DE VEL OPMENT. 537 



Excretory system.- -The kidneys are very long bodies, 

 extending above the swim-bladder under the vertebral 

 column. The largest parts lie just in front of and just 

 behind the swim-bladder. From the posterior part an 

 unpaired ureter extends to the urinary aperture, before 

 reaching which it gives off a small bilobed bladder. 

 The pronephros degenerates ; the functional kidney is a 

 mesonephros. 



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 with the cavity of the 

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

 but very close to, that of the ureter. According to some 

 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, 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 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 

 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 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 



