ALTERNATION OF GENERATIONS I 73 



CHAPTER V 



ALTERNATION OP^ GENERATIONS IN ITS RELATION TO 



THE IDIOPLASM 



Starting from the germs specially adapted for amphimixis 

 (sexual intermingling), we liave designated as ge^'Jii-plasni the 

 definitely arranged group of determinants which must be con- 

 tained in the sexual cells. By this term is meant an idioplasm 

 which contains all the determinants of the species. At the 

 same time a large number of species exist in which the sexual 

 cells are not the only ones which contain all the determinants, 

 and in which the development takes place, for tJie second time 

 during the course of the life-cycle, from a single cell ; the idio- 

 plasm of this cell must therefore also be composed of all the 

 determinants of the species. This is the case in most of the 

 lower plants, such as mosses, horse-tails, and ferns, — in all of 

 which sexual reproduction alternates with the formation of 

 asexual ' spores,' — as well as in those groups of animals in which 

 that form of alternation of generations which is known as 

 heterogeny occurs. But even in the case of alternation of gen- 

 erations in the more restricted sense, — i.e., the alternation of 

 sexual reproduction and gemmation, — the development of an 

 individual may take place twice successively from a single cell, 

 as was mentioned above with regard to the stocks or colonies 

 of plants and of Hydroid-polypes. In all these a cell, the idio- 

 plasm of which contains all the specific determinants, occurs 

 twice in the course of the life-cycle from one fertilised ovum to 

 the next one. The question therefore arises as to whether the 

 idioplasm in each case is to be considered identical, and may 

 merely be described as germ-plasm. 



This question has already been discussed in the section on 

 the process of gemmation in plants ; and it was there concluded 

 that the idioplasm of the apical cell and that of the fertilised 

 ovum cannot be assumed to be perfectly identical, owing to the 

 fact that the course taken by embryogeny — in which process the 

 first shoot and roots are formed — is different from that followed 



