On the Nature of Fertilization and Sex. i 7 i 
It must not be forgotten that cases are known in which egg- 
cells develop regularly without fertilization, just as the majority 
of plant embryos develop without the stimulus of fungal infection ; 
but ova which behave in this way are exceptional and in most cases 
not the cytological equivalents of normal ova. The rule and 
exception in the two cases are inverted. 
Let us consider for a moment the position which has now 
been reached. 
The point of view advanced is that the male and female 
gametes in fertilization stand to one another in a relation cor¬ 
responding to that occupied by parasite and host, or perhaps more 
accurately to that existing between the two components in cases of 
balanced parasitism or “ symbiosis,” many examples of which can 
be cited from the plant and animal kingdoms . 1 Using the 
language of pathology, it may be said that the egg-cell is sus¬ 
ceptible to attack by the male gametes of the same species, 
but that the zygote is immune following entry of a male cell into 
the ovum. One is almost tempted to say that the egg-cell after 
fertilization has been “ vaccinated ” against a further attack from 
spermatozoids ! 
The stimulus to cell division which the egg-cell receives from 
union with the spermatozoid has been likened to the stimulus to 
development the embryos of certain plants receive from infection 
by the mycorhizal fungus—in neither case can growth occur with¬ 
out the application of the respective stimuli, except in special and 
rare cases. 
It is possible to extend this comparison of fertilization 
phenomena with the general mutual reactions of protoplasm. 
There are many cases of plants with dimorphic flowers in which 
legitimate unions are fertile and illegitimate unions more of less 
sterile. The relationship in the case of Primula may be expressed 
in the following way according to the present hypothesis. 
The egg-cells produced by plants with “ pin eyed ” flowers are 
susceptible to attack by the male gametes from plants with 
“thrum-eyed” flowers, but immune from attack by those from 
“ pin-eyed ” flowers. Conversely, the egg-cells produced by “ thrum* 
1 Every stage may be found of a series having at one end a purely 
destructive parasite, against the attack of which the host plant seems able to 
offer little resistance, and the “parasite” that lives side by side with its 
“ host,” the relations between the two being so nicely balanced that the 
attacking power of the one is just compensated for by the limited immunity of 
the other. A further development of the immunity of the “host” leads to 
the partial or entire exclusion of the potential parasite. 
