ARTIFICIAL PARTHENOGENESIS IN ANNELIDS 679 
result may be due to the impossibility of the spermatozoon 
of the one species entering the egg of the second species, or 
to the fact that the spermatozoon of Cheetopterus brings 
about the development of the Chetopterus egg by substances 
which are ineffective in the Arbacia egg, and vice versa, or 
the spermatozoon of the one species is poisonous for the egg 
of the other species, or vice versa.’ The second possibility 
is of interest to us on account of the fact that we can bring 
about the parthenogenetic development of the Cheetopterus 
eggs by means which have no effect upon the Arbacia egg. 
When we intend to produce artificial parthenogenesis in 
the eggs of Echinoderms, it is only necessary to put them for 
from one and one-half to two hours in sea-water, the osmotic 
pressure of which has been raised about 374 to 75 per cent. ; 
that is, into sea-water to which has been added 124 to 25 per 
cent. of its volume of a 24” NaCl solution or of a solution 
isosmotic with the latter. We have not yet determined the 
osmotic pressure of the sea-water at Woods Hole, and on 
indirect data assume that it is about isosmotic with a 3n 
NaCl solution. The optimal increase of osmotic pressure 
varies for different species and even for different females of 
the same species. It may be that the temperature of the 
water and the degree of maturity of the eggs play a rdle. 
In making experiments of this kind, it is necessary to use 
always a series of solutions of different osmotic pressure and 
to take the eggs out at various intervals, from one-half to two 
hours or more, until the optimum concentration and time 
have been ascertained. 
An increase in the osmotic pressure of the sea-water is 
also able to cause artificial parthenogenesis in Chetopterus. 
The chief difference between the Chetopterus and Arbacia 
eggs is that at the same temperature the Chetopterus eggs 
1Certain constituents of the blood (globulins, enzymes?) frequently destroy the 
blood corpuscles of other species that are not closely related. 
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