138 THE PHILOSOPHY OF BIOLOGY 



and develops like it ; and (2) each of the first few 

 blastomeres is different from the others, and from the 

 entire ovum, and develops differently from the others, 

 and from the entire ovum. 



Let us try to construct a notion of what this organi- 

 sation in the developing ovum must be. In the 

 16-blastomere stage of the sea-urchin egg we have a 

 ' system ' of parts. In the case of normal develop- 

 ment each of these parts has a certain actual fate — 

 it will form a part of the larva into which the embryo 

 is going to develop : It has, as Driesch says, a pro- 

 spective value. But let the normal process be inter- 

 fered with, and then each of these parts does something 

 else. In the extreme case of interference, when the 

 blastomeres are separated from each other, each 

 blastomere, instead of forming only a part of a larva, 

 forms a whole larva. The prospective potency of the 

 part, that is its possible fate, is greater than its pro- 

 spective value. Normally it has a limited, definite 

 function in development, but if necessary it may greatly 

 exceed this function. 



What any one blastomere in the system will become 

 depends upon its position with regard to the other 

 blastomeres. When the egg of the frog is floating 

 freely in water it lies in a certain position with the 

 lighter part uppermost, and then development is 

 normal, each of the two first blastomeres giving rise 

 to a particular part of the body of the larva ; that is, 

 each of them is affected by the contact of the other 

 and develops into whatever part of the normal embryo 

 the other does not. But let the egg in the 2-cell stage 

 be turned over and held so that the heavy part is 

 uppermost : the protoplasm then begins to rotate so 

 as to bring the lighter part uppermost; but the two 

 blastomeres do not, as a rule, adjust themselves to the 



