194 ARTIFICIAL PARTHENOGENESIS AND FERTILIZATION 
It was observed in the experiments with the blood of 
mammals that, just as with the Dendrostoma blood, it was not 
the eggs of every sea-urchin that would respond, but only the 
eggs of about 10 per cent of the females. I am inclined to 
attribute this to differences in the permeability of the eggs of 
different females. In order that the blood may cause membrane 
formation, it 1s necessary for its effective constituent to diffuse 
into the egg. It seems then that the necessary degree of per- 
meability will not be found in the eggs of every female, but 
only in those of a certain percentage. Moreover, the blood of 
mammals is less effective than that of Dendrostoma. Whereas 
the latter produces membrane formation when diluted 100 to 
1,000 times, the former is effective only in 2 to 10 times dilution.! 
The experiments succeed best when the eggs are taken fresh 
from the ovary. 
2. The fact that not the eggs of every female reacted with 
foreign blood made it necessary to find methods of sensitizing 
the eggs to the effects of foreign blood. Various methods were 
tried. A rise in temperature seemed at first promising. In the 
following experiment the eggs of a female were used of which 
about 3 per cent formed membranes with ox. serum at room 
temperature. 
The eggs of this female were put in a beaker with sea-water; 
the ox serum was put in a second beaker, and both were heated 
slowly in a water bath. At certain temperatures 0.5 ¢.c. of 
sea-water+eges and 0.5 c.c. of serum were mixed in a watch 
glass, and the percentage of eggs that formed membranes 
was estimated. The result of one such experiment is given in 
Table XXXVI. 
1T. B. Robertson has recently found that ox serum acts best in a dilution of 
1:16 with sea-water. He even obtained results with a greater dilution (Robertson, 
Archiv f. Entwicklungsmechanik, XXXV, 70, 1912). Wasteneys and I did not 
notice such an effect of dilution on the egg of Arbacia, where the results agreed 
with the writer’s former observations on S. purpuratus. Robertson ascribes 
the beneficial effect of dilution to the presence of an inhibiting factor in the 
serum. 
