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CHAPTER V 



DEVELOPMEXT AND GROWTH. 



The eggs of fishes do not develop unless acted upon by the milt of the male. In all 

 species of marine bony fishes, with very few exceptions, the eggs do not come into 

 contact with the milt until after they have been discharged from the body ot the 

 female; the whole process of development takes place outside the body of the parent in 

 the water of the sea. Wlien the egg of a bird is laid it requires only to be kept at a 

 certain constant temperature to develop into a chick. When the egg of a snake or lizard 

 is laid it develops at the natural temperature of the air without any additional warmth 

 derived from the mother's body, and after a certain time a young snake or lizard is 

 hatched from it. When the egg, or "purse," of the skate is laid, or taken from the 

 body of the mother, it develops into a young skate as it lies at the bottoiii of the sea, 

 or in an aquarium tank. In the spiny dog-fish {Acanthias vulgaris), the eggs develop 

 within the oviducts of the female, and do not escape tiU they have reached the 

 condition of actively moving young dog-fish, in all respects except size resembling 

 their parents. But in all these cases the egg has been acted upon by the milt of the 

 male while still within the body of the female. When ripe eggs are pressed 

 from the ovary of the female sole into clean sea water they do not develop into 

 young fishes, but after floating for a few days in the condition previously described as 

 that of the unfertilised ovum they die, sink to the bottom, and decompose. This 

 proves that the eggs of the sole are not fertilised or acted upon by the male repro- 

 ductive elements within the body of the mother, but only after extrusion. 



I have not been able hitherto to observe the natural process of shedding and 

 fertilising the eggs of the sole in living specimens in our tanks, and it is of course 

 impossible to observe the process in living soles in the sea. It is kno^vn that in 

 pelagic fish, like the herring, the fish spawn while collected in crowded shoals, 

 males and females being mingled together, and that the females simply shed their eggs, 

 and the males their milt into the water near the bottom simultaneously. The water 

 into which the eggs pass is thus teeming with spermatozoa, and none of them 

 can escape fertilisation. But in the herring the testes are as large as the ovaries, 

 and the ([uantity of milt produced by a single fish is very large. In most flat-fishes 



