PROTOPLASMIC MOVEMENT. 71 



factors of development may some time be harmonized and uni- 

 fied by the discovery of some common causal principles. 



I. MECHANICAL FACTORS OF DIFFERENTIATION. 



Of the many factors of differentiation which have been pro- 

 posed within recent years, the majority are in the nature of 

 simple physical causes. Thus the polar differentiation of the 

 egg has been attributed to differences in the specific weight of 

 protoplasm and yolk ; e.g., Hertwig 1 says : " Polar differentiation 

 consists in this, that the lighter protoplasm collects at one pole 

 and the heavier yolk substance at the other." Experiments on 

 the frog's egg led Pfliiger, Born, and Schultze to essentially the 

 same conclusion. 



The differentiations of cleavage are commonly attributed to 

 factors of a similar character ; thus the direction of cell division 

 is said to be due to the fact that the mitotic spindle lies in the 

 direction of least resistance (Pfluger) or in the longest axis of 

 the protoplasmic mass (Hertwig) ; the shape and position of 

 cells, and consequently to a certain extent the direction of divi- 

 sion, are said to be due to the rectangular intersection of cleav- 

 age planes (Sachs), or to the principle of smallest surfaces 

 (Berthold) ; the rate of division and the relative size of daughter- 

 cells are commonly attributed to the mechanical influences of 

 inert yolk (Balfour, Hertwig). Finally, the differences in the 

 quality of cells have been referred to intercellular reactions 

 (Hertwig, Wilson),* which, in some cases at least, are regarded 

 as of a physical rather than of a physiological character. 



It has been repeatedly shown that none of these principles 

 are of universal application, and it seems doubtful whether in 

 any case they are the real causes of the phenomena in ques- 

 tion. How little gravity has to do with polar differentiation 

 is well shown in the eggs of many gasteropods where the eggs 

 lie in all possible positions in the egg capsules with their pri- 

 mary axes turned in all possible directions, and yet the polar 

 differentiation occurs as perfectly and as rapidly in one position 

 as in another. That gravity can have nothing directly to do 



i The Cell, p. 215. 



