434 FISHES CHAP. 



but in viviparous forms, whose embryonic development is 

 completed within special uterine dilatations of the oviducts, 

 additional means of nutrition are provided for the young. Such 

 Elasmobranchs as Spinax, Acanthias, Centrina, Scymnus, Trygon, 

 Torpedo, and Myliohatis have long filaments (villi or trophone- 

 mata) developed from the inner surface of the uterus, which 

 secrete a nutritive fluid, and this fluid is either absorbed by the 

 blood-vessels of the embryonic yolk-sac, or it is taken up by 

 the embryo in some more direct manner. In some of the 

 Trygonidae and Myliobatidae of the Indian Ocean it seems prob- 

 able that the secretion is taken into the alimentary canal of 

 the embryo either through the mouth or through the open 



Fig. 246. — Egg of the Spotted Dog-Fish {Scyllium canimla), showing its mode of 

 attacliuieut after extrusion. (Prom Hertwig, after Kopsch. ) 



spiracles.^ One species, Pteroplatect mierura, has its long and 

 highly vascular and glandular trophonemata gathered into two 

 bundles, which are thrust through the huge spiracles into the 

 pharynx of the embryos, of which there may be from one to three, 

 and the nutritive secretion is apparently digested in the 

 alimentary canal of the embryo and absorbed by the foetal 

 blood-vessels (Fig. 247). A few Sharks, like most species of 

 Mustelus, develop a placenta when the food-yolk in the yolk- 

 sac of the embryo is nearly used up. Folds or projections from 

 the highly vascular wall of the yolk-sac interlock with. similar 

 vascular folds of the lining membrane of the uterus, and a 

 diffusion of nutrient material takes place from the maternal 

 blood in the uterine blood-vessels to the foetal blood in the 



1 Wood-Mason and Alcook, Proc. Roy. Soc. 49, 1891, p. 359. 



