70 BULLETIN: MUSEUM OF COMPARATIVE ZOGLOGY. 
as much. But in view of the frequent. departures from the principle of 
least resistance, it appears necessary that the object of any given method 
of cleavage should be judged in each case for itse If; so that Braem’s con- 
tention that his principle is fitted “den Verlauf der normalen F urchung 
in wesentlichen Punkten zu erklüren " must be considered unsuccessful. 
It may, by calling attention to one method in which cells react to a 
stimulus, “explain” the cleavage of some cells in the same sense that 
the growth of the stem and root of a plant may be said to be “explained” 
by saying that the protoplasm of one is so constituted as to react to light 
by growing toward it, the other, on the contr ary, so as to react by grow- 
ing away from it. But in other organisms the deter mining stimuli are 
of an entirely different nature, and the “explanation” must be sought 
anew for every organism. 
I am of course fully aware that the view here put forth, that the 
position of the spindle must be interpreted teleologically, Wer in many 
cases as a reaction to stimulus, is not an *explanation. But I see no 
a priori ground for expecting a simple mechanical explanation for the 
direction of cell division, any more than we should of the direction of 
growth of a plant. As a matter of fact, the phenomena show that such 
an explanation is not at present possible for either. 
(4) Rouzs theory of a compromise between tho tendency immanent in 
the nucleus and the tendency due to the form of the protoplasmic mass. 
(Compare page 6.) — As remarked above, this theory is not definite, in 
the same sense as the three foregoing, inasmuch as one of its factors — 
the immanent tendency of the nucleus — is of an entirely unknown 
character. From the foregoing description and discussion it is evident 
that I must agree fully with this conception. The further question 
comes, In how far do the phenomena in Asplanchna lead to a recogni- 
tion of the second factor, — the tendency due to the form of the cells 
It is evident that the form of the cell does not determine the main fea- 
tures of the direction of division, — the question as to whether the 
spindle shall be dorso-ventral or lateral in direction. But are there sub- 
ordinate features in which the form of the cell does affect the position 
of the spindle, as held by Roux? In other words, does the spindle 
always lie in either the long or the short axis of the cell, and never 
oblique to both? 
An examination of the figures will show that in the large majority of 
cases the latter question is to be answered affirmatively. In the earlier 
stages, before the great changes in the positions of cells have occurred, the 
dorso-ventral axis of the egg commonly coincides with either the greater 
