PAR THENOGENESIS 



that Mead's work is too often overlooked and deserves 

 greater appreciation than it has received. 



Turning to that class of eggs which are fertilizable in the 

 stage of the intact germinal vesicle, I may cite the results 

 of experiments to induce parthenogenesis in three eggs: 

 those of Mactra, of Thalassetna and of Nereis. 



Eggs of Mactra, a marine clam, exposed to lo cc. of 23-2 

 N KCl plus 90 cc. of sea-water for 45 minutes complete 

 maturation and develop as far as the six-cell stage. Longer 

 exposures, i^-? to 4 hours, and to stronger solutions, give 

 development without formation of polar bodies. KCl- 

 treated eggs may also develop into swimming forms by 

 nuclear divisions only, i.e., they differentiate without cleav- 

 age as do the eggs of worms mentioned above that are 

 similarlv treated.' 



By the action of either mineral or organic acids, including 

 carbon-dioxide, in sea-water, one can induce the develop- 

 ment of 50 to 60 per cent, of unfertilized eggs of Thalas- 

 sema mellifa,'' a marine worm, to the larval stage; this is 

 scarcely to be distinguished from that of the normal, i.e., 

 developed from fertilized eggs. Because of these positive 

 results obtained with acids, the effects of hypertonic sea- 

 water were not thoroughly investigated. Those reported 

 are not only too incomplete but also too contradictory to 

 permit the drawing of a conclusion. In view of the effi- 

 ciency of hypertonic sea-water on so many other eggs, how- 

 ever, one may hazard the opinion that it is efficacious on 

 this egg also. One is strengthened in this opinion by recent 

 work on the eggs of another species of Thalassema, T. 

 neptuni.^ 



In the egg of Nereis by shaking or centrifugal force ecto- 

 plasmic break-down, with extrusion of jelly, and matura- 



^ Kostanecki, igii and earlier. 

 ' Lefevre, igoi. 

 ^ Hobson, ig2S. 



217 



