96 The American Naturalist [February, 



development of the fertilized ovum begins with an excessive 

 proliferation of the nuclei, by which numerous cells are cre- 

 ated, each with little protoplasm. Histogenetic differentiation 

 begins later. The asexual rejuvenation has a similar course, 

 but needs more thorough investigation. 



Now differentiation is the sign of inheritance, and this mor- 

 phological inheritance cannot develop itself fully until the 

 senescence of the cells becomes recognizable by the growth of 

 their protoplasm. On the other hand, we see complete inher- 

 itance develop itself, after preceeding rejuvenation. Accord- 

 ingly we gain two conceptions : first, the hereditary impulse 

 belongs to the inherent and constant properties of cells in gen- 

 eral ; second, the activity of their impulse may be inhibited by 

 the condition of the cells. My view may be expressed in the 

 following way : Somatic cells are simply cells in which the 

 activity of the hereditary impulse is inhibited in consequence 

 of their senescence, or, in other words, differentiation ; but 

 under suitable conditions the somatic cells may pass over into 

 the rejuvenated stage, and thereupon develop the most com- 

 plete hereditary possibilities. 



The importance of rejuvenation must also be recognized 

 when we consider the phylogenetic origin of single organs. 

 Let us take a simple example. We may safely assume that 

 the ancestors of mammals possessed a smooth skin, and that 

 the covering of hairs is a new acquisition. Each hair is the 

 product of a local growth. If we investigate the germ of a 

 hair, we find that it consists of rejuvenated cells, that is to say, 

 of cells with little protoplasm, or, as we are accustomed to say, 

 of the embryonic type. Thus the formation of hairs depends 

 on numerous centers of rejuvenation. In the multiplication 

 of striped muscle fibres we find the agents to be the muscle 

 buds, which are small, protoplasmatic structures, with rela- 

 tively numerous nuclei. If we observe a developing gland, 

 let us say a pancreas or a sweat gland, we find the rudiment 

 to consist of rejuvenated cells; the cells multiply rapidly, and 

 after the organ has its essential form, the histogenetic differenti- 

 ation begins. It would be easy to multiply such examples a 

 thousandfold. 



