CH. IV] LIFE IN THE SEA 83 



occurred during development. The blood of the mother must 

 therefore supply some material to the embr^-o other than that 

 provided in the egg. Other British dogfishes and rays lay large 

 eggs containing a quantity of food yolk, thus the eggs of the 

 spotted-dog {Scylliiim) and those of the skates and rays are fairly 

 large and are enclosed in a leathery looking capsule, and are 

 deposited on the sea bottom. Development takes a long time, 

 perhaps the greater part of a year, and while the embryo is 

 growing inside the egg-capsule it is being nourished by the food 

 yolk originally present in the Qgg. In fact the general course of 

 development is very like that exhibited by the egg of the fowl, 

 except that the latter develops in a much shorter time, because 

 it is incubated at a temperature of about 38' C. while the egg 

 of the skate or ray is exposed to a temperature which does not 

 usually rise above 15'' C. In other respects the reproduction of 

 these fishes is curiously similar to that of the fowl, for the fish 

 probably " lays " in much the same manner and the spawning 

 period probably extends over a considerable part of the year. At 

 the end of the incubation period the little fish which is hatched 

 from the egg is a large and perfectly formed animal and one 

 Avhich has probably no difficulty in escaping from its enemies, or 

 in finding its food. 



But the eggs of the other fishes are quite different from those 

 of the skates, rays and dogfishes. Most bony fishes produce a 

 great number of ova, and these are very small, usually one to two 

 millimetres in diameter, and they are therefore provided with a 

 very small quantity of food yolk. Some, like the eggs of the 

 herring, are laid on the sea bottom (demersal eggs), but most 

 fishes shed their ova into the sea and the latter then develop 

 floating about among the plankton : these are pelagic eggs. On 

 account of the limited amount of food yolk development is a rapid 

 process and the little fish usually hatches out fi^om the egg in a 

 week or two, but is a very feeble and helpless creature. It is 

 quite unable to feed for itself, indeed the gullet is usually not an 

 open tube for some time after it is hatched. The larva, for such 

 it is, depends for its food on the remains of the yolk sac which is 

 still attached to its abdomen when it is hatched, and it is only 

 after this is absorbed that it begins to catch diatoms and copepods 



6—2 



