198 ARTIFICIAL PARTHENOGENESIS AND FERTILIZATION 
While it was thus easy to cause the membrane formation of 
the unfertilized sea-urchin egg with blood or extracts of tissues 
from foreign species, it was almost impossible to bring about 
the membrane formation in the sea-urchin egg with extracts 
from the tissues of the sea-urchins. Only in one among many 
attempts did the writer succeed in causing membrane formation 
in the eggs of S. purpuratus by the watery extract from the 
coecum of S. franciscanus. The extract had been standing five 
days; newly prepared extract was without effect upon the same 
eggs. About 5 per cent of the eggs formed membranes. 
We may ask the question why it is that blood or the extracts 
of tissues of foreign species will readily cause membrane forma- 
tion in the unfertilized eggs of sea-urchins, while the extract 
from tissues of the sea-urchin remains ineffective. From the 
experiments with acids and alkali it became evident that a 
necessary prerequisite of the efficiency of a substance for the 
causation of membrane formation is its diffusion into the egg, 
or at least into its cortical layer. The same can be shown to 
be true for the hydrocarbons, ether, alcohols, and the other 
substances which cause membrane formation. It is therefore 
possible that the inefficiency of the blood and tissue extracts 
of the same animal and the efficiency of the blood of foreign 
species for the causation of membrane formation is due to the 
fact that the foreign lysins can diffuse into the egg while the 
lysins of the same species cannot. 
We know that the blood of all animals carries lysins which 
may cause haemolysis in foreign forms, but are harmless to 
the cells of the animal itself. What causes this immunity ? 
According to our investigations the immunity of our cells 
against the lysins contained in our blood may be due to the 
fact that the lysins of our blood cannot diffuse into our own 
cells while they may diffuse into the cells of foreign species. 
This view is supported by some data which will be given in 
the next chapter and to which we may refer briefly here. The 
