318 STUDIES IN GENERAL PHYSIOLOGY 
ever, ina 6.5 per cent. solution. In fresh water the embryos 
hatch just as rapidly as in normal sea-water. The fish is 
able to live in fresh water. 
I know of no other animal which is able to stand such 
great and sudden variations in the concentration of the sea- 
water as the Fundulus embryo. The question might arise 
whether this depends upon a peculiar characteristic of the 
egg membrane or the protoplasm. It can be shown that the 
protoplasm is relatively insensitive to variations in concen- 
tration, while diffusion through the egg membrane occurs 
very promptly. In order to prove the latter I have to call 
attention to my earlier experiments on the action of KCl 
upon the Fundulus embryo.’ The addition of 3 g. of KCl 
to 100 c.c. of sea-water brought the heart of one of the older 
Fundulus embryos to a standstill in a minute. A consider- 
able amount of this salt must, therefore, diffuse through the 
egg membrane in a very short time. That the protoplasm is 
very insensitive to variations in concentration can be shown 
on the spermatozoa. I convinced myself, first of all, of the 
fact that the unfertilized Fundulus egg can form neither an 
embryo nor segment. I then introduced eggs, under bac- 
teriological precautions, into sea-water to which 5 g. of NaCl 
had been added to 100 ¢.c. When I added spermatozoa to 
so concentrated a solution, the eggs developed as in normal 
sea-water. The movements and the power of fertilization of 
the spermatozoa must, therefore, be retained in such a solu- 
tion. The spermatozoa also retain their power of fertiliza- 
tion in fresh water. TI believe also that they still penetrated 
the egg in a 13.5 per cent. solution. I neglected, however, 
to follow this experiment more accurately, and so must leave 
this fact undecided for the time being. The fact that the 
spermatozoon retains its functions in an 8.5 per cent. NaCl 
solution is sufficient, however, to show that the independence 
1 Part I, pp. 297, 298. 
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