32 THE GROWTH OF THE BRAIN. 



on so magnificent a scale. By repeated removal of the 

 limbs Spallanzani obtained from a single newt in the 

 course of three months 687 new bones. In the adult 

 animals, to be sure, the regenerated portion is at first 

 small, and only gradually reaches the normal size, but it 

 ultimately does so, and the ease of this process, the 

 rapidity of it, as well as the number of times it can be 

 repeated, are all highly suggestive. This capacity for 

 the reproduction of parts lost is much diminished in 

 the higher vertebrates, and the decrease in this power is 

 explained by their higher specialisation, or, in other 

 words, the greater the number of generations between 

 any group of somatic cells and the ovum from which 

 they are derived, the less the capacity in them for 

 regeneration. 



If the organism, as a whole, is highly specialised, it is 

 due to the fact that the constituent cells are also 

 specialised ; so that in the animal series progressive 

 specialisation may be expressed either in terms of 

 the entire animal or its structural elements. It has 

 just been indicated that the embryonic condition of 

 cells is that most favourable to growth, and it therefore 

 follows that the processes of regeneration would be least 

 perfect in the higher vertebrates whose cells had 

 departed furthest from the embryonic condition. It is 

 recognised, moreover, that the growth changes in cells 

 are but incompletely reversible. Thus, when a cell has 

 once become in a high degree modified, it loses the 

 capability of reverting, under new conditions, to its 

 primitive state. Here, again, in the animal series a 

 gradation is evident, for those cells which are least 

 highly modified and those composing the less specialised 

 animals are the ones in which this reversion is most 

 readily accomplished. In a broad way, then, the 

 capacity for regeneration implies a latent youthfulness. 



