MENDELISM AND OTHER METHODS OF DESCENT I93 



fusion of the parent sex-cells or gametes, but this is only par- 

 tially true among the higher plants and animals. Among the 

 lower groups of sexual organisms no distinction is to be made 

 between conjugation and fertilization, but among the higher 

 types fertilization (the union of sex-cells which initiates the 

 development of a new plant or animal), no longer represents 

 the whole process of conjugation, but only the first part of it. 

 The outer protoplasms of the sex-cells have fused, and also 

 their nuclei, but the conjugation is not yet complete, for the 

 fertilized egg-cell still retains a double equipment of chromatin, 

 derived from the two parents. And since this chromatic material 

 represents the most highly specialized and active form of pro- 

 toplasmic substance, it is proper to hold that the fertilized egg- 

 cell and all its subdivisions are double cells as long as they 

 contain this double equipment of chromatin. 



The gametes can be thought of as persisting through the 

 whole life-history of the individual organism, and do not become 

 finally and completely united until the chromatin has fused in 

 mitapsis, preceding the formation of sex-cells for the next gen- 

 eration. All the cells formed between fertilization and mitapsis 

 are double in the above sense, that they contain two complete 

 sets of chromatin elements, derived from those of the parent 

 gametes. Organisms built up of cells formed while conjuga- 

 tion continues, that is, between fertilization and metapsis, may 

 be called conjugate organisms. After the chromatin has fused 

 in mitapsis new gametes are formed with single equipments of 

 chromatin, capable of undertaking new conjugations. Then 

 the number of chromosomes is agayi doubled, and the vegeta- 

 tive subdivisions of the conjugating cells are resumed. 



CONJUGATE AND EXJUGATE REPRODUCTION. 



There is thus a fundamental difference between the repro- 

 ductive processes of the lower types and the higher. The cells 

 which compose the bodies of the higher organisms are of this 

 double or persistently conjugate character. Reproduction is 

 still a process of cell-divisions, since it is only by cell-divisions 

 that the cellular bodies of plants and animals can be built up. 

 But in the Higher types com*"gation is not finished before these 



