INTRODUCTION ik 
begin to develop, but then disintegrate unless the harmful 
secondary effects of membrane formation are counteracted 
by some further manipulation. Hence some of the sea-urchin 
eggs treated with starfish sperm behave as though membrane 
formation had only been caused by virtue of some haemolytic 
substance carried by the sperm. But if these eggs are first 
treated with starfish sperm and then, after membrane forma- 
tion, exposed for a short while to hypertonic sea-water, they 
can develop into larvae. The other sea-urchin eggs, however, 
that formed membranes with starfish sperm developed into 
larvae without the necessity for any further exposure to hyper- 
tonic sea-water or to lack of oxygen. 
These facts are intelligible on the following assumption. 
The spermatozoon causes membrane formation by a substance 
which is comparatively soluble in the egg; in addition to this 
membrane-forming substance the spermatozoon carries a 
second substance into the egg which prevents the disintegration 
which follows after mere membrane formation. Those eggs 
of the sea-urchin which upon contact with the sperm of the 
starfish formed membranes but then began to disintegrate 
absorbed only this membrane-forming substance; while into 
those eggs which formed a membrane and developed into larvae 
the whole spermatozoon had entered. 
These observations upon the effect of starfish sperm on the 
sea-urchin egg lead to the interesting problem of why the treat- 
ment of sea-urchin eggs with their own sperm never causes 
membrane formation alone (without complete development 
following). For when sea-urchin eggs are fertilized with sea- 
urchin sperm, all the eggs that form membranes invariably 
develop, and it never happens that some begin to develop and 
afterward decompose. The answer is that the “lysins” of for- 
eign sperm can penetrate into the egg in two ways, by diffusion 
or by being carried by a spermatozoon; the “‘lysins” of the 
spermatozoa of the same species, however, never can get into 
