JENNINGS: DEVELOPMENT OF ASPLANCHNA HERRICKII. 78 
the asters accompanied by motions not in a straight line, the same 
result is attained that would otherwise be produced by a rotation after 
the asters were fully separated. 
In the larger cell at the two-cell stage, the change of position of the 
nucleus and spindle is not due to a rotation after the two asters are 
formed, but to a change of position of both nucleus and aster before the 
aster has begun to divide. 
In d*? (Plate 1, Fig. 7) the asters and nucleus at first rotate into a 
position whieh is not the final one, so that subsequently rotation in 
a different manner is necessary to bring the spindle into its definitive 
position (Plate 2, Fig. 16). 
In d*? (Plate 5, Fig. 37, Plate 6, Fig. 46, Plate 7, Figs. 53 and 54) 
the definitive position taken by the spindle is at right angles to that of 
the preceding spindle, so that the simplest method of formation would 
be the natural separation at right angles to the axis of the preceding 
spindle, as commonly takes place in such cases. But, owing appar- 
ently to the peculiar form of the cell (Plate 5, Figs. 37 and 38, surface 
and section), the asters separate obliquely, taking up a position such 
that the lino uniting them is parallel to the axis of the preceding 
spindle; and the definitive position is reached only by a later rotation 
through an angle of 90 degrees (Plate 7, Figs. 53 and 54). 
There is thus no regularity about the method by which the asters come 
to occupy their definitive positions at the ends of the spindles. Appar- 
ently the early position of the asters is influenced or determined by the 
mechanical conditions within the cell, whereas the later position of the 
spindle is largely independent of such conditions. 
On the other hand, in the divisions of the ectoderm of Asplanchna, 
there is a regularity in the final angle between the axes of two succes- 
sive spindles in any given “layer” of cells, the second spindle being in 
all the cells of a given layer either parallel with or else at right angles 
to the preceding spindle. It is thus quite possible that there may be 
whole systems of tissues where there is such a regularity. But the fact 
that the definitive position may be reached by such various means 
renders the phenomenon of little significance for a mechanical theory 
such as that presented by Heidenhain. 
(6) Sachs’s view that the walls separating the cells meet one another at 
right angles. — An examination of the figures, especially the sections, 
shows that the condition above stated is not generally complied with in 
the cleaving cells of Asplanchna, so that it is not necessary to enter upon 
a discussion of this view. Neither is the regular alternation of spindles 
