WILLIAM ROUX'S THEORY 165 



cytotropism and Hartog as adelphotaxy and for which 

 both have found a chemical explanation. 



To demonstrate the existence of cytotropism Roux 

 isolated in a neutral fluid the blastomeres of a segmen- 

 tated ovum and noticed that whenever the space sepa- 

 rating them did not exceed their diameter they moved 

 towards one another and became clustered. 



Herbst attributes the motions of the cells, beginning 

 with the first blastomeres and continuing through all 

 ontogenesis, to a certain chemical attraction he calls 

 chimiotactism. This chemical action attracts certain 

 cells toward the surface of the organism where they 

 become epidermic cells and others toward the internal 

 regions where they constitute the digestive apparatus. 

 For instance, when a nerve is in formation, its axis- 

 cylinder first penetrates the surrounding tissue; the 

 mesodermic cells, which later constitute Schwann's 

 sheath, cluster around it, evidently attracted by a force 

 originating from the chemical composition of the axis- 

 cylinder. 



Likewise, perimysium forms around the muscles, the 

 successive layers of the vascular walls grow around the 

 endothelial sheath wherein the blood circulates, etc.; 

 a conjunctive cell, at first undifferentiated, may be- 

 come a neurilemma cell, or a perimysium cell or a peri- 

 ostis cell, according to the peculiar tactism which at- 

 tracts it. 



Besides this special tactism, whose physico-chemical 



