APPENDIX I 



VIVIPAROUS FISHES 



The fact that in certain species among cartilaginious fishes {i.e.., sharks 

 and rays) the whole development of the egg takes place within the body 

 of the mother, has been mentioned in the text. In these cases there is 

 a quantity of yolk in the egg, as in cases in which the eggs are laid, and 

 when the embryo is advanced in development the yolk is contained in a 

 globular bag or umbilical vesicle, attached to the embryo by a stalk. In 

 many species of sharks there is no attachment or connection between 

 the oviduct in which the embryo is contained, and the embryo or yolk- 

 sac. But in these cases the inner surface of the oviduct is furnished 

 with finger-like processes, called villi, and these secrete a nutritious liquid 

 which aids to some extent in the nourishment of the embryo. This is 

 the case in the spur-dogfish and others. In several other sharks, includ- 

 ing the large ferocious species of tropical seas, the yolk-sac, when the 

 yolk is nearly consumed, becomes attached to a certain area of the inner 

 surface of the egg-tube or oviduct, and the blood-vessels of the yolk-sac- 

 belonging to the embryo come into close proximity with the blood- 

 vessels of the adjoining surface of the oviduct. Projections and depres- 

 sions are formed on the two surfaces or membranes, maternal and 

 embryonic, and the blood-vessels of the yolk-sac take up nourishment 

 and oxygen from the blood-vessels of the oviduct. This condition forms 

 what is called a placenta, very similar to that which is formed in the 

 gestation of /naiiiiiials, or animals which have hair and which give milk. 



The smooth hound {Miisfelus vulgaris) of British seas has no pla- 

 centa, the yolk-sac remaining always free and separate from the walls 

 of the oviduct but in a closely allied species {Mustelics Icevis) occurring 

 in the Mediterranean, a well-developed yolk-sac placenta is formed. In 

 the latter case the egg is at first surrounded by a thin flexible egg-shell, 

 between which and the egg proper there is a wide space filled with 



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