328 A HISTORY OF FISHES 



(Sebastes), a member of the family Scorpaenidae, may have as 

 many as one thousand embryos in each ovary. The eggs of 

 the Eel Pout hatch in about twenty days, but the young do 

 not leave the body of the mother until some four months after 

 fertilisation: they are then about one and a half inches in length, 

 and, externally at least, closely resemble their parents. 



The accompanying figures (Fig. 1 1 7a) have been selected to 

 show three of the main stages through which the developing 

 embryo passes in a typical Selachian, and to a large extent 

 explain themselves. As in any other vertebrate, the head of 

 the early embryo is very large in proportion to the body, and is 

 bent downwards at an angle, forming the cranial or cephaHc 

 flexure (I). As development proceeds, this angle becomes less 

 and less marked (II). The median fins appear at an early 

 stage, and take the form of a continuous membranous fold 

 surrounding the trunk, which later becomes spHt up into the 



Fig. ii8. 

 Viviparous Blenny or Eel Pout {Zoarces viviparus), X |. 



dorsals, anal, and caudal of the mature fish {cf. p. 55). Con- 

 currently with this differentiation of the median fins, the first 

 rudiments of the pectoral fins make their appearance, and soon 

 afterwards the pelvics also appear. At the same time, the head 

 commences to take on the form of that of the adult, accompanied 

 by marked changes in the form of the mouth and gills, and 

 the lateral line appears. An interesting feature of Selachian 

 embryos is the presence of the so-called external gills {cf. p. 43), 

 long, filamentous processes developing from the walls of the 

 branchial clefts and protruding through the external gill- 

 openings (II) . Their function appears to be twofold : they assist 

 the embryo to breathe, and may also aid in the absorption of 

 nutriment. Having served their purpose, they completely 

 disappear. The last figure shows a final larval stage, in which 

 all the external features of the mature fish are recognisable 

 and only the pendent yolk-sac remains as a legacy from the 

 embryonic life (III). 



