SEA-FISHERIES LABORATORY. 123 
different in all probability. But it would, perhaps, be difficult 
to account for the rather rich fat-contents of Copepods merely 
by the fat contained in Diatoms and Algae, and we seem to 
be compelled to postulate a synthesis of fat from either the 
carbohydrates or proteids of the latter organisms. 
The seasonal metabolic phases. 
_In the case of the herring, that is, in the case of the in- 
dividuals of a differential race, or “elementary species”’ of 
herring, the phases are to be arranged into two categories 
(1) those which make up the annual reproductive cycle, and 
(2) those which are to be related particularly to the annual 
_ wave of sea-temperature. In reproduction the gonads (ovaries 
or testes, that is, ““ hard and soft roes’’) gradually enlarge. 
First there is rapid proliferation from the germinal epithelium, 
_ then growth in size of the eggs, or multiplication with diminution 
in size of sperm mother-cells. The ova remain opaque for 
_ a considerable time, then acquire their fat globules, and absorb 
water, becoming glassy clear. Then they dehisce from the 
germinal epithelium, lie freely in the cavity of the ovarian sac, 
and are finally extruded. The ovary then shrinks up and be- 
comes very much smaller than it was before spawning. 
Corresponding changes occur in the testes. Both organs 
are about the same size, and have very much the same ratio 
of weight to total body weight. In most other fishes the 
mature testes are smaller than the mature ovaries, and in fishes 
like the sole very considerably smaller. 
In the case of the herring (but in that of no other British 
marine food fish) spawning may occur at any time during the 
_ year. Most British caught herrings spawn during the Summer 
and Autumn months, but a large fraction spawn during the 
late Autumn and early Winter, while those caught off the north 
west coasts of Scotland spawn in the early months of the year. 
_ There is little spawning during the late Winter and Spring 
