Summary 505 



lasts at most for a few hours. The phenomenon may be reproduced 

 several times m the same animal. "But," remarks Lt^pine, "when these 

 experiments are continued for several days or several weeks, striking 

 always at the same level, we soon observe that the resistance of the 

 animals to the blows increases very rapidly, and that excitations 

 which, in normal animals, produce paraplegias of several hours' 

 duration, produce no effect upon those which have been under 

 experiment for several days." We have in this example a real 

 adaptation of the spinal region when subjected to concussion. 



Similar facts are known to everyone as an experience of daily life. 

 We can become habituated more or less easily to all kinds of violent 

 sensations. Light and very intense noises which, at first, excite 

 exaggerated reflex actions are ultimately perceived without setting up 

 the least movement. Even in the psychical sphere habit dulls painful 

 feelings, and it is very probable that a whole gamut of adaptation, 

 starting from unicellular organisms which accustom themselves to 

 live in an unsuitable medium, up to cultured human beings who 

 habituate themselves to a disbelief in human justice, will be found 

 to rest upon one and the same fundamental property of living matter. 



Regarded from this point of view, immunity becomes a very 

 general phenomenon, passing far beyond the resistance offered by the 

 animal to infective diseases. After all is said and done, it invariably 

 reduces itself to that cellular susceptibility [irritability] which governs 

 so many of the vital phenomena in plants and in animals. It is this 

 susceptibility which impels the branch towards the light and the root 

 towards the ground, and which guides the spermatozoon towards the 

 ovum. From the very commencement of embryonic life the cells 

 derived from the segmentation of the egg exhibit a marked suscepti- 

 bility. Wilhelm Roux^ observed that the earliest cells of the frog 

 embryo, if they are separated by artificial intervention, guided by 

 their positive chemiotaxis again come together. In the formation of 

 the tissues cellular susceptibility plays an important undoubted rule. 

 The prolongations of the nerve cells direct themselves towards the 

 organs of sense or towards the muscular fibres, according to their [590] 

 specific susceptibility'. The mother-cells of the capillary vessels are 

 also guided by susceptibility, when they go towards a new-formed 



1 "Ueber die Selbstordnung der Furchungszellen," in Berichte d, naturwisi. 

 Vereins zu Innsbruck, 1893, Bd. xxi. 



2 Herbst, Biol. Centralhl, Erlangen, 1894, 1895, Bde. xiv, xv; Forssniann, Ziegler'a 

 Beitr. z. path. AnaL, Jeua, 1898, Bd. xxiv, S. 56. 



