86 MAKKKTABLE BRITISH MARINE FISHES CHAP. 



The development of the young fish in these large eggs, 

 whether inside or outside the mother, takes several months. 

 Except in a few species, none of which are common in British 

 waters, there is no difference in the course of the development 

 between the eggs which develop in the egg-tube or uterus and 

 those which develop in sea-water. The process com- 

 mences by the growth of the germ into a membrane or skin which 

 spreads round the yolk. At one point this skin is, as it were, 

 pinched up and thickened to form the commencement of the 

 young fish, which is called the embryo, and at first is very small 

 compared to the ball of yolk on which it lies. The embryo as 

 it grows and develops is nourished by the yolk, which therefore 

 gradually decreases. When the gill-slits are formed, long 

 thread-like gills develop and hang out a long way beyond the 

 slits as delicate fringes. When the embryo has acquired 

 generally the shape of the young fish it is connected with the yolk 

 by a narrow hollow stalk, and the interior of the yolk-sac com- 

 municates with the interior of the stomach. This stalk is 

 situated just behind the front pair of fins, and when the young 

 fish is hatched or born the yolk and the stalk have disappeared, 

 the last sign of them being a sort of navel or soft place in the 

 belly where the yolk-stalk was previously situated. No part of 

 the yolk is separated, it simply dwindles away as it is gradually 

 consumed to nourish the developing fish. The gradual growth 

 of the little fish in the egg of the smaller spotted dog-fish can be 

 seen through the partially transparent shell, and has been on 

 view from time to time in the aquarium of the Association at 

 Plymouth. 



The young fish when hatched or born has all the characters 

 of the adult fish, and immediately enters upon the same mode of 

 life. 



The eggs of the bony fishes consist only of the yolk con- 

 taining the germ, and surrounded by the enclosing membrane. 

 It is better to avoid calling the latter the shell, because it is 

 different in nature from the shell of the bird's egg or of that of the 

 dog-fish. The eggs of different kinds of these fishes differ in si/.e, 

 in proportional weight, some being heavier and some lighter than 

 sea-water, in the character of the membrane, in the proportion 

 which the quantity of the germinal matter bears to that of the 

 yolk, and lastly in the appearance and structure of the yolk 



