AuGusT, rgrs | THE ORCHID REVIEW. 227 
z 7 : 
Ea THE MECHANISM OF HEREDITY. ee 
(Concluded from page 200). 
E have now seen how the process of sexual reproduction grew directly 
out of the earlier process of reproduction by vegetative bipartition, 
and by a series of progressive modifications, beginning with the fusion of 
two specialised motile cells that are externally indistinguishable, and ending 
with those between which the widest diversity exists. In its initial stages 
the new process probably began as a reaction against seasonal exhaustion, 
for it has been observed in primitive groups that when these specialised 
cells (gametes) fail to conjugate they are subsequently able to vegetate 
independently, though they do not acquire the vigour of individuals formed 
by the new method. It is the union of these specialised cells into a new 
body, called the zygote, which thenceforth is able to develop into a new 
individual, that introduces the sexual process. All earlier cells’ are 
produced by vegetative bipartition, a process that ultimately leads to 
exhaustion, and against which the sexual method may be regarded as a 
reaction, for in the higher groups the gametes completely lose the power of 
independent growth. 
The union of the gametes takes place by the fusion of the nuclei and 
the contained chromosomes, and here we see the significance of the 
reduction division that has already been mentioned. The union of two 
nuclei doubles the number of chromosomes, and were the process repeated at 
ach conjugation it would soon lead to an impossible condition of things. 
This is provided against by the particular division of the nucleus known 
as the reduction division, which results in the gamete having only half as 
many chromosomes as the vegetative cell, and at the same time limits the 
power of independent growth in the gamete until the normal number of 
It is a fundamental distinction, 
chromosomes is restored in the sexual act. 
that has been retained by the nucleus through its ever-increasing complexity 
in the higher groups. The change is itself an adaptation, and it is not 
difficult to form an inference as to the way in which it has been brought 
about. ; 
The next consideration is that the two conjugating cells, however 
diverse in size and appearance they may be, take an equal part in forming 
the characters of the new individual, and this results from the fact that it is 
the nucleus that is the bearer of hereditary qualities. The female cell is 
chiefly differentiated by possessing a mass of nutrient matter, called 
cytoplasm, whose chief function is the nourishment of the new organism. 
In the male cell no such provision is necessary, hence the cytoplasm is 
