66 MARKETABLE BRITISH MARINE FISHES CHAP. 



examination shows that the roc, or hard roe as it is often called, the 

 ovary as naturalists call it, is in the form of a sac, which is smooth 

 on the outside and hollow inside. This sac has only one opening, 

 the opening by which the spawn escapes, on the lower side of 

 the fish, behind the vent. There is a roe on each side, and the 

 two open at a common opening (Fig. 32). The inner wall of 

 the sac is not smooth but thrown into a number of ridges run- 

 ning along the length of the sac. These ridges consist almost 

 entirely of eggs, and together they correspond to the ovary of 



FIG. 31. Diagram of the interior structure FIG. 32. Similar diagram of the hard 

 of the soft roe or testis in the Herring. roe or ovary of the same fish. 



a dog-fish, a skate, or a hen. The great difference is that the 

 single eggs are very much smaller, and very much more numerous. 

 The eggs burst through the surface of the ridges on the inside 

 of the roe, and so become free in its interior, and then they are 

 squeezed out through the external opening into the water. 

 Now it was mentioned above that the egg as it escaped from 

 the ovary in the dog-fish consisted only of the round yellow 

 yolk, surrounded by a thin delicate skin. The outer parts of 

 the complete egg, the white and the shell, are added in the 

 oviduct or egg-tube. In the herring the eggs which are set free 



