RELATIVE SENSITIVENESS OF FisH Emspryos' 817 
mation of the blastoderm. Some eggs were introduced into 
the 13.5 per cent. solution one hour after fertilization; after 
five hours’ cleavage had gone on in these eggs to about the 
thirty-two-cell stage. Half of these eggs were then returned 
to normal sea-water, while the remainder were left in the 
concentrated solutions. Three hours later the latter had lost 
their power of development, with the exception of three 
among hundreds of specimens. The other portion of the 
eggs, after having remained for eighteen hours in normal sea- 
water, were returned again to the 13.5 per cent. NaCl solu- 
tion. A large percentage of these eggs formed embryos 
which remained alive in this concentrated solution for more 
than a week. 
A remarkable phenomenon was observed in the embryos 
which developed in the very concentrated solutions; namely, 
that the maximum concentration of the sea-water in which the 
embryo is able to develop completely is much higher than the 
maximum of the solution in which the fully developed embryo 
is able to hatch from the egg. When normal Fundulus em- 
bryos are introduced in the second week of their development, 
into sea-water to which 10 per cent. NaCl was added they con- 
tinue their development, but they do not hatch, but die 
within the egg. This phenomenon shows itself still more defi- 
nitely when more dilute solutions are used. If 5 g. of NaCl 
are added to 100 c.c. of sea-water, the embryos develop in a 
normal way in this solution, and remain alive for more than 
five weeks without hatching. If, however, the eggs are intro- 
duced into normal sea-water as soon as the embryo is fully 
developed (after about two or three weeks), the embryo 
hatches in one or two days. This fact may be connected in 
some way with the fact that the fish which has just hatched 
is more sensitive to the concentrated sea-water than the two- 
day-old embryo. The former dies in a 13.5 per cent. NaCl 
solution in less than twenty-four hours. It can live, how- 
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