THE HERRING. 503 



they have been laid, and undergo meroblastic segmentation. 

 The eggs float, i.e., are pelagic ; while those of the herring 

 sink, i.e., are dimersal. 



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 peri- 

 blast 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 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 a while on 

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

 suspended in the water back downwards. 



Third Type of Fishes. The Herring — Clupea karengus. A 

 type of those Teleosteans 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. 



E.xternal Characters. 



The herring has the typical "fish" shape. Externally it 

 differs from the haddock in the following features : — there is 

 no barbule ; the maxilla is divided into 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 fin ; the body is 

 more compressed ; the ventral edge is covered by sharply 

 keeled bony scales. 



