PROTOPLASMIC MOVEMENT. 73 



these so-called "laws" are repeatedly set at naught, and this 

 fact has led those who have studied such cleavage to the con- 

 clusion that no simple physical explanation of these processes 

 of differentiation is possible, and that their cause must be found 

 in the structure of the protoplasm or in some physiological 

 factor. 



II. PHYSIOLOGICAL FACTORS. 



The causes of differentiation are frequently referred to the 

 structure of the germinal protoplasm, as if this were a sat- 

 isfactory explanation. But to say that polarity and differen- 

 tiations of cleavage are due to the constitution of the egg is 

 merely a form of words which means little or nothing. In the 

 same way it might be said that all the multifarious aspects of 

 the universe are the results of the constitution of matter. To 

 refer vital phenomena to the constitution of protoplasm and 

 there to rest is merely to juggle with words. The phenomena 

 in question must be analyzed and their immediate causes deter- 

 mined step by step before any " explanation " can be thought of. 

 If, then, those who attempt to explain differentiation as the re- 

 sult of simple physical factors have taken too narrow a view 

 of the problem, and have unduly simplified it, those who suggest 

 the structure of protoplasm as an explanation propose a factor 

 so vague and remote that it has no real value. The immediate 

 causes of differential cleavage have been located, by different 

 authors, in various parts of cells. 



a. Nucleus. According to the views of Weismann, Roux, 

 De Vries, and many others, the nucleus is the prime mover in 

 all processes of differentiation ; and hence we find that they 

 attribute the direction, size, and rate of division, and the quality 

 of cells, to the action of the nucleus. According to Roux ('95) 

 there is immanent in the nucleus a direction of division which 

 may be independent of the chief dimensions of the protoplasmic 

 body. Almost all observers agree that the direction and size 

 of division depend upon the direction and position of the mitotic 

 spindle, and many consider that the position of the spindle is 

 not determined by the cell body ; thus Jennings says of As- 

 planchna ('96, p. 71), "The form of the cell is greatly influenced 



