348 THE NATURE OF EVOLUTION 



can be rejuvenated by adequate environmental changes. The spores 

 or germinal cells must be the splendid rejuvenated specimens produced 

 by the environmental change. The spores or germinal cells may be 

 comparable to the form a in Fig. 33 which was cited in the previous 

 chapter, and somatic cells to ^i, and accordingly h^ is the differentiated 

 structure of a, to which h^ can be transformed if the environment is 

 changed from B to A. And further the form a, the spore or the ger- 

 minal cell, thus recovered from h^ can again be changed to hx, the 

 somatic cell or the vegetable form, when the environment is again 

 changed to B, and if hx can take cell form while a only virus-like 

 particle, the cell composed of hx will be decomposed to the virus-like 

 particles when brought under the condition A. Thus the spores or 

 the germinal cells are looked upon as the completely rejuvenated form 

 of the organisms. 



As for higher plants, it seems generally accepted that germ cells 

 can be directly originated from somatic cells, whereas in animals the 

 cells, from which germ cells are to be produced, are usually believed 

 to be distinctly separated from the somatic cells at their early stage 

 of the development. It should be born in mind, however, that the 

 cells which seem to be thus separated from somatic cells are not the 

 germ cells themselves but cells from which germ cells are to be 

 produced. Not only gonads but also other organs are generally de- 

 veloped separately from an early stage of the development. At present 

 the majority of embryologists hold the view that the cells constituting 

 gonads in which the germ cells become differentiated are not essen- 

 tially different from usual somatic cells. 



Many species of planarians have great power of regeneration, and 

 when an individual of them is divided by cut into a head- and a tail- 

 piece, each of which heals the wound and forms the missing parts by 

 cell division, whereby germ cells are subsequently produced from the 

 individual regenerated from the head-piece in which gonad was origi- 

 nally absent, indicating clearly that germ cells are produced from 

 somatic cells. 



In short, the germ cell thus produced from the somatic cell by the 

 reduction in its structure to primitiveness can return to the differ- 

 entiated structure of the somatic cell, if a proper stimulus is given 

 under suitable conditions favourable for the development of the differ- 

 entiated, original structure. This return process in itself is on- 

 togeny. 



