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INTRODUCTION 7 
stages without it being necessary to expose them in addition to 
hypertonic sea-water. At room temperature, on the other 
hand, the eggs after artificial membrane formation go to pieces 
in the course of afew hours. Hence it is the artificial membrane 
formation which starts the development, but starts at the same 
time a tendency to disintegration. The latter, however, can 
be counteracted by a short exposure of the eggs to a hypertonic 
solution; but in order to produce this effect the hypertonic 
solution must contain a sufficient quantity of free oxygen, and 
the higher the concentration of hydroxylions, within certain 
limits, the more effective is the hypertonic solution. 
In 1906 I discovered still another method of overcoming the 
injurious secondary effect connected with membrane formation, 
which consists in arresting the development of the eggs for two 
or three hours. This is effected by putting the eggs after mem- 
brane formation into sea-water from which the oxygen has been 
displaced by a current of hydrogen, or to which some KCN has 
been added. As long as the eggs are in such a solution they 
cannot develop on account of the cessation of their oxidations. 
If the eggs are replaced some two or three hours later (at 15° C.) 
in normal aerated sea-water, practically all the eggs segment 
and develop in a perfectly normal manner. Hence there must 
have occurred during that time a change in the egg which allows 
it to develop normally. 
<Further proof can be given that membrane formation is 
really the essential step in the activation of development] 
for in the eggs of many forms membrane formation is sufficient 
to allow them to develop into normal larvae, at room tempera- 
ture. I found that if membrane formation is produced in the 
eggs of a starfish, Asterina, by the use of a fatty acid, some 
of the eggs are able to develop into normal larvae; and I 
afterward found the same to be the case with Polynoe, a poly- 
chaet, while Lefevre established the same fact for the eggs of 
another marine worm, Thalassema. Now these eggs differ from 
