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MANUAL OF ZOOLOGY 



ening, which is moulded, step by step, into the form of 

 the embryo fish. The head, trunk, and tail acquire dis- 

 tinctness, and become more and more clearly separated off 

 from the bulk of the egg, the latter taking the form of a 

 yolk-sac attached by a narrow stalk to the ventral surface of 

 the embryo (Fig. 230). 



In this condition the various parts of the adult fish can 

 be recognised, but the proportions are different, and the 



Fig. 230. — A, embryo of Scyllium with yolk-sac (x 1.5); V>, under-side of head 

 enlarged. br. f t branchial filaments protruding through gill-clefts; br. f\ 

 branchial filaments projecting through spiracle; cd J, caudal fin; d f, dorsal 

 fins; e, eye; ejc.br. ap external branchial apertuies. mt/i. mouth; na, nostrils; 

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

 yolk-sac. (After Balfour, slightly altered ) 



head presents several peculiarities. The gill-filaments (br./) 

 are so long as to project through the external branchial 

 apertures and the spiracle {br. f) 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 ovi- 

 duct in the viviparous forms. Besides this mode of nutrition 

 the yolk-sac communicates with the intestine by a narrow 

 duct, through which absorption of its contents is constantly 



