MULTIPLICATION- BV FISSION 



153 



tion in worms, beginning with tlie formation of segments at the 

 growing tail-end. and then passing from the regeneration — first 

 of the tail, and then of the head-end. to the actual fission of 

 Lujnbriculus, and finally to that of Nais. And accordino- to 

 our view, this course of development depends on the regular 

 distribution of certain accessory determinants to particular 

 tissue-cells, and the gradual increase in the complexity of this 

 distribution. 



The regenerative process which renders fission possible must 

 be traced to the doubling of certain groups of determinants in 

 the idioplasm, so that the half of them remain latent. I imagine 

 that this doubling need not necessarily take place and be fol- 

 lowed by the subsequent multiplication of the inactive groups 

 of determinants in the germ-plasm itself: such a multiplication 

 would, in fact, be a useless encumbrance to the germ-plasm. 

 The latter need only contain the determinants for fission when 

 this process leads to an alternation of generations ; that is to 

 say, when the animals formed by division have a different struc- 

 ture from those which arise directly from the ovum : for in the 

 latter case the forms which arise by fission are independently 

 variable hereditarily. This must be the case in the alternation 

 of generations in certain marine annelids, such as the Syllida; 

 and also in the strobilation of polypes, which will be discussed 

 later on. In all such cases, two kinds of ids must be assumed 

 to exist in the germ-plasm. In the ordinary kind of fission 

 which occurs in the fresh-water annelids, on the other hand, the 

 separation of the groups of determinants necessary for the 

 regeneration of a part may take place during embryogeny : 

 nothing definite, however, can be said on this point at present, 

 but in any case the process of splitting off of the accessory deter- 

 minants may conceivably be thrown back from the later to the 

 earlier stages of ontogeny, until it finally takes place in the 

 fertilised ovum, so that double ids are present in the germ- 

 plasm. It will be assumed in the next section that certain forms 

 of budding owe their origin to the presence of these double ids. 



