DEVELOPMENT 



471 



apparently serving for the absorption of nutriment 

 the albumen in the egg-shell in the case of Scyllium, 

 secretions of the oviduct in the viviparous forms referred 

 to on p. 469. Besides this mode of nutrition, the yolk- 

 sac communicates with the intestine by a narrow duct (st), 

 through which, as well as by means of the blood-vessels, 

 absorption of its contents is constantly going on. By 

 the time the young fish is ready to be hatched, the 

 greater part of the yolk-sac has been drawn into the 

 ccelome, a mere remnant of it still dangling from the 

 ventral surface of the body. 



FIG. 129. A, embryo of Scyllium with yolk-sac (x i) ; B, underside of head, 

 enlarged, br. /. branchial filaments protruding through gill-clefts ; br. f . 

 branchial filaments protruding through spiracle ; cd. f. caudal fin ; d. f. dorsal 

 fins ; e. eye ; ex. br. ap. external branchial apertures ; mth. mouth ; na. nostril ; 

 pet. f. pectoral fin ; pv. f. pelvic fin ; st. stalk of yolk-sac ; v. f. ventral fin ; 

 yk. s. yolk-sac. (From Parker's Biology, after Balfour slightly altered.) 



