558 PISCES FISHES. 



formed from an anterior cardinal from the head and a 

 posterior cardinal from the body. The posterior cardinals 

 extend along the kidneys, and are continuous with the caudal 

 vein, but the middle part of the left cardinal is obliterated. 

 The circulation of the blood seems to be helped, in some 

 fishes at least, by the respiratory movements and by the 

 muscular contractions in swimming. 



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. 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 ccelom, 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 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 



