i 74 W. Neilsoti Jones. 
It is believed that a good case lias been made out for 
regarding many of the phenomena that accompany fertilization 
as manifestations of the same general protoplasmic reactions 
which are exhibited in the behaviour of parasite and host towards 
one another in cases of parasitism or in the mutual reactions of the 
two components of a symbiotic alliance. 
If this view be accepted, it becomes of interest to speculate 
as to what lengths it is possible to push the comparison. For 
instance, a lichen consists of the balanced association of a 
“host-like” and a “ parasite-like ” partner. In what degree can 
one regard the zygote as an organism built up of two partners— 
the male and female gametes—whose association is more intimate 
and the balance between which is even more delicately poised ? 
Before dismissing such an idea as fantastic let us examine 
the points of resemblance more closely. 
A lichen is built up of two partners, and although the associa¬ 
tion is an intimate one, the two components can always be 
recognised histologically, since each retains its own cell walk In 
cases where symbiosis occurs between an animal and a plant, as in 
the association between an animal such as Convoluta roscoffensis 
and a unicellular green plant, the delimitation of the two components 
one from the other is often readily to be observed owing to the 
well defined cell wall possessed by the plant cells. In this case, had 
the cells of the plant partner been naked and amoeboid, it is 
conceivable that the association would have been much more 
intimate and more difficult to resolve into two separate organisms. 
In point of fact there is frequently considerable difficulty in 
recognising histologically that two organisms are involved in cases 
of parasitism or symbiosis, so close may be the adaptation between 
the individual cells, or so great the reduction of one or other of 
the components in certain regions. This is the case, for instance, 
with the “ smut” fungus when growing in the vegetative tissues of 
its host, or with the mycorhizal fungus in the stem and leaf tissues 
of Calluna vulgaris. In other cases, again, it has been suggested 
that two organisms are involved although no proof can at present be 
adduced of their existence. The somewhat fantastic suggestion 
that the chloroplasts of the higher green plants are phylogeneti- 
cally derived from invading algal cells, or—in the region of animal 
pathology—the view that an individual suffering from small-pox, or 
measles, or scarlet fever is a victim of parasitic, invasion would be 
cases in point. 
