On the Nature of Fertilization and Sex. 169 
microscope in a drop of sea water in which male gametes are 
swimming, the latter are seen to cluster closely round the egg¬ 
cell (no doubt attracted chemically) often in such numbers as to 
cause it to undergo considerable movement. Eventually one 
spermatozoid enters the egg-cell, whereupon the egg-cell ceases 
to attract the remainder of the spermatozoids. One explanation 
offered for this change of mutual reaction is that after fertilization 
the egg-cell excretes a cell wall, thus cutting off communication with 
the outside world, It would seem unlikely, however, that a cell 
wall of such a nature as would prevent the egress of the 
attractive substance from the egg-cell could be formed so instan¬ 
taneously, and it is more probable that the change in reaction is 
the result of chemical causes, e.g., gametic fusion brings to an end 
the production of attractive substances, or even leads to the 
production of substances whose action is repellant to the remainder 
of the male gametes. 
This explanation receives support from results obtained in 
experiments with sea-urchins’ eggs conducted with the object of 
bringing about double fertilization. Prom such experiments it 
appeal s that it is possible to induce more than one spermatozoid 
to enter the egg-cell if the vitality of the latter be suitably 
lowered either by chemical or physical means. 1 One may com¬ 
pare the results obtained by Salmon in his experiments on the 
susceptibility of grasses to fungal attack under varying conditions. 
It was found that a variety of grass which was immune to 
attack by a particular fungus might become susceptible if the 
vitality (and therefore resistance) of the individual were lowered 
artificially. So far as double fertilization of the egg-cell is con¬ 
cerned, the facts may be stated in the following way: the 
fertilized egg-cell is immune to attack from spermatozoids under 
normal conditions; if, however, its vitality is lowered, resist¬ 
ance is decreased and it may be rendered susceptible to attack by a 
second spermatozoid. 
The “ double' fertilization ” occurring in the embryo-sac of 
angiosperms may be described in somewhat similar terms. The 
1 Compare results obtained in experiments on hybridization of Echinoids. 
Thus Doncaster (Experiments in Hybridization, Phil. Trans. B. Vol, CXCVI., 
1903), states that “cross fertilisation is assisted by conditions which tend to 
reduce the vitality of the eggs.” Similarly Loeb (“ Ueber die Befruchtung 
von Seeigeleiern durch Siesternsamen,” Pfltiger’s Archiv. Vol. XCIX., 1903, 
also Vol. CIV., 1914), found that cross fertilisation between Strongylocentrotus 
and Asterias, which occurred very rarely in normal sea water, could occur to 
the extent of 50 per cent, if the sea water were made slightly alkaline by 
addition of Na,CO, or NaOH. 
