JENNIXGS: DEVELOPMENT OF ASPLAXCHNA HERRICKII. 73 



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 c?*-^ (Plate 1, Fig. 7) the asters and nucleus at first rotate into a 

 position which 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. Eut, owing appar- 

 ently to the peculiar form of the cell (Plate 5, Figs. 37 and 38, suj-face 

 and section), the asters separate obliquely, taking up a position such 

 that the line 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 loithin the cell, whereas the later position of the 

 spindle is largely independent of such conditions. 



On the otlier hand, in the divisions of the ectoderm of Asplanchua, 

 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. Xeither is the regular alternation of spindles 



