JEXXIXGS: DEVELOPMENT OF ASPLAXCHXA HEPiPvICKII. 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 ceU moves inward, it very probably 

 accomplishes a pai-tial 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 d^-'^ 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 c?®--^ had already moved some distance toward the in- 

 terior of the egg (Plate 5, Fig. 38). As the cells c?®-^ and d^-° now 

 withdraw successively their deeper parts and increase their surface 

 extension during division, they push ventrad, displacing 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 e^g, occupying the space made vacant by the ventral extension 

 of the other cells. Soon after, the anterior cells, belonging to quad- 

 rants A, B, and C also enter upon the karyokinetic 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 occupied 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 d''-^, the cloud of granules 

 previously described changes its position still further. We had traced 

 it, after the 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 d'-^. 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 rf®-\ thus 

 displacing the cell inward. Pmt such an impulse from one side only 



