RFXiENERATION T I T 



which does not influence its primary (embnonicj ontoji^cny. 

 but only its secondary formation by regeneration. A vertebral 

 column is formed primarily ; but if the re-formation of a part of 

 it becomes necessary, in consequence of the loss of the tail, the 

 secondary reduced process for the development of the axis 

 comes into play, and a simple cartilaginous tube is formed. 

 This process recalls the phenomena of ' dichogeny ' which take 

 place so frequently in plants, and in which the same group of 

 cells may develop in either of two different ways, according to 

 the nature of the external stimulus which is applied to them. 

 Thus a shoot of ivy will produce roots on a certain side if it is 

 shaded, and leaves if it is exposed to light. The determination 

 of the sex of an animal may perhaps be referred to similar 

 causes, — if, at least, we may assume that the sex is not always 

 universally decided by the act of fertilisation, and that influences 

 exerted upon the organism subsequently may have an eiTect in 

 this determination. In the case of certain parasitic Crustaceans, 

 the Cyinathoidcp^ the male sexual organs are developed first : 

 and when the animal has fulfilled its function as a male, the 

 female organs become developed, and give the animal the char- 

 acter of a female. The two developmental tendencies here come 

 into operation temporarily, one after the other ; just as in the 

 case of the lizard's tail, in which the tendency to form the verte- 

 bral column first comes into play, and then that to form the sec- 

 ondary cartilaginous tube. The necessity for the formation of 

 this tube certainly need not arise at all ; just as that side of the 

 shoot of ivy from which the roots arise need not necessarily be 

 subsequently exposed to the light, and give rise to leaves ; the 

 possibility of such an occurrence is, however, foreseen by Nature. 

 It might be urged that there is an important difference between 

 the regeneration of a lizard's tail and the successive development 

 of the two kinds of sexual organs in the Cymathoids, since in 

 the latter case the rudiments of these organs are present in the 

 embryo, and it is only their final development which takes place 

 successively. This is 'certainly a difference, but it is just such 

 a one as to indicate to us how these cases of supplementary 

 substitution may be explained theoretically. The cells in the 

 tail of a lizard which give rise to the secondary cartilaginous 

 tube must contain determinants which differ from those of 

 the embryonic formative cells of the caudal vertebra?, just as 

 the idioplasm of the formative cells must contain ihfVerent 



