438 FISHES. 



authorities the genital canals in Teleosteans are secondary 

 structures, unconnected with the archinephric or segmental 

 ducts, but the researches of Jungersen have made this very 

 doubtful. 



Development. — The ova of the haddock, hke those of other 

 Teleosteans, contain a considerable quantity of yolk, are 

 fertilised after they have been laid, and undergo meroblastic 

 segmentation. 



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 vitelline membrane. After fertilisation, the formative disc divides 

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

 derm. 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 hypoblast or 

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

 aids the mesoblast which appears between epiblast and hypoblast ; the 

 medullary canal is formed as usual in the dorsal epiblast. It is likely 

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

 gastrula, much disguised by th^cresence of yolk. 



The newly hatched larva iflUl mouthless, and lives for a while on 

 the residue of yolk, which by ^^^oyancy causes the young fish to be 

 suspended in the water back do^Hvards. 



The Herring — Clupea haretlfus. A type of those Teleo- 

 steans which have the swim-bladder communicating 

 with the gut (Physostomi). 



In habit the herring is pelagic and gregarious. It is found 

 in the North Sea, the temperate and colder parts of the 

 Atlantic, the Baltic, and the White Sea. A similar species 

 lives in the N. Pacific. 



External Characters. — The herring has the typical " fish " 

 shape. Externally it differs from the haddock in the fol- 

 lowing features :— there is no barbule ; the maxilla is divided 

 irito three parts ; the nostrils have a single aperture on each 

 side ; there is no lateral line ; the pelvic fins are abdominal, 

 not jugular in position ; there is one dorsal and one anal 



