40 CLASS XIV. 



the males are attracted by the olfactory sense, by which they collect 

 around the females of their species at the season of spawning and 

 come into their neighbourhood. Amongst those which copulate, 

 some species are also oviparous. Only few bony fishes are vivi- 

 parous, as, for instance, Anableps, Blennius viviparus. Of the carti- 

 laginous fishes, on the contrary, amongst the Plagiostomes the Rajce, 

 in a proper sense, and the species of sharks of the genus ScylUum 

 alone, are oviparous. The egg in these rays and sharks has a tough, 

 horny, flat, elongated shell, of which the four corners terminate in 

 the rays in long tortuous filaments, wound round each other. 



The development of fishes is distinguished from that of the 

 scaly reptiles, the birds and mammals, in that neither amnion nor 

 allanto'is is formed. In the beginning that dividing or cleaving of 

 the yolk is perceptible, which we have already spoken of in different 

 classes of invertebrate animals 1 . When the yolk has again become 

 smooth, the germinal disc appears, and as it grows spreads itself 

 over the yolk until it quite surrounds it. After it has thus become 

 a vesicle, or in other fishes even before this period, there arises, in 

 that part of the germ-disc which is first formed, a longitudinal groove 

 as the first commencement of the embryo. Two projecting edges 

 surround this groove and approach each other, whilst at the bottom 

 of the groove the dorsal cord, as the first commencement of the 

 skeleton (comp. above, pp. 5, 8), is formed. The innermost layer of 

 the germ-membrane (the mucous layer) presents a constriction, and 

 is thus divided into a canal situated beneath the dorsal cord and into 

 a vitelline sac. In some fishes this vitelline sac is included in 

 the ventral cavity with the intestinal canal by the walls of the 

 abdomen, formed from the serous layer ; there is thus an internal 

 vitelline sac present in these, and the abdomen of the embryo 

 presents an unusual projection ( Cyprinus, Perca, Salmo) ; in others 

 the abdominal covering is drawn together by constriction like the 

 mucous layer, and the vitelline sac hangs on the outside of the 

 ventral cavity, being attached to it by a short pedicle (Blennius 



1 Observations on this subject, in eggs of fishes impregnated artificially, have been 

 published by EUSCONI, MUELLER'S Archiv, 1836, s. 278 288, Taf. xin. The eggs 

 acquire an elevation or protuberance, and this, not the entire yolk, is the seat of the 

 regularly increasing grooves. Later observations have shewn that this phenomenon 

 depends upon the development and change of the germinal vesicle. 



