68 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY, 
the direction of growth of plants and animals, such a conclusion would 
be reached with regard to almost every agent that ever exercises a 
determining power. The effect of light is to cause certain parts to grow 
towards it, others to grow away from it; while in other organisms the 
direction of growth is not affected by it at all. The same is true of 
gravity, of heat, and of various chemical and physical agents. (See the 
extended list of such cases given by Herbst, ’94.) In all these cases 
we are dealing with reactions to stimuli. It is only when we attempt to 
make one of these agents the only determining factor, and expect to sce 
it act always in the same way, as a simple mechanical cause, that our 
result becomes unsatisfactory. 
It is evident that in this question of the relation of the form of cells 
to the direction of cleavage, we are dealing with a problem of a nature 
similar to those which I have briefly stated above. Some organisms are 
so constituted as to react to rays of light by growing toward them, 
others are not. In the same way, some cells or nuclei are so constituted 
as to react to the influences determining form by bringing the spindle 
into the longest axis of the protoplasmic mass ; others are not. In each 
case the result is due to a reaction to stimulus, or to an action of similar 
nature, and not to à simple mechanical action of the agent. 
In almost or quite all cases of reaction to stimuli, the result may be 
shown to be the accomplishment of a certain end that is of importance 
for the existence of the organism. The immediate explanation always 
takes a teleological form. It is not diffieult to perceive a teleological 
aspect of this tendency of many cells to divide with the spindles in the 
longer axes. In cases where the purpose of cell division is merely to 
double the number of cells without alteration of form, division with the 
spindle in the longer axis is obviously the simplest method. A con- 
sideration of what would be the result if the opposite method prevailed 
shows this clearly. By continued division of a cubical cell and its 
products with the spindle in the shortest axis, a series of flat plates 
would be produced ; every cleavage would bring about a greater modifica- 
tion in the form of the resulting cells, and cells of such form would 
undoubtedly be very inconvenient for the purposes of the organism. 
Continued division with the spindles in the long axis would result, on tho 
other hand, in the production of cells of the same form as the parent 
col. It is not remarkable, therefore, if in many cases the cells are so 
constructed as to respond to a chauge of form by a corresponding change 
of position of the spindle, so that the resulting cells shall be as nearly as 
possible of the form of the parent. 
