xvi] DIFFERENTIATION 259 



extent, the choice between a vitalistic and a mechanistic 

 theory of life as a whole. The solution of this great problem 

 is probably very distant, but clearly one of the conditions 

 of approaching it is to discover the actual steps by which 

 differentiation is accomplished. The answer to the question 

 "In what manner?" does not necessarily give that of the 

 question, "By what cause? " ; it may be possible to describe 

 every step in the process and yet to be ignorant of whether 

 the causes at work are purely mechanical or not. But at the 

 present time very little indeed is known of the simple stages 

 of differentiation. Sometimes, as in the cleavage of Styela, 

 some steps in the process can be seen, but even in such 

 cases as this it is possible that the apparent clearness with 

 which the changes can be watched is really misleading, and 

 merely hides the more fundamental factors that lie behind. 

 And in many cases in which a cell divides unequally it is 

 possible to say no more than that a differential division has 

 taken place; the nature of the unlikeness between the two 

 cells is entirely beyond our present powers of analysis. 



The study of differentiation leads the cytologist to the 

 borderland between his territory and that of the experi- 

 mental embryologist, and another subject that has attracted 

 much attention in recent years may perhaps be regarded 

 as carrying him across the frontier. This is the problem of 

 the stimuli that lead to cell-division, and to the initiation 

 of development in an egg. For a full elucidation of such 

 questions the methods of the experimental embryologist 

 are doubtless needed, but the subject is essentially one of 

 cytology, and the chief hope for advance lies in the co-opera- 

 tion or combination of the two lines of investigation. In this 

 respect science is like the boring of a tunnel; the workmen 

 start far apart, and those at one end have no immediate 

 knowledge of what those at the other end are doing, but 



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