70 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



as nmch. But iu 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 itself; so that Braem's con- 

 tention that his principle is fitted " den Verlauf der normalen Furchung 

 iu wesentlichen Punkten zu ei-kliiren " 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 tliat 

 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 contrary, so as to react by grow- 

 ing away from it. But in other organisms the determining 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, and 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) Roux's theory of a compromise between the 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 factoi'S — 

 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 lea^ 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 featui-es 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 



