592 THE BIOLOGICAL BASIS OF INDIVIDUALITY 



to finer differentials and towards individualization, but a parallel evolution 

 takes place also during the ontogenetic development. Here, also, the organs and 

 tissues evolve from a relatively simple substratum and this process is accom- 

 plished through the interaction of chromosomes and their genes with preformed 

 cytoplasmic structures with the aid of evocators and organizers. 



Likewise during embryonal development, the organismal differentials under- 

 go a transition from less individualized precursors to more individualized 

 differentials. The processes which lead to this ontogenetic differentiation 

 exhibit certain remarkable similarities to those noted in the phylogenetic 

 evolution, or expressed differently, the embryonal development of phylo- 

 genetically farther advanced organisms resembles and repeats the embryonal 

 development of phylogenetically earlier stages, as though the number of 

 mechanisms which living matter can use in attaining the advanced stages of 

 development is limited and determined by the actual stages through which 

 the phylogenetic development has passed. 



In the same way in which organs and tissues and their differentials, as well 

 as organismal differentials, develop during embryonal processes, so also during 

 regeneration of some of the more primitive organisms certain aspects of the 

 phylogenetic evolution may be repeated, although to a still more restricted 

 degree than during ontogeny. Regenerating tissues of adult urodele amphibians 

 behave in some respects like embryonal tissues. Parallel to the increase in the 

 extent of organ and tissue differentiation, with advancing regeneration there 

 is a decrease in transplantability. The earlier, less differentiated embryonal 

 and regenerating tissues are still more plastic and adaptable to environmental 

 factors than the farther advanced stages, in which the organ and tissue 

 differentials are more fixed and in which there is a greater tendency on the 

 part of organs and tissues to develop by means of self -differentiation. In 

 higher organisms the ability to produce new organs during regeneration is 

 lost. There is again, therefore, noticeable here a relation between the differ- 

 entiation and fixity of organs and tissue differentials and the increasing refine- 

 ment of organismal differentials, both of these processes leading to a greater 

 immutability and fixity of the organism as a whole. 



However, notwithstanding these similarities there is one very significant 

 difference between phylogenetic and ontogenetic evolution. The former starts 

 with a primitive substratum, in which the specialized organs and tissues and 

 the finer organismal differentials of the higher organisms are not yet pre- 

 formed. Chromosomes and genes, as well as cytoplasm of the primitive organ- 

 isms, differ greatly from those of the higher ones and this is true of the germ 

 cells as well as of the cells of the adult tissues. On the contrary, in the fertilized 

 ovum of a higher organism, all the chromosomes and genes and the precursors 

 of organ (tissue) differentials and of organismal differentials are present, 

 and these precursors of organismal differentials differ from those of other 

 individuals and species. The organs and tissues, and also the organismal 

 differentials, merely mature in the course of ontogenetic development, whereas 

 they are newly created in the course of phylogenetic evolution. 



Various types of specificities in chemical and morphological structure and 



