398 



THE DOGFISH 



sphere of yolk. The edge of the blastoderm becomes 

 invaginated at one point (gastrula-stage, p. 295) and its cells 

 become differentiated into the three embryonic layers — 

 ectoderm, mesoderm, and endoderm. At the same time 

 the blastoderm extends in a peripheral direction so as 

 gradually to cover the yolk, and its middle part becomes 

 raised up into a ridge-like thickening which is moulded. 



Fjg. 107.— a, embryo of Scylliuni with yolk-sac ( x i§) : b, underside 

 of head, enlarged 



br. f, branchial filaments protruding through gill-clefts : br. /', 

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

 dorsal fins ; e, eye ; ex. br. ap, external branchial aj)ertuies ; mth, 

 mouth ; na, nostrils ; pet. /, pectoral fin ; t>v. /, pelvic fin ; st, yolk- 

 stalk ; V. f, ventral fin ; yk. s, yolk-sac. 



(After Balfour, slightly altered.) 



Step by step, into the form of the embryo fish. The head, 

 trunk, and tail acquire distinctness, and become more and 

 more clearly separated off from the bulk of the egg, the 

 latter taking the form of a yolk-sac (Fig. 107, k, 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 



