FECUNDATION IN FISHES. 



5D9 



420 



The structure and formation of the ovum in scaled and scuted 

 Reptiles are essentially the same as in the cartilaginous Fishes. 

 The germ-cell, with a single nucleus, is first formed in a delicate 

 ovisac imbedded in the stroma of a solid ovarium. A yolk of 

 large size is added, of which 

 the greater part consists of 

 large non-nucleated oil-vesi- 

 cles, and the smaller part 

 of the vitelline granules and 

 cells with a granulated nu- 

 cleus ; these originally sur- 

 round the germ-cell, then 

 indicate its tract from the 

 centre to the periphery of 

 the yolk, and form with the 

 matter of the germ-cell the 

 cicatricula, or blastoderm. 

 This occupies a much smaller 

 extent of the surface of the 

 yolk than in the small- 

 yolked eggs of Batrachians 

 and Osseous Fishes, and 

 segmentation is limited 



thereto, the rest of the yolk being nutritive. The ovum, con- 

 sisting of the above parts, inclosed in a vitelline membrane, quits 

 the ovary and is received into the oviduct: here it acquires a 

 certain proportion of soft albumen, upon which is condensed a 

 thin tough layer, called ' chorion.' In the ovo-viviparous Snakes 

 (Vipera) and Lizards (Zootoca) this membrane is thin: in the 

 oviparous species it acquires a crust of calcareous matter before 

 exclusion. This crust is very thin and scanty in most Serpents 

 and Lizards, but is thicker in Cliclonia and Crocodilia, forming a 

 shell. The egg is spherical in some Chelonia, spheroidal in others, 

 fig. 420, f, elliptical in Emys, fig. 399 : in the Crocodiles the egg 

 is a long ellipse, fig. 420, e. In no reptile does it show the 

 oval form which prevails in Birds, ib. C and D. 



§ 114. Fecundation in Fishes. — Certain changes and peculiar 

 phenomena attend the increase of size of the soft and hard roes 

 during these primary processes of generation. The colours of 

 the fish become more marked and brilliant: the different sexes 

 are often distinguished by peculiar tints, as the male Stickleback 

 by his bright red throat, for example. The cutaneous crest on 

 the head is developed in Salarias and many other Blennioids, e.g., 



External forms of different eggs of Reptiles and Birds. 



CCCVIII. 



