FISHES 



425 



Fig. 952. — Floating Egg of 

 Sea- Bass {Serrafius atrarius\ 

 enlarged. M, Enclosing mem- 

 brane ; Etnh, embryonic patch; 

 o.g,, oil -globule imbedded in 

 yolk [yk). 



and attached to its under surface will be seen a sort of bag (yolk 

 sac) in which the yolk is contained, serving as a supply of food 

 until hatching takes place (fig. 951). 



Ordinary Bony Fishes (Teleostei). — A few members of this 

 order are viviparous, but the large majority lay eggs, which are of 

 much smaller size than those of Sharks and 

 Rays, varying from -^ to f of an inch in 

 diameter. They are produced in correspond- 

 ingly large numbers, this fecundity being an 

 important means of protecting the species, 

 since but a very small percentage develop 

 into adults. Gunther says, in The Study of 

 Fishes, " The small-sized roe in the Herring, 

 Lump-fish, Halibut, and Cod-fish have been 

 estimated at, respectively, 25,000, 155,000, 

 3,500,000, and 9,344,000". In individual Turbots, weighing 18, 

 21, and 23 lbs., the number of eggs respectively produced has 

 been estimated at 5,612,000, 10,114,000, and 14,311,000. There 

 is usually no special provision for the protection of the eggs, 

 which in those forms which live in the open sea are usually found 

 floating on the surface (fig. 952). Although so 

 small, the eggs contain a large proportion of 

 food-yolk, and cleavage is consequently partial 

 (see p. 346), the body of the embryo develop- 

 ing from a small patch situated at one end. 



It is clearly advantageous that the patch 

 which becomes the embryo should be below, 

 and the &<g^ naturally floats with this part 

 downwards, the lighter yolk being on top, and 

 in many cases containing an oil-drop which 

 ensures the maintenance of this favourable 

 position (fig. 952). 



In some marine fishes, notably in the Herring {Clupeus 

 karejtgus), the eggs do not float, but are sticky, and adhere to 

 stones and other substances on the sea-bottom. Some of the 

 fishes known as Blennies {BleniidcB), several species of which are 

 common on our shores, lay their eggs in little horny capsules, 

 which are attached to rocks (fig. 953). One of our native species, 

 the Viviparous Blenny [Zoarces viviparus), is, as its name indicates, 

 exceptional in the nature of its life-history. It is a comparatively 



VOL. III. 90 



^■'^'"iimw^ 



Fig, 953. — Embryo of a 

 Blenny [Blennius] enclosed in 

 its Capsule, much enlarged. 



yk, yolk. 



