DEVELOPMENT 321 



eighty millimetres in length and about one hundred and forty 

 millimetres in width, the smallest sixty-three millimetres and 

 thirty-seven millimetres respectively. They are generally de- 

 posited on muddy or sandy flats, and the more convex surface 

 is provided with a sticky substance, to which small pieces of 

 stone, shell, seaweed, etc., adhere, and thus help to anchor 

 the capsule. 



The capsules of the Chimaeras [Holocephali) are essentially 

 similar to the above, but of somewhat different form, being 

 spindle-shaped in outline and bordered by a broad fringe 

 (Fig. ii6d). In the Californian Chimaera or Spook-fish 

 {Hydrolagus) they are about six inches long, and one end is 

 produced into a lengthy "tail," which sticks into the mud of the 

 sea-floor when the tgg is deposited. The shape of the 

 Chimaeroid capsule is adapted, not to the ^gg as it exists when 

 the capsule is formed, but to the later developed embryo, and 

 the interior cavity may be divided into three distinct chambers, 

 each of which corresponds in shape and size to a definite portion 

 of the embryo fish. There is a row of small slits along each side 

 of the envelope, closed by membrane when the ^gg is first 

 extruded, but by a process of gradual weathering and decay 

 these become open at a later stage of development, thus allowing 

 currents of air to flow through the case and provide the growing 

 embryo with the necessary oxygen. The upper and lower 

 halves of the capsule are at first united by a membrane, but at 

 the time of hatching they separate at one end, leaving an 

 opening through which the fish can make its escape. 



The eggs of Bony Fishes are enclosed only in a vitelline 

 membrane, formed from the ovary itself, and varying from a 

 tough and almost leathery structure to a very fine and fragile 

 membrane. As a rule, the shape of the egg is spherical, and 

 although always provided with some yolk, is never as large as 

 that of the Selachians. Two main types of eggs may be recog- 

 nised, according to their structure and the manner in which 

 they undergo development: pelagic eggs, which are buoyant 

 and generally provided with a thin and non-adhesive membrane; 

 and demersal eggs, which are heavy and sink to the bottom, 

 and have a hard and smooth or adhesive membrane. Marine 

 fishes may produce eggs of one type or the other, but with very 

 few exceptions, the eggs of fresh- water fishes are demersal. 

 Most of our well-known food -fishes have pelagic eggs, only the 

 Herring {Cliipea harengus), Wolf-fish [Annarrhichas) , Sand Eel 

 {Ammodytes) , and a few others depositing their spawn on the 



