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 alge 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 
algz 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 alge, 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- 
