REACTIONS OF NEWBORN AND ADULT ORGANISMS 529 



stages, but merely that the ontogenetic structural development is accompanied 

 by a parallel chemical development and that therefore different stages of the 

 developing organisms possess their own characteristic substances, which under 

 certain conditions may function as antigens. Furthermore, the organs and 

 tissues of embryos of phylogenetically higher, more differentiated organisms 

 may resemble the organs and tissues of embryonal, and, perhaps, even of 

 adult, phylogenetically more primitive forms. But, the embryonal organs and 

 tissues of the higher organisms differ from those of phylogenetically less 

 developed species, in that the former possess specific precursor substances 

 of the organismal differentials present in the corresponding adult forms, 

 which are lacking in the phylogenetically lower embryonal or adult 

 organisms. 



