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 Myliobatis 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 
Fia. 246.—Ege of the Spotted Dog-Fish (Scyllium canicula), showing its mode of 
attachment after extrusion. (From Hertwig, after Kopsch.) 
spiracles.| One species, Pteroplatea micrura, 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 Alcock, Proc. Roy. Soc. 49, 1891, p. 359. 
