136 



THE PHILOSOPHY OF BIOLOGY 



but purely a matter of chance. Nevertheless, each of 

 the parts into which the larva is cut becomes an entire 

 embryo. For a time the partial blastula — approxi- 

 mately a hollow hemisphere in form — goes on develop- 

 ing as if it were going to become a partial embryo, but 

 soon the opening closes up and development becomes 

 normal. It does not matter even if the two parts into 

 which it is divided are not alike in size ; provided that 

 a part is not too small, it will follow the ordinary course 

 of development. 



Suppose the blastula opened out on the flat, like 



the Mercator pro- 



j" 



M 



r 



J 



.H 



o 

 Fig. 17. 



-JT 



jection of a globe on 

 a flat map. Suppose 

 that a is a small ele- 

 ment of it. Suppose 

 - that the rectangles 

 hcdCy FGHe, I JcL, 

 MNoe, and as many 

 more as we care to 

 make, represent the 

 pieces of the blastular wall separated by our operation 

 — they all contain the element a, but this is in a 

 different position in each case. There are really an 

 infinite number of such parts of the blastula and a 

 occupies an infinitely variable position in each of 

 them. 



This demonstration is very important, so let us 

 make it as clear as possible : Driesch's logical proof of 

 vitalism may be stated as follows : — 



The different parts of the blastula are going to 

 become different parts of an embryo. 



The part a, occupying a definite position in the 

 entire blastula, is going to become a definite part, 

 having a definite position, in the embryo ; 



