JEXXIXGS: DEVELOPMENT OF ASPLAXCHXA HEKPJCKII. 61 



long axis of the cell (Plate 3, Fig. 17), then a rotation takes place 

 (Fig. 18) b}^ which the line joining the asters, i. e. the axis of the future 

 spindle, is brought into the shortest axis of the cell (Figs. 19-24). The 

 six spindles are then formed in the shortest axes of the cells (Figs. 20- 

 28), and the planes of cleavage accordingly coincide with the long axes 

 of the cells. 



The simple fact that there are divisions in which the spindles lie in 

 the shortest axis of the cells is of course a direct contradiction of Hert- 

 wig's law. The case becomes even more striking, however, when the 

 movements of the asters are taken into consideration. They at first lie 

 in the position demanded by the law, but move from this position to 

 that which directly contradicts the law. (See pages 25, 26.) 



Hertwig ('93, p. 175) has cited a^ similar phenomenon, described by 

 Auerbach, as proof of his law. Auerbach ('74) observed in the eggs of 

 Ascaris nigrovenosa, at the time of fertilization, that the two pronuclei 

 often come together in such a way that the plane separating them lies 

 in the short axis of the egg. Since the axis of the first cleavage spindle 

 commonly coincides with the plane separating the pronuclei, the result 

 in the eggs of this species of Ascaris would be that the spindle would 

 occupy the short axis of the egg. But the two pronuclei after meet- 

 ing undergo a rotation through an angle of 90 degrees, thus bringing 

 the spindle into the loiig axis of the egg. Ziegler has recently observed 

 with even greater clearness the same phenomenon in the eggs and 

 cleavage cells of other nematodes (Ziegler, '95, Taf. XVIII. Figs. 40-42), 

 and in the eggs of echinoderuis ('94). He observed in nematodes in 

 some cases that the line joining the two asters on opposite sides of the 

 nucleus lies in the short axis of the egg, and that then follows a rota- 

 tion of the whole complex, till the line joining the asters — the axis of 

 the forming spindle — occupies the longest axis of the egg. Ziegler, 

 like Hertwig, has interpreted this change of position as a confirmation 

 of Hertwig's law, and the interpretation is certainly the most natural 

 and apparently well grounded that could be given. 



Nevertheless we have in Asplanchna an entirely similar phenomenon, 

 but occurring under such circumstances as to give a direct contradic- 

 tion, instead of a confirmation, of Hertwig's law. 



It is instructive also to notice that in the eight-cell stage of As- 

 planchna, notwithstanding the gi-eat variety in the form of the cells, 

 the direction of the cleavage spindles is the same in all the cells. Thus 

 rf*-^ (Plate 2, Fig. 15), though irregular in shape, is of such a form 

 that it is possible to be confident that the spindle does lie in the greatest 



