0.3 G ANNUAL OF SCIENTIFIC DISCOVERY. 



When the yolk lias 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 some fishes even before this period, there 

 arises, in that part of the germ-disc which is first formed, a longitudi- 

 nal 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, 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 then 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 viiipams, Cottus gobio, Syngnathus). In the Playiostomes 

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

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

 with viUi. 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, which are at 

 first smooth, suddenly present on each side five (or six) fissures of equal 

 width. Between these fissures four small streaks are formed as the com- 

 mencement 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 which hang freely from the branchial fissures 

 productions of the internal leaflets of the gills, reminding us of the exter- 

 nal gills of larvae of Salamanders are especially deserving of regard. 

 Van der Hoeven's Zoology. 



NEW FACTS IN EMBRYOLOGY. 



At a late meeting of the Boston Society of Natural History, Prof. Agassiz 

 stated that Dr. Augustus Miiller had recently published a paper on the 

 Embryology of Petromyzon (the Lamprey), presenting facts hardly to be 

 credited if they had not emanated from such authority. In the family of 

 Cyclostome Fishes there have been placed two characteristic genera, viz. 

 Petromyzon and Ammocetes. From the egg of Petromyzon Miiller says he has 

 raised Ainmocetcs, and he has likewise seen the latter become a Petroinyzon. 

 It is now well established that fishes undergo a form of metamorphosis, as 



