102 Artificial Parthenogenesis 



extracts of a large number of animals, especially of 

 mammals (rabbit, pig, ox, etc.), had the same effect, 

 though it was necessary to use higher concentrations, 

 one-half sea water and one-half isotonic blood serum. 

 The eggs of every female sea urchin, however, did not 

 give the reaction and not all the eggs even of sensitive 

 females formed membranes. The writer found, how- 

 ever, that it was possible to increase the susceptibility 

 of the eggs against foreign blood serum by putting them 

 into a 3/8 m solution of SrCl 2 for from five to ten 

 minutes (or possibly a little longer) before exposing 

 them to the foreign blood serum. BaCl 2 acts similarly. 

 The fact that SrCl 2 alone can cause membrane forma- 

 tion in unfertilized eggs if they are left long enough in 

 the solution suggests that the sensitizing effect of the 

 substance consists in a modification of the cortical 

 layer similar to that underlying membrane formation; 

 and that the subliminal effect of a short treatment 

 with SrCl 2 and the subliminal effect of the foreign 

 serum when combined suffice to bring about the 

 membrane formation. 



Not only the watery extract of foreign cells but also 

 that of foreign sperm, induces membrane formation 

 in the sea-urchin egg. The watery extract of sperm 

 of starfish is especially active, but the degree of activity 

 varies considerably with the species of starfish from 

 which the sperm is taken. The eggs of different species 

 of sea urchins also show a different degree of suscepti- 



