48 The Science of Life. 



Pringsheim extended the recognition of alternating gen- 

 erations to Algae (CEdogonium and Coleochcete\ and in 

 1866 Haeckel gave a clear generalized account of the 

 subject in his Generelle Morphologie, introducing the 

 convenient term metagenesis for true alternation of 

 generations as opposed to such cases as the succession 

 of vegetative and reproductive shoots. 



In 1868 Celakowsky introduced a theoretical distinc- 

 tion between two kinds of alternation homologous and 

 antithetic. Homologous alternation was illustrated 

 among the Algae, where there may be an alternate occur- 

 rence of sexual and asexual forms otherwise similar. 

 Antithetic alternation was illustrated by mosses and 

 ferns, where there are two fundamentally distinct genera- 

 tions, e.g. the prothallus and the " fern-plant". With 

 this Braun essentially agreed (1875), and it is interesting, 

 in view of recent zoological discussions by Beard and 

 others, to notice his opinion that antithetic alternation 

 is confined to plants. 



Of much importance was the discovery of apogamy 

 (Farlow, 1874), the direct production of the asexual from 

 the sexual without the intervention of ova and sperma- 

 tozoa, and the converse apospory (Pringsheim, 1876), 

 or the vegetative production of the sexual from the 

 asexual without the intervention of spores. 



An extended recognition of alternation of generations 

 among Algae and Fungi, the further study of apospory 

 and apogamy, the interesting discovery that in many 

 cases the number of nuclear bodies or chromosomes in 

 the dividing nucleus of the sporophyte is twice as great 

 as in the cells of the gametophyte, and a few experi- 

 mental studies, have influenced the development of the 

 theories of antithetic and homologous alternation, but 

 as yet no decision has been arrived at. 



" On the homologous theory, the sporophyte is to be 

 traced back to a generation of originally independent 

 individuals similar to those from which the gametophyte 

 has arisen, the almost invariable alternation and the 

 permanent or temporary dependency of the spore-bear- 

 ing on the sexual generation being subsequent adapta- 

 tions, On the antithetic theory, the sporophyte is not 



