ARTIFICIAL PARTHENOGENESIS IN STARFISH 255 
of lack of oxygen or of addition of KCN upon the sea-urchin 
egg after membrane formation, and hence has not recognized 
that his results harmonize with my experiments. 
4. Some eggs of the starfish possess a certain. tendency to 
develop spontaneously into larvae without any evident external 
stimulus. The percentage of these “naturally” partheno- 
genetic eggs fluctuates, and is always very small. It is also 
possible that this tendency toward natural parthenogenesis 
is found only in the eggs of certain females. Mathews dis- 
covered that in the case of A sterias forbesit the number of these 
eggs that reach the larval stage can be increased by shaking 
or agitating them. The amount of shaking necessary varies 
with different cultures: sometimes a very vigorous shaking 
in a test tube is required, at others the mere transference of 
the eggs from one dish to another by means of a pipette is suffi- 
cient. The most favorable time for obtaining this result is 
about three hours after the liberation of the eggs from the ovary, 
probably because they are then ripe. 
The key to the explanation of this experiment is perhaps to 
be found in Mathews’ observation that these eggs after being 
agitated or shaken form a fertilization membrane and then 
have the appearance of fertilized eggs. Hence we must regard 
this membrane formation as the probable impulse to develop- 
ment in these experiments. But how can membrane formation 
be produced by mere agitation? It seems to me that this can be 
understood if we suppose that it depends upon the destruction 
of an emulsion in the cortical layer of theegg. It is conceivable 
that in the eggs of certain forms the stability of this emulsion 
is so small that mere shaking would be enough to destroy it 
and thus induce membrane formation, and so development. 
This hypothesis is supported by an observation recently 
made by the writer. If the ovary of a starfish is subjected to 
moderate pressure and the eggs are afterward removed, it is 
1A. P. Mathews, “ Artificial Parthenogenesis Produced by Mechanical Agita- 
tion,’”’ Am. Jour. Physiol., VI, 142, 1901. 
