XIII PHYLUM CHORDATA 241 



union of the two folds a cavity is enclosed, which is the lumen 

 of the ovary. The ovidxict is developed as a backward continuation 

 of these folds of peritoneum, and appears to be quite unconnected 

 with the embryonic renal system, and therefore not to be 

 homologous with the oviducts of Elasniobranchs and Holocephali, 

 which, as we have seen, are Miillerian ducts. In tlie Salmonidse 

 and the Eels oviducts are absent, and the ova are discharged 

 by genital pores, which are probably to be looked upon as 

 degenerate oviducts. True abdominal pores are present in 

 Ganoids and in some Physostomi. Most Teleostomi are dioecious, 

 but Serrarms, one of the Perch family^ is hermaphrodite and 

 self-impregnating ; Ghrysophrys is hermaphrodite and successively 

 male and female ; and there are many well-known species, such 

 as the Cod and the Herring, which exhibit the hermaphrodite 

 condition as an occasional variation. 



Reproduction and Development. — Most Teleostomi are 

 oviparous, the eggs being impregnated after they are laid, but 

 in some Teleostei, such as the Viviparous Blenny {Zoarces), internal 

 impregnation takes place ; tlie young are developed in the hollow 

 ovary and are brought forth alive. Many instances of parental 

 care of the young are known, the most familiar being that of the 

 male Stickleback (Gasterosteus), which constructs a nest of weeds, 

 fastened together by a glutinous secretion of the kidneys, and 

 jealously guards the developing young. In the Sea-horse {Hippo- 

 campus) and the Pipe-fish {Syngnathus) the young are developed 

 in a pouch (Fig. 881, hrd. p.) on the abdomen of the male. lu 

 the Siluroid Aspreclo the eggs are pressed into the soft spongy 

 skin of the belly and thus carried about by the parent. The 

 ova are always small as compared with those of Elasmobranchs, 

 never exceeding 5 to 10 mm. in diameter, and being usually 

 much smaller. They are rarely pro- 

 tected by an egg-shell. They are 

 produced in immense numbers, a 

 ■ single female sometimes laying several 

 millions : in such cases the mortality 

 among the unprotected embryos and 

 young is immense. The eggs may be 

 pelagic, i.e. so light as to float when laid, 

 as in the Cod, Haddock, Turbot, Sole, 

 &c. ; or demersal, i.e. so heavy as to sink 

 to the bottom, as in the Herring, Sal- 

 mon, Trout, &C. In some cases {Ghilo- Fig. SOS.— segmentation in tepi- 



hranchus) they become cemented to the parto"*' 

 surface of a rock. 



In all the Ganoids hitherto investigated (Polypterus, Lepidosteus, 

 Amia, and Acipenser), segmentation is complete, but very 

 unequal (Fig. 898) : the megaraeres are immense as compared 



