128 THE GERM-PLASM 



the mouth, and it is certainly not easy to explain why the de- 

 terminants which cause their formation become active at these 

 points only. It will, however, be shown later on that the 

 cells of Hydra — and probably those of all animal tissues 

 — are in a certain sense polarised ; that is to say, they are 

 differently constituted in the three directions of space. The 

 fact that the determinants of the tentacles — which we must sup- 

 pose to exist in all regions of the body — only become active in 

 certain cells around the margin of the mouth, may be due to the 

 polarisation of the cells as well as to the peculiar conditions of 

 pressure within the cellular dome of the oral disc. 



What has just been said can certainly not be looked upon as 

 anything more than the merest provisional explanation of the 

 facts, but it appears to me to be impossible to give a better one 

 at present. It nevertheless, I think, penetrates somewhat further 

 into the problem than does Herbert Spencer's hypothesis, in 

 which regeneration is compared in general to crystallisation, 

 and the capacity of arranging itself on every occasion under 

 the influence of the whole aggregate in the manner required 

 for the renewal of the missing part, is attributed to every ulti- 

 mate particle. If we take the fresh-water polypes alone into 

 consideration, one of these explanations seems just as good as 

 the other ; but if other groups of animals are included, it is at 

 once apparent that this capacity is not by any means always 

 possessed by the particles, but that even the cell may give rise 

 bv regeneration sometimes to various parts of the whole aggre- 

 gate, at other times only to one certain part, and at others again 

 only to those similar to itself, and that it must therefore contain 

 something which makes it specially capable of one or of the 

 other kind of regeneration. This something is the group of 

 supplementary determinants. 



If a polype or worm is cut through transversely, or if a loss of 

 substance is caused artificially in any organism, the conditions 

 of pressure previously existing in the cell in the region of the 

 injury become changed, the pressure previously exerted by 

 the lost part suddenly ceasing. This induces a change in the 

 vital conditions of the cells tluis affected, which must have a 

 definite morphological and physiological result. We are unable 

 at present to state more precisely what this change is ; but as we 

 know that such losses of substance are followed by the multi- 

 plication of the cells, we may safely assume that it exerts a stimu- 



