626 PHYLUM CHORDATA : CLASS PISCES — FISHES 



Reproductive system. — The testes are long lobed 

 organs, conspicuous in mature males at the breeding season ; 

 there is no epididymis. The ovaries of the female are 

 more compact sacs, more posterior in position. 



Two vasa deferentia combine in a single canal. The 

 likewise single oviduct is continuous w'ith the cavity of the 

 ovaries. The genital aperture in either sex is in front of, 

 but very close to, that of the ureter. The oviducts of 

 most Teleosts seem to be backward extensions of the 

 ovarian sacs, but they may be disguised Miillerian ducts. 

 In salmonids the eggs are shed into the coelom, and escape 

 by a pair of pores opening together behind the anus. 



Development. — The ova of the haddock, like those of 

 other Teleosteans, contain a considerable quantity of yolk, 

 are fertilised after they have been laid, and undergo 

 meroblastic segmentation. The eggs float, i.e. are pelagic ; 

 while those of the herring sink, i.e. are demersal. 



At one pole of a transparent sphere of yolk lies a disc of formative 

 protoplasm of a light terra-cotta colour. The ovum is surrounded by a 

 firm vitelhne membrane. After fertihsation the formative disc divides 

 first into two, then into four, then into many cells, which form the 

 blastoderm. From the edge of the blastoderm certain yolk-nuclei or 

 periblast-nuclei are formed, which afterwards have some importance. 

 At the end of segmentation the blastoderm lies in the form of a doubly 

 convex lens in a shallow concavity of the yolk. 



The blastoderm extends for some distance laterally over the yolk ; 

 the central part raises itself, and thus forms a closed segmentation 

 cavity ; one radius of the blastoderm becomes thicker than the rest, and 

 forms the first hint of the embryo ; an inward growth from the edge of 

 the blastoderm forms an invaginated layer — the dorsal endoderm or roof 

 of the gut ; the periblast forms the floor of the gut, and afterwards aids 

 the mesoblast, which appears between ectoderm and endoderm ; the 

 medullary canal is formed as usual in the dorsal ectoderm. It is Hkely 

 that the edge of the blastoderm represents the blastopore or mouth of 

 the gastrula, much disguised by the presence of yolk. 



The newly hatched larva is still mouthless, and lives for awhile on 

 the residue of yolk, which, by its buoyancy, causes the young fish to be 

 suspended in the water back downwards. 



General Notes on the Functions, Habits, and 

 Life-Histories of Fishes 



Movement. — A fish may well compare with a bird in its mastery 

 of the medium in which it lives. Thus a salmon travels at the rate of 

 about eight yards in a second, or over sixteen miles an hour. The 



