224 PLANT LIFE 
are watched under the microscope while the 
sperms are swimming about them, they will 
be seen, one by one, suddenly to change 
their form—they swell up, and at some one 
spot on their surface they become “ prickly.” 
This prickliness spreads with great rapidity 
over the surface of the egg. The onset of 
this curious appearance marks the entrance 
of a sperm into the egg. The immediate effect 
of that sperm on the egg protoplasm is to 
render it not only no longer attractive to 
the rest of the sperms, but actually poisonous 
to them. An explanation is therefore at once 
furnished as to how the entrance of more 
than one sperm is prevented. The change is 
a sudden one, resulting from the interaction 
of the substance of the egg and sperm—a 
circumstance which sufficiently emphasises 
the physiological difference existing between 
them. Under unfavourable conditions, e. g. 
badly aérated water, or by the. addition of 
certain substances to the water, the sudden- 
ness of this reaction can be slowed down, and 
then it may happen that more than one 
sperm effects an entrance. But it seems to 
be a general rule that if more than one of 
them fuses with the nucleus of the egg, 
either no further development takes place, 
or monstrous embryos are produced which 
commonly die during the earlier stages of 
development. 
It is evident, then, that the act of sexual 
fusion produces striking and immediate change 
