 50, 
and 51.) The large cell therefore pushes dorsad into the interior of 
the egg, occupying the space made vacant by the ventral extension 
of the other cells. Soon after, the anterior cells, belonging to quad- 
rants A, D, and C also enter upon the karyokinetie process, and in so 
doing likewise push ventrad (Plate 8, Fig. 64) at the same time vacat- 
ing of course a portion of the space before oecupied by them near the 
animal pole, The cell d therefore continues to move dorsad, so that 
at the end of this cleavage it is almost completely enclosed (Fig. 65). 
During this inward movement of the cell du, the cloud of granules 
previously described changes its position still further. We had traced 
it, after tho sixth cleavage, until it occupied a position between the nu- 
cleus of d" and the two vesicles formed at the fifth and sixth cleavages 
(Plate 6, Fig. 51). As gastrulation continues, the cloud of granules 
migrates still further dorsad, and later even crosses the dorso-ventral 
axis, so as to lie posterior to it, surrounding the dorsal aster at the next 
division of d" (Plate 8, Fig 64). 
This movement of the cloud of granules possibly gives the key to the 
change of axis of division in the cell diu. When the posterior cells 
extend ventrad during division, as previously described (Plate 6, Figs. 
48 and 51), they push against the posterior side of the cell ., thus 
displacing the cell inward. But such an impulse from one side only 
