FISHES. 41 



viviparus^ Cottus gohio, Syngnathus). In the Plagiostomes (sharks 

 and rays) an external vitelline sac is similarly observed, which 

 here, however, has a longer pedicle, which in some sharks is beset 

 externally with villi. In most of these fishes the umbilico-intestinal 

 duct is continued within the abdomen into a second internal vitelline 

 sac : a blind sac, which occupies a large part of the ventral cavity, 

 and is inserted into the anterior bladder-like portion of the intestinal 

 canal above the commencement of the spiral valve \ The lateral 

 walls of the body of the embryo, wliich are at first smooth, suddenly 

 present on each side 5 (or 6) fissures of equal width. Between 

 these fissures four small streaks are formed as the commencement of 

 the branchial arches. In front of the first fissure and behind the 

 mouth arises a wider arch divided by a groove into two parts. 

 The anterior half of this is changed into the under jaw and the 

 various bony pieces which unite it with the cranium. From 

 the posterior half arise the horns of the tongue-bone ; at the 

 posterior margin of these parts in bony fishes the gill-covers and 

 the branchial rays are developed at a later period only, the branchial 

 arches being at first unprotected. The unpaired fins arise at first 

 as a long fold of skin, which surrounds the body, and is much 

 more extensive than the future pinna dorsalis and analis. All the 

 bony fishes whose development has been hitherto observed, quit 

 their egg-covers at a very early period and whilst still imperfectly 

 formed. In the embryos of sharks and rays the filaments Avhich 

 hang freely from the branchial fissures, productions of the internal 

 leaflets of the gills, reminding us of the external gills of larva3 of 

 Salamanders, are especially deserving of regard '\ 



^ This vesicular part of the intestinal canal was named by Collins h%i,Tsa Entiana, 

 after George Ente, by whom it was first discovered; System of Anatomy, 1685, Tab. 

 33, fig. 2g. In Carcharias and Miistelus Icevis MuELL. an external vitelline sac alone is 

 present, and this is attached by many folds to the inner surface of the wider inferior 

 part of the oviduct named uterus. These sharks have thus a placenta, which however 

 differs from that of mammals, and is formed by the vitelline sac. Compare especially 

 J. Mueller Ueber den glatten Hai cles Aristoteles, &c. Abh. d. Akad. zu Berl. Jahrg. 

 1840, Physik-math. Kl. s. 187 — 257, with fig. 



^ On the development of fishes, amongst other works may be compared H. Rathke 

 Abhandlungen zur Bildungs- mid Entwickelungs-geschichte, ir. Leipzig, 1833, 4to, s. 

 r — 68 (on the development of Bhnnius viviparus) ; V. Baek Untersuchungen iiher 

 die Entvjiclelungs-gesckicJite der Fische, Leipzig, 1835, 4to; the same, Ueber Ent- 

 wiclcelungs-gcscliichte der Thiere, II. Konigsberg, 1837, 4to, s. 295 — 315 ; Rathke in 

 Bdbdach's Physiologie, 2te Aufl. 11. 1837, s. 276 — 296 ; C, VOGT Embryolog'ie des 



