SIGNIFICANCE OF THE SEXUAL PROCESS. 57 



of the power of growth and cell-division. The influence of the 

 hereditary characters of each parent upon each other by their intimate 

 association in the same nucleus seems to be the physical basis of 

 phylogenetic variation, but the manner in which this influence acts to 

 bring about variation, or to impart a more vigorous character to the 

 product of fecundation still remains a matter of speculation. 



It is well to consider the blending of the two lines of descent as a 

 consequence of fecundation in a relative sense or as a correlative 

 phylogenetic process. In certain of the lower cryptogams, Ulothrix 

 and Basidiobolus for example, in which the gametes arise from 

 adjacent cells of the same filament and in which a sexual differentia- 

 tion is not at all or only scarcely recognizable, there does not seem to 

 be two lines of descent to blend, yet it is conceivable that the sexual 

 character of the nuclei may have been determined before the stage of 

 ontogeny is reached in which the sexual cells manifest themselves as 

 such. If in such forms a reduction in the number of chromosomes 

 occurs, the sexual character of the nuclei is determined at that time. 

 It is well known that among the simpler forms of the algae and fungi, 

 the development of gametes depends to a certain extent upon external 

 conditions, which effect transpiration, atmospheric pressure, food 

 supply, and so forth, yet no one would suppose for one moment that 

 sexuality is the outcome of these external conditions. 



We have now to touch briefly upon the category of phenomena by 

 which a growth stimulus, or the power of growth and cell-division, is 

 imparted to the product of fecundation. Among many of the lower 

 algae about the only important difference which seems to exist between 

 a gamete and an asexual swarm-spore is the ability of the latter to 

 develop into a normal individual of the adult size. It is true that the 

 iso-gametes of algae, such as Ulothrix, are capable of developing into 

 small dwarf individuals a fact which indicates that here, at least, the 

 gametes possess the power of independent growth sufficiently to enable 

 the resulting plantlet to develop to a limited extent. As soon, how- 

 ever, as the sexual elements have attained any marked degree of 

 bisexual differentiation in the plant kingdom, the individual gametes 

 are quite incapable of independent development even into the most 

 rudimentary individuals, cases of normal and artificial parthenogenesis 

 excepted. 



The stimulus to growth and division in bisexual reproductive cells 

 is imparted normally only by the fusion of male and female elements, 

 and the question naturally arises, is this stimulus due to the fusion of 

 the cytoplasm of the male cell with that of the female, or is it due 

 merely to the fusion of the respective nuclei ? Experiments upon arti- 



