54 WORK IN THE HATCHING-HOUSE. 



hyoid arch, which gradually grows backwards over the arches 

 following, and gives rise to the operculum. There appear . . . 

 shortly before hatching double rows of papillae on the four anterior 

 arches behind the hyoid ; they are the rudiments of the branchiae. 

 They reach a considerable length before they are covered by the 

 operculum membrane. . . . The unpaired fins arise as simple 

 folds of the skin along the dorsal and ventral edges, continuous 

 with each other round the end of the tail. The ventral fold ends 

 anteriorly at the anus. The dorsal and anal fins are developed 

 from this fold by local hypertrophy. The caudal fin, however, 

 undergoes a more complicated metamorphosis. It is at first 

 symmetrical, or nearly so, on the dorsal and ventral sides of the 

 hinder end of the notochord. This symmetry is not long retained, 

 but very soon the ventral part of the fin, with its fin rays, be- 

 comes much more developed than the dorsal part, and at the 

 same time the posterior part of the notochord bends up towards 

 the dorsal side. . . . Owing to the simultaneous appearance of 

 a number of fin rays on the dorsal and ventral side of the noto- 

 chord, the external symmetry of the tail is not interfered with. 

 . . . The yolk-sac persists long after hatching, and is gradually 

 absorbed. There is ... just before hatching ... a rich vascular 

 development in the mesoblast of the yolk-sac. The blood is at 

 first contained in lacunar spaces, but subsequently it becomes con- 

 fined to definite channels." l 



In practical fish-culture there are five stages which pre- 

 eminently command attention. The first stage is sometimes 

 called the 



MULBERBY STAGE, 



and occurs about the close of the segmentation. The round disc 

 rises in all ova during the first twenty-four hours, but during the 

 period of segmentation the disc, in properly fertilised eggs, becomes 

 hard, and in unimpregnated eggs annular. 



At about the close of the period of segmentation the disc in 

 properly fertilised eggs enlarges, and has a soft appearance to the 

 eye, so that it is perfectly easy to remove all ova likely to produce 



1 Abridged from Balfour's Embryology. 



