104 '^"Hf" GERM-PLASM 



in each differential cell-division a certain number of determinants, 

 which ripen later on, become split off from the rest, and are 

 retained in the cell as accessory idioplasm. The mechanism for 

 regeneration is certainly a very complicated one, for each separate 

 bone is controlled by a number of different determinants, and 

 not by a single one ; and all these special determinants are 

 contained in the accessory idioplasm. As far as we can judge 

 from the investigations made hitherto, the bones are at any rate 

 regenerated in detail fairly exactly. The complexity of the 

 mechanism accounts, in my opinion, for the fact that the fore- 

 limb, which has such a marked power of regeneration in the 

 salamander, has lost this power completely in the higher 

 Vertebrates, for in them the mechanism would have become 

 too complex. 



A simpler mechanism than that which we have supposed to 

 exist can only be conceived, if, with Herbert Spencer,* we attri- 

 bute to each of the units composing the body the power com- 

 bining to form any necessary organ just when it is wanted. We 

 might then compare the entire animal to a large crystal, in the 

 individual parts of w'hich ' there dwells the intrinsic aptitude 

 to aggregate into the form of that species ; just as in the atoms 

 of a salt there dwells the intrinsic aptitude to crystallise in a 

 particular way.' The only difference between the particles of 

 the crystal and those of the organism w^ould be that the former 

 are all permanently alike ; and that the latter, in order that 

 regeneration may be possible, are arranged in many different 

 ways, according to whether an entire limb, a tail, a gill, or a 

 single toe, fore-arm, or finger is to be replaced. How are the 

 ' units ' shown in each individual case wdiat part is missing, and 

 what form their arrangement is to take in order to produce the 

 part anew? We are thus once more brought back to Blumen- 

 ])ach's ' iiisusforinativtis.'' Spencer himself says : — 'If in the 

 case of the crystal we say that the whole aggregate exerts over 

 its parts a force which constrains the newly-integrated atoms to 

 take a certain definite form, we must, in the case of the organism, 

 assume an analogous force.' This force would correspond to what 

 was formerly spoken of as the ' spiritus rector'' or ' nisus forma- 

 tivus ; ' and even supposing it to exist, it does not in the least 

 help us in the attempt to explain the mechanism of the phenom- 



* Herbert Spencer, ' The Principles of Biology,' Vol. i, p. i8i. 



