42 MARVELS OF FISH LIFE 
bottom. At first the yolk sac fills up most of the egg 
case, but as time goes on the embryo ray absorbing the 
food from the sae grows in size, and the sac diminishes 
in proportion. 
In the photograph of embryo thornbacks removed 
from the egg case, the growth of the young fish at the 
expense of the yolk sac is well illustrated. 
As soon as a fish hatches, it enters upon the larval 
stage, and, as will be seen later, the appearance of the 
larve of bony fishes, such as the salmon, roach and 
plaice, is totally unlike the parent fish, and the yolk 
sac is only partially absorbed when they leave the egg. 
The ray, however, when he emerges from the egg case 
is a perfectly formed fish, and very little remains of the 
sac. The back is already coloured and marked so as 
to render him inconspicuous on the bottom, and protect 
him while he is still of tender age. 
The under-surface is colourless, for lying on the 
bottom there is no need for this part to be concealed. 
The yolk sac has practically gone, the fins are as they 
will appear in after life, and the mouth being open, the 
young ray is able to feed at once. The gill slits are seen 
on each side of the middle line, between the mouth and 
the remains of the yolk sac. At fourteen days old the 
black spots seen on the back of the newly hatched ray 
blend together, and give it a more uniform shade. 
The top photograph of the second plate shows a 
ray three weeks to a month old, and it will be seen that 
there are several rings round the edge of the large fins. 
