SPECIALIZATION AND ORGANIZATION. I3 



observation of Trembley, and of others who repeated his 

 experiments ; and hence the unity or individuaUty of the 

 Hydra community of cells has generally been estimated 

 too low. 



That the two layers carry functions fundamentally dis- 

 tinct and non-interchangeable, and that the co-operative 

 combination of the two sets of functions is necessary to 

 existence, — is, in fact, the very essence of the Hydra 

 personality, — is shown by still another fact brought out 

 by Mr. Ischikawa. It is generally taught that Hydra 

 may be divided ad libitum, and that each fragment will 

 have the power to regenerate the whole individual. But 

 it turns out that there is a very definite limit to such 

 possibilities, which cannot be overstepped without anni- 

 hilating individuality and extinguishing even the germ 

 of it. An isolated piece of either layer is incapable of 

 regenerating the individual. The mutual dependence 

 of these two layers is such that they must go together 

 or perish. Carry artificial division in any direction that 

 does not sunder these complementary parts, and repeat 

 the operation as often as you like, the smallest frag- 

 ments in which this vital connection is preserved will 

 represent, potentially at least, the personality of Hydra. 

 This personality comprises a certain number of func- 

 tional powers ; and hence the minimum number of cells 

 combining these powers in vital relations represents all 

 the essential elements of individuality. The fertilized 

 ovum unites all these powers as potentialties, and it is 

 therefore the individuality in germ. If the ectoderm 

 cells, like the entoderm, were all alike, the essentials of 

 a Hydra might be said to exist in a single pair of cells, 

 one from each layer ; but of course we could not hope 

 to isolate such a pair of cells in vital union. 



