402 EB. J. ALLEN. 
observations for each kind of fish lie very nearly in a straight line, which 
means that the increase in rate of development, over the range of tem- 
perature examined, is directly proportional to the increase in temperature. 
It follows, therefore, that the actual time, under natural conditions 
in the sea, which an egg takes to develop, from the time it is spawned 
until it is hatched, is by no means constant. There are, I believe, no 
actual observations on the point and direct evidence as to the time occu- 
pied would not be easy to get. If we suppose that temperature is the 
only factor that need be considered, it is possible to deduce from the 
data given by the laboratory experiments the time taken in particular 
instances. Thus, in the waters off Plymouth, Plaice spawn from Decem- 
ber to March. Taking the temperature at 9° C., which is the mean for 
February, the coldest of the four months, the Plaice egg would, according 
to Dannevig’s figures,! take thirteen days to hatch. In the southern part 
of the North Sea, on the other hand, which is a great spawning-ground for 
Plaice, the mean temperature for February is 7° C., at which temperature 
the eggs would take about sixteen days to hatch. 
When they first emerge from the egg, the young fishes are small, 
transparent larvee, whose form is very different from that of the adult 
fish, and the next points to consider are the time occupied by the period 
of transition from the larval to the adult form and the increase in size 
which accompanies this change of form. 
In treating of this period of the life-history we will consider separately 
the ordinary round fishes such as Cod, Whiting, Mackerel and Herring, 
and flat-fishes like the Plaice, the Sole and the Turbot. In the case of 
the round fishes it is a little difficult to draw a sharp line of demarcation 
between the end of the larval and the commencement of the adult life, 
since both as regards structure of body and habits of life the one passes 
very gradually into the other. 
The time occupied by the larval period may be illustrated by one or 
two examples. The principal spawning time of the Cod in the North Sea 
begins in January and is at its height in February and March. The larve 
when first hatched have a length of about 4 mm., the length after absorp- 
tion of the yolk being 4-5 mm. Ata length of 25 mm. the adult form has 
practically been reached. Little cod of this size begin to appear, often 
hiding under the bell of a jelly-fish, from the middle of May onwards, 
that is to say about four months after the beginning of the spawning in 
January. We may say therefore with some confidence that these small 
cod of 25 mm. or 1 inch in length are from three to four months old. 
Some observations on larval mackerel taken off Plymouth in the 
summer of 1914 will illustrate the method, a somewhat laborious one, it 
1 \;¢ Ls [ee * ones . : 
Cf. Apstein (1, p. 366). According to Apstein’s figures the time would be nearer 
fourteen days at 9° C. and seventeen days at 7° C. 
