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 Asterias forbesii 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. 1 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 the egg. 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 



1 A. P. Mathews, "Artificial Parthenogenesis Produced by Mechanical Agita- 

 tion," Am. Jour. Physiol., VI, 142, 1901. 



