EFFECT OF FOREIGN BLoopD AND CELL Extracts 197 
Arbacia.! It was found that if the eggs were sensitized against 
ox serum they are sensitized against other foreign blood and 
tissue extracts. 
The question is: How does SrCl. (or BaCl,) cause these sensi- 
tizing effects? This is possibly answered by the observation 
that if the unfertilized eggs of purpwratus remain permanently 
in the SrCl, solution, they will ultimately form membranes 
without requiring any further treatment. The time required 
for this effect differs for the eggs of different females. If eggs 
which have formed a membrane upon treatment with SrCl, 
(or BaCl,) are subsequently exposed to a hypertonic solution 
they will develop into larvae. The fact that the SrCl, alone 
can cause membrane formation if the eggs are exposed to it 
for a sufficiently long time suggests that the sensitivation con- 
sists in a modification of the cortical layer of the egg of a char- 
acter similar to that which leads to membrane formation. SrCl, 
thus facilitates the subsequent membrane formation by serum. 
The following facts are of interest. I had already noticed 
in my experiments on heterogeneous hybridization that the eggs 
of the sea-urchin can be fertilized in larger numbers with the 
sperm of A sterias than with the sperm of Pycnopodia or Asterina. 
It was found that the extracts of the coecum of these three 
species of starfish showed the same relative difference in their 
power of causing membrane formations in the unfertilized 
ege of S. purpuratus that had previously been sensitized with 
SrCl,. This agrees with the fact, which we shall prove in the 
next chapter, that the membrane formation by the spermato- 
zoon is caused also by a cytolytic agent—a lysin; and that the 
lysins contained in the coecum show the same relative efficiency 
as the lysins contained in the spermatozoa of the three species. 
It may be of interest that the extracts of all kinds of starfish 
cells, even of the eggs, were able to bring about the membrane 
formation in the sea-urchin egg. 
1 Loeb and Wasteneys, Science, XXXVI, 255, 1912. 
