DEVP:LOPMENT 



471 



fish. The head, trunk, and tail acquire distinctness, and 

 become more and more completely separated off from the 

 bulk of the egg, the latter taking the form of a yolk-sac 

 (Fig. 129, A, yk.s) attached by a narrow stalk to the 

 ventral surface of the embryo. 



In this condition the various parts of the adult fish can 

 be recognised, but the proportions are different and the 

 head presents several peculiarities. The gill-filaments (pr.f} 



FIG. 129. A, embryo of Scyllitim with yolk-sac (xi); B, under-side of head, 

 enlarged. br. f. 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 ; with, mouth ; na. nostril ; 

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

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



are so long as to project through the external branchial 

 apertures and spiracles in the form of long threads 

 abundantly supplied with blood-vessels, and 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. 470. Besides this 

 mode of nutrition, the yolk-sac communicates with the in- 

 testine by a narrow duct (st), through which absorption of 



