226 ARTIFICIAL PARTHENOGENESIS AND FERTILIZATION 
fluids mixed with the sperm. Control experiments, however, 
showed that this was not the case with the eggs of S. purpuratus. 
In one experiment, all the eggs of a female purpuratus formed 
membranes when placed in a mixture of 50 c.c. of sea-water+- 
0.6¢.c. N/10 NaOH, to which was added only a few drops 
of living Asterias sperm. However, only a few of these eggs 
(only a fraction of 1 per cent) developed to larvae; the others 
disintegrated in the manner characteristic of eggs which have 
not been treated with a hypertonic solution subsequently to 
artificial membrane formation. This membrane formation, 
however, was brought about in this case through the living. 
spermatozoa and not by the body fluids of the starfish; this, 
point, which is important, was established by adding to the 
same eggs sperm from the same Asterzxas male after heating it 
to 50°, in which case no egg formed a membrane. Even on 
addition of ten times the amount of (dead) sperm which was 
sufficient in the living condition to cause all the eggs to form 
membranes, membranes were formed by only a few of the 
eggs, even after a duration of several hours and when the eggs 
were thoroughly shaken and mixed with the sperm. Hence 
the membranes in the above-mentioned experiments were 
formed by the living spermatozoa and not by any admixture 
with the sperm. 
While all the eggs of S. purpuratus form membranes under 
the influence of living sperm within an hour, only a fraction of 
the eggs (which varies in different cases) develops into larvae. 
The rest of the eggs behave as if the sperm had only brought 
about the artificial membrane formation by the giving-off to 
the eggs of a substance which acts like butyric acid. The 
explanation of this result is probably as follows. The sperm of 
the starfish penetrates only slowly into the sea-urchin egg; the 
starfish spermatozoon lingers for a long time in contact with 
the protoplasm of the sea-urchin egg before it gets into the in- 
terior. The time during which the spermatozoon lingers in 
