JENNINGS: DEVELOPMENT OF ASPLANCHNA HERRICKII. 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 
des 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. c. 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 the developing embryo, but without regard to the form of the 
cells, scems 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 are 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 either 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 scems possible 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 eases either in the dorso-ventral axis or at 
right angles to it. 
Comparison of the conditions in Asplanehna 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 enses in which the direction of the in- 
falling rays of light determines the position of the spindle (Stahl, '85). 
But the result is not universal for either agent, so that we must hold 
that the effect in both eases is of the nature of a reaction to stimulus. 
