JENXINGS: DEVELOPMENT OF ASPLANCHNA HERRIGKII. 85 



lateral extension to a dorso-ventral position, and thereby from the greater 

 into the lesser axes of their respective cells (Figs. 17-22, Plate 3). In 

 d^-^ the asters do not separate at right angles to the previous spindle, as 

 commonly occurs, but the line joining them is parallel to the preceding 

 spindle, i. e. lateral (Figs. 37 and 38, Plate 5, and 46, Plate 6) ; later, 

 by a rotation into the short axis of the cell, the dorso-ventral position is 

 attained (Plate 6, Fig. 48, Plate 7, Figs. 53 and 54). It is not possible 

 to refer these and the other changes described in the general account 

 of the development to any simple factors. We can refer the changes in 

 position of the asters, and consequent manner of cleavage, only to the 

 structure of the protoplasm and the (molecular?) processes occurring 

 within it. 



The fact that the spindles take definite positions with relation to the 

 axes of tiie developing embryo, but without regard to the form of the 

 cells, seems to indicate that there is some influence governing the egg 

 as a whole, which is related to its form, and that the position of the 

 spindles is regulated by this. The determining factors in the position 

 of the spindles would therefore lie, not within any given cell itself, but 

 outside of it. But there ai'e certain facts which seem to render this 

 very doubtful. As discussed on pages 70, 71, in later stages the cells 

 become displaced by the changes taking place during gastrulation, and 

 there is a corresponding change in the position of the spindles ; they 

 are no longer eitlier parallel with or at right angles to the dorso-ventral 

 axis of the egg. This is shown especially in Figures 68 (Plate 8) and 

 83 (Plate 10). If a changed position of the cell with regard to the 

 axis of the embryo results in a correspondingly changed position of the 

 cleavage spindle, it seems to follow that the position of the spindle is 

 determined within the cell. 



I do not, however, consider this conclusion as well established. It 

 still seems joossi'^/e that the spindles are all placed with reference to some 

 influence resulting from the axial relations of the egg as a whole, — 

 though not necessarily in all cases either in the dorso-ventral axis or at 

 right angles to it. 



Comparison of the conditions in Asplanchna with those reported by 

 other observers for other organisms shows that there are cases in which 

 the form of the cell does determine the position of the spindle. In the 

 same way we know that there are cases in which the direction of the in- 

 falling rays of light determines the position of the spindle (Stahl, '85). 

 But the result is not imiversal for either agent, so that we must hold 

 that the effect in both cases is of the natin-e of a reaction to stimulus. 



