ARTIFICIAL PARTHENOGENESIS 641 



contained practically all the constituents of normal sea-water. 

 Yet if unfertilized eggs of Arbacia are left in such a solution 

 for from one and one-half to two hours, as many as 50 per cent, 

 of the eggs may reach the blastula stage when put back into 

 normal sea-water. Many of these eggs die in the blastula 

 stage and only a small number reach the gastrula or pluteus 

 stage. The blastulse are like those which I described in one 

 of my former papers. 1 In the majority of cases more than 

 one blastula develops from one egg. I have seen as many 

 as six moving blastulaB arise from one egg. The tendency 

 to give rise to more than one embryo is greater in the egg 

 of Arbacia than in the egg of Strongylocentrotus. This 

 difference is probably due to the fact that even the unferti- 

 lized egg of Strongylocentrotus often forms a fine membrane 

 which is much thinner than the one produced through the 

 entrance of a spermatozoon, but which is sufficient to keep 

 the blastomeres together. The addition of NaCl or KC1 to 

 sea- water favors the formation of this membrane. 



4. In all the experiments mentioned thus far the increase 

 in the osmotic pressure had been brought about by the 

 addition of electrolytes. This might be considered as an 

 indication that the electrically charged ions in the sea-water 

 played an important role in the production of partheno- 

 genesis. I myself was originally inclined to such an assump- 

 tion. I have convinced myself, however, that an increase in 

 the osmotic pressure of the sea-water through the addition 

 of cane-sugar or urea can produce parthenogenesis. My 

 stock solution of cane-sugar (rock candy) was Zn and con- 

 tained 684.3 g. in a liter, while the stock solution of urea 

 was 2^n and contained 150.31 g. in a liter. I found that 

 the unfertilized eggs of Arbacia were able to develop after 

 they had been for from one and one-half to two hours in one 

 of the following solutions: 



1 Part II, p. 576. 



