used by the yolk-bag. Over the whole are
scattered a few patches of colour, in the shape of spidery lines and
blotches, as yet only just dense enough to attract attention.

At six days old, as you will see (fig, 1, C), he has grown darker, and
has developed a mouth and a tiny pair of breast-fins; but beautiful he
certainly is not, judged by human standards of beauty. It often happens,
however, that the outward mark of ugliness is but the sign of hidden
peculiarities of unusual interest. Up to this point this baby sole is
very like any other fish-baby; but from now onwards it enters on a most
remarkable career. At six days old he shows all the promise of a
well-grown fish; that is to say, his body is round and tapering, he has
an eye in each side of his head, and both sides of the body are alike in
colour--in other words, he is symmetrical.

The beginning of the change (fig. 1, D) is indicated by a disposition of
the growing fish to lie on one side--the left--and at the same time the
left eye begins to change its position, moving from the side of the head
towards the crown of it! In a short time this point is reached, and
passed, and not until the left eye has approached its fellow of the
right side fairly closely does its progress stop! By this time the habit
of lying on one side has become fixed, and the body has taken the
characteristic shape of the sole. Thus, then, what appear to be the
upper and under surfaces of the sole, are really the right and left
sides, and this can easily be proved by a careful examination of the
body, which, if it be placed on edge will be found to have a back or
dorsal fin, and a pair of breast fins--one on either side, as in
ordinary 'round' fishes.

[Illustration: Fig. 1.--Egg of Sole, and Stages in its Growth.]

The difference in the colouration of these two sides is a matter to
which we must now refer. As everybody knows, the upper side is
dark-coloured, while the under side is white. Why is this? Why are not
the colours reversed, or why are not both sides coloured? These
questions open up a most fascinating study--the use and meaning of the
colours of animals. And you will find, when you come to look into the
matter, that there is a very close relation between the colour of an
animal and the nature of its surroundings. In the case of the sole, the
brown upper surface, from its resemblance to the mud and sand at the
bottom of the sea, serves to conceal it from the sharp eyes of prow