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 mammals, or animals which have hair and which give milk. 



The smooth hound (Mustelus 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 (Mustelus 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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