130 THE GERM-PLASM 



placed with their opposite poles as close together as possible, the 

 entire magnet is once more formed. Similarly, if the root of a 

 poplar is cut in half transversely, each half produces buds and 

 roots at the corresponding poles ; but if, on the other hand, the 

 two portions are joined together in the same relative position as 

 that which they occupied originally, they become united together, 

 so that a single piece of root, with its two poles, results, quite 

 similar to the original piece. 



These important results which Vochting has obtained by his 

 experiments on transplantation, are mentioned in this place 

 because they can be utilised in considering the phenomena of 

 regeneration in animals, which have just been discussed. We 

 may in this respect compare a fresh-water polype with a poplar 

 root. After a Hydra has been cut in half transversely, the dis- 

 tal portion gives rise to a new foot at its proximal end. and the 

 proximal portion produces an oral region at its distal end. We 

 might therefore in this case speak of pedal- and oral-poles, instead 

 of root- and stem-poles, as in the case of the poplar. And. in 

 fact, if a Hydra is cut transversely into three portions, the distal 

 part or oral pole of the middle piece develops a new oral region, 

 and its proximal part or pedal pole gives rise to a new foot. It 

 might not be impossible for a clever experimenter to cause this 

 middle piece to unite with the two terminal portions of the body 

 before the former had had time to develop into a complete ani- 

 mal, by joining the three portions together with bristles. This 

 would result in a union just as in the case of the poplar. 



It would be a mistake to try to deduce that one of the poles of 

 the poplar root must grow shoots and the other roots merely 

 from the fact of its polarisation : one might as well try to deduce 

 it from the fact of the polarisation of a real magnet. Something 

 more is required before this can take place : — the cells of the 

 poplar 7-oot must contain the primary constituents for the forma- 

 tion of shoots and roots ; that state of the cells which Vochting 

 describes as polarisation only produces the conditions under 

 which one or other of the primary constituents becomes active, 

 and thus undergoes development. The hypothesis of the polar- 

 isation of the cells does not, therefore, relieve us in the least 

 from the necessity of making a theoretical assumption to explain 

 how it comes about that the primary constituents of different 

 kinds of structures are present in one and the same cell. 

 According to my view, we must assume in the case of the poplar 



