IX 



DEVELOPMENT 



455 



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 {br.f) 

 are so long as to project through the external branchial 

 apertures and the spiracle 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 



Fig. I20. — A, embryo o^ Scyllimit with yolk-sac ( x i^) ; B, under-side of head, 

 enlarged. b7; 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. hr. ap. e.vternal branchial apertures ; vith. mouth ; iia. nostrils ; 

 pet. /. pectoral fin ; pv.f. pelvic fin ; sf. yolk-stalk ; v.f. ventral fin ; yk. s. 

 yolk-sac. (From Parker's i>/tp/('^_j', after Balfour, slightly altered.) 



in the viviparous forms referred to above. Besides this 

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

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

 its contents is constantly going on. By the time the young 

 fish is ready to be hatched or born, 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. 



