JENNINGS: DEVELOPMENT OF ASPLANCHNA HERRICKTI. 61 
long axis of the cell (Plate 3, Fig. 17), then a rotation takes place 
(Fig. 18) by 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 ege. Since the axis of the first cleavage spindle 
commonly eoineides with the plano separating tho pronuclei, the result 
in the eggs of this species of Ascaris would be that the spindle would 
occupy tho 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 dong 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. 40-12), 
and in the eggs of echinoderms (94). Te 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 ego, 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 great variety in the form of the cells, 
the direction of the cleavage spindles is the same in all the cells. Thus 
d (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 
