JENNINGS: DEVELOPMENT OF ASPLANCHNA HEERICKII. 37 



where thev were given off. (Compare Figure 50, Plate 6, with Figures 

 56, Plate 7, aud 65, Plate 8.) 



Other changes occur at the same time in the large ventral cell. After 

 the fifth cleavage the granular cloud in the cytoplasm gathered into the 

 region of the anterior margin of the free surface of the cell (Plate 5, 

 Fig. 38). As the spindle for the sixth cleavage is formed, the cloud 

 becomes concentrated over a small area, at a slight xiistance from the 

 anterior margin of the cell (Plate 6, Fig. 48). Then, as division takes 

 place, the cloud moves up to the anterior margin, at the same time 

 spreading out, and begins to pass beneath the cells of the quadrants A 

 and B (Figs. 49 and 50). As the large nucleus moves away from the 

 wall of the cell where the vesicle was formed, the granular cloud moves 

 inward (dorsad) and spreads out between the nucleus of the large cell 

 and the two vesicles (Figs. 51 and 52). The granules at this time have 

 become very coarse and distinct. 



Meantime, cleavage is taking place in the cell d^-^. In this cell the 

 changes occurring in the asters are peculiar. 



Immediately after the preceding cleavage, the cell, having passed 

 into the resting stage, has been pressed into an irregular wedge-sliaped 

 form by the processes occurring in the surrounding cells (Plate 5, Fig. 

 37, surface view, and Fig. 38, section, from the same egg). The cell 

 has become very narrow at the level at which the nucleus lies, so that, 

 apparently, there is not room for the asters to separate at right angles 

 to the foregoing spindle. The nucleus is pressed closely against the 

 ventral wall of the cell (Fig. 38), and the aster begins to extend 

 obliquely along the dorsal side of it, between the nucleus and the dor- 

 sal wall of the cell. When the aster has become completely divided 

 and the products are on opposite sides of the nucleus, their common 

 axis is already in the same direction as the axis of the spindle at the 

 previous division. The same result is obtained as in the rotation nt the 

 fourth cleavage, though in a different manner. £ut the final position is 

 not yet reached. 



As now situated, the asters lie in the long axis of the much elongated 

 cell (Plate 6, Fig. 46). As the active condition preparatory to di- 

 vision comes on, the cell withdraws its deeper parts (shown in Plate 5, 

 Fig. 38), and its dorso-ventral dimension increases. Accompanying this 

 change is a rotation of the nuclear complex, from a position with the 

 axis in the greatest dimension of the cell, to a position with axis in the 

 shortest dimension. This change is shown in progress in Plate 7, 

 Fig. 53. A later stage is shown in Figure 54; here the spindle is 



