268 C. U. Ariens Kappers 



per (p. 236-246) was the first to realise that very definite tropistic factors 

 apparently influence the arrangements in the nervous system ^ His 

 theory — it may be emphasized — is based on embryologic material. 



According to him different chemical secretions (of the glia tissue, the 

 muscles, the skin, as well as of the nervous cells themselves), stimulate 

 the neuroblasts to send out axis-cyhnders and dendrites, and have to be 

 considered as the factors of tropism, during embryonic development of 

 the nervous system. 



He called these secretions «substances attractives» and «substances 

 repoussantes», the former of which should be secreted first by gHacells, 

 thus effecting the outgrowth of the primary axis-cylinders. 



The great amount of new facts, discovered by Cajal, as well in the 

 organs of sense, especially in the optic system, as in the rest of the cen- 

 tral nervous system, are a lasting profit to science. His hypothesis of 

 «substances attractives», is however considered by himself, but a shifting 

 of the point of interrogation, because it does not point the factors de- 

 termining the secretions of these «substances attractives >. He expres- 

 sed his opinión as follows: 



«Cette hypothese présuppose l'existence de certaines conditions pré- 

 alables chimiques et morphologiques tout á fait inexplicables, par exemple, 

 la production dans des parties différentes des centres de substantes atti- 

 rantes ou repoussantes d'apres des lois pré-établies, la suspensión ou la 

 transformation de l'état chimiotactique, etc. On peut diré que cette théo- 

 rie éloigne la difficulté sans cependant parvenir a la résoudre» -. 



Meanwhile in his excellent handbook of neurology, he made a state- 

 ment which I found to be of still greater importance than his chemotactic 

 doctrine of greater importance even than Cajal himself perhaps realised 

 at that time. I refer to the following lines: 



«Una vez fijada la dirección y posición de las dendritas y axo- 

 nes, el estado secretorio cesa en la mayoría de las células, y con ella 

 toda contracción quimiotáctica. Si en tal situación de relativo reposo 

 llegasen al territorio nervioso axones nuevos, la neurona no deshará 

 los contactos ya creados ni retirará dendritas, sino que, echando mano 



^ About the same time Strasser (Ergebn. der Anatomie und Entwicklungsges- 

 chichtc, Bnd. I, 1892, p. 721 ) emphasized this point, without however giving perso- 

 nal researches. It is interesting that he considered electric forces as the chief factor. 



2 Ramón y Cajal: La retine des vertebres. La Cellule, tome IX, 1892, p. 240. 



