1 68 LEO LOEB. 



homoioreaction as well as the heteroreaction exists, but again 

 both are probably less pronounced than in mammals. 



There has, therefore, as far as we can judge from the experi- 

 ments so far recorded, taken place an ontogenetic as well as a 

 phylogenetic evolution in the formation of the individuality dif- 

 ferential. Ontogenetic and phylogenetic evolution has apparently 

 led to an individualization of the organisms. There is added 

 gradually to the reaction against the species differential the re- 

 action against the individuality differential. 



If we inquire into the mechanism on which this gradual refine- 

 ment depends, several factors would have to be considered : 



1. The individuality differential might as yet be absent in the 

 lower forms, especially in invertebrates, and appear only in ver- 

 tebrates. 



2. The individuality differential might be present in all animal 

 organisms, but the supplementary substance in the body fluids 

 might develop only in certain ontogenetic and phylogenetic stages 

 and consequently the reaction on the part of the host might be 

 very weak. 



3. The individuality differential might be present and the host 

 might react towards it, but somehow the tissues of lower or- 

 ganisms show a very low degree of sensitiveness to these re- 

 actions. In other words, is this individualization due to the 

 gradual acquisition of the individuality differential or to the de- 

 velopment of a reaction on the part of the host against the indi- 

 viduality differential or to a greater sensitiveness on the part of 

 the higher tissues to this reaction? Authors have so far not dif- 

 ferentiated between these three possibilities, and the data on hand 

 do therefore not permit us to decide the question definitely. 



There exist, how r ever, some facts which have a bearing on this 

 question. In the first place we find a very interesting exception 

 to the statement that individuality reactions are absent in lower 

 forms: in a certain rhizpocl. Orbitolithes, Max Schultze observed 

 as early as 1863 a peculiar reaction which was further analyzed 

 by P. Jensen. Two pseudopodia belonging to the same individual 

 unite, the protoplasms flowing together at the point of contact ; 

 but if two pseudopodia belonging to two different individuals 

 touch each other a contraction occurs and a reunion fails to take 



