ss of evolution
we have to reckon with the possibility that parthenogenetic propagation
may have preceded sexual reproduction. This suggests also the
possibility that at that period outside forces may have supplied
the conditions for the development of the egg which at present the
spermatozoon has to supply. For this, if for no other reason, a brief
consideration of the means of artificial parthenogenesis may be of
interest to the student of evolution.

It seemed necessary in these experiments to imitate as completely as
possible by chemical agencies the effects of the spermatozoon upon
the egg. When a spermatozoon enters the egg of a sea-urchin or certain
starfish or annelids, the immediate effect is a characteristic change of
the surface of the egg, namely the formation of the so-called membrane
of fertilisation. The writer found that we can produce this membrane in
the unfertilised egg by certain acids, especially the monobasic acids
of the fatty series, e.g. formic, acetic, propionic, butyric, etc.
Carbon-dioxide is also very efficient in this direction. It was also
found that the higher acids are more efficient than the lower ones,
and it is possible that the spermatozoon induces membrane-formation by
carrying into the egg a higher fatty acid, namely oleic acid or one of
its salts or esters.

The physico-chemical process which underlies the formation of the
membrane seems to be the cause of the development of the egg. In all
cases in which the unfertilised egg has been treated in such a way as
to cause it to form a membrane it begins to develop. For the eggs of
certain animals membrane-formation is all that is required to induce a
complete development of the unfertilised egg, e.g. in the starfish and
certain annelids. For the eggs of other animals a second treatment is
necessary, presumably to overcome some of the injurious effects of
acid treatment. Thus the unfertilised eggs of the sea-urchin
Strongylocentrotus purpuratus of the Californian coast begin to develop
when membrane-formation has been induced by treatment with a fatty acid,
e.g. butyric acid; but the development soon ceases and the eggs
perish in the early stages of segmentation, or after the first nuclear
division. But if we treat the same eggs, after membrane-formation, for
from 35 to 55 minutes (at 15 deg C.) with sea-water the concentration
(osmotic pressure) of which has been raised through the addition of a
definite amount of some salt or sugar