REGENERATION I 2 7 



It appears to me that the regenerative processes which have 

 been observed in the fresh-water polype Hydra and in the sea- 

 anemones (^Actinice) are instances of this kind of regeneration. 

 If a worm is cut through in the median or any other longitu- 

 dinal plane, neither part grows again, and each soon dies. The 

 case is different in Hydra. If this animal is cut through 

 longitudinally, the two parts grow again into entire individuals, 

 irrespective of the plane of section. As the transverse section 

 of the animal at any point is likewise followed by the restoration 

 of each part, it follows that Hydra, in every part of the body, 

 must be capable of a threefold regeneration, i.e.^ of regeneration 

 in the three directions of space. And as the body is differently 

 constructed in these three directions, we are compelled to assume 

 that each cell contains groups of determinants of three different 

 kinds, viz., those which are concerned in the formation of the 

 pro.ximal and distal ends, and in the completion of the body-wall. 

 An individual cell * must therefore be capable of dividing in 

 three different planes, and of giving rise to a part of one of three 

 different regions of the body ; and, moreo\er, the plane in which 

 division actually occurs, and consequently the kind of deter- 

 minants whicn become active and control the cell, is decided 

 not by the quality, but by the kind of division resulting from the 

 stimulus produced by the injury. 



The processes of regeneration in Hydra can, I think, to a 

 certain extent be understood on this assumption. If, for in- 

 stance, the group of supplementary determinants of the proxi- 

 mal end of the body becomes active, it will cause the develop- 

 ment of linear rows of cells extending in the direction of the axis 

 of the body and united laterally so as to give rise to a tube ; these 

 cells, moreover, will have the tendency to close in towards the 

 centre as soon as possible, so as to form the disc or foot, and will 

 also cause the differentiation of the ectoderm cells of the foot 

 into glandular cells which secrete slime : the determinants for 

 the formation of tentacles are wanting in this group. If, again, 

 the group of supplementary determinants of the distal end 

 becomes active, rows of cells arise which will tend to close in to 

 form the oral disc, lea\ing a large space in the centre for the 

 mouth. Tentacles will then grow out from certain points around 



* I shall not refer to the histological details with regard to the process 

 of regeneration in Hydra, as the necessary data appear to be too uncertain 

 and incomplete. 



