213 
naturally exposed to many dangers. They are probably 
eaten by many pelagic animals, though there is not much 
published evidence on this point. In Loch Fyne, how- 
ever, the floating fish eggs are closely associated with great 
numbers of Copepoda, and this results in the eggs being 
eaten by the herring in the search of the latter after Cope- 
poda and other pelagic Crustacea, and pelagic fish eggs 
have been found in the stomach of that fish. It is possible, 
however, that physical events are at least as fruitful causes 
of the destruction of floating eggs and larvee as predaceous 
pelagic animals. The change from the pelagic to the 
demersal mode of living, for instance, happening when the 
larva is still in deep water. Remarkable conditions are 
present in the Baltic. Petersen* has shown that young 
Plaice (up to 2-3 inches in length) are entirely absent in 
that sea, though spawning fish are abundant, and, as 
Hensen has shown, fertilized eggs are there in enormous 
abundance. The low specific gravity of the water affords 
the explanation. Ata specific gravity of 1:0140 (at 99°C.) 
a great number of Plaice eggs sink to the bottom, and at 
a specific gravity of 1:0120 (at 10°C.) all sink. The lowest 
specific gravity at which the eggs can drift about without 
any sinking is 10152 (at 9°8°), and if in their migration 
they enter water of less than this density their destruction 
follows. Now it happens according to Hensen that about 
once every month there occurs such a low specific gravity 
of the Baltic water that all the Plaice eggs sink. There is 
of course a possibility that the eggs may go on developing 
at the bottom, but this is unlikely. It is possible too that 
a low salinity of the water may prejudicially affect the 
processes of development, but this subject has not been 
adequately investigated. It is possible also that Plaice 
eggs entering the estuaries on the Lancashire coast may 
* Rep. Danish Biological Station, 1V., 1894, p. 27. 
