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 powei*. 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 see 

 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 thera, 

 others ai'e 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 a 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 difficult 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 obviouslj' 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 the 

 other hand, in the production of cells of the same form as the parent 

 cell. It is not remarkable, thei-efore, if in many cases the cells are so 

 constructed as to respond to a change 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. 



