REGENERATION 1 29 



lus on the cell, and more especially on its idioplasm, which 

 forces the latter to undergo multiplication. This view is main- 

 tained by those who have the greatest opportunity of investi- 

 gating the details of such processes, — I refer to the pathological 

 anatomists. The proliferation which ensues in the surrounding 

 tissue after a loss of substance, is not explained by them as 

 being due indeed to a stimulus — in the ordinary sense of the 

 word — exerted on the surrounding cells, but rather to a cessa- 

 tion of the * resistance to growth,^ and this may in one sense also 

 be described as a ' stimulus,' inasmuch as it is an ' incitement ' 

 to growth. 



If the cells were constituted alike in the three directions of 

 space, the effect on the idioplasm would be the same whether 

 the stimulus due to the loss of substance acts from before, from 

 behind, or from the side. One of the three groups of deter- 

 minants could not possibly be alone effected by the stimulus and 

 thus rendered active in one case, the second only in another, and 

 the third only in a third instance. We have, however, every 

 reason to suppose that the structure of one of these tissue-cells 

 is not the same in the three directions of space, and that they 

 are, in fact, variously differentiated according to each of these, 

 and consequently respond to stimuli in different ways according 

 to the direction in which the latter act upon them. Vochting * 

 has proved that at any rate in higher plants, ' a different upper 

 and lower, anterior and posterior, and right and left half, can be 

 distinguished in each living cell in the root and stem."' Portions 

 of the root of the poplar transplanted on to the stem, or por- 

 tions of the stem transplanted on to the root, only grew and 

 flourished when they were fixed in a certain position ; in the 

 reverse position they sometimes indeed gre\v, but soon showed 

 phenomena of degeneration. Vochting infers from this obser- 

 vation that the cells are ' polarised,' this term being taken 

 merely in an analogous sense to that in which it is generally 

 used. The root and stem behave in a certain sense like a 

 cylindrical magnet, which is composed of sections equally mag- 

 netised in the radial and longitudinal directions. Such a mag- 

 net, like the stem and root, may be separated into pieces. If 

 the smooth adjoining surfaces of the portions of the magnet are 



* H. Vochting, ' Uber Transplantation am Planzenkorper,' Tubingen, 

 1889, p. 400. 



