22 BULLETIN OL' THE UNITED STATES FISH COMMISSION. 



PREMMIIVARV NOTICE OF THE .IIORE IMPORTANT SCIENTIFIC 

 RESUI>TS ORTAIi\CD FROM A STUDY OF THE E.TIRRVOLOOV OP 

 FISHES. 



By J. A. K¥I>JBR. 



At the last meeting of the Academy of Natural Sciences of Philadel- 

 phia (April 10), Mr. Ryder remarked that, as a result of his studies 

 with the United States Fish Commission on the development of the 

 bony or Teleostean fishes, he had learned that they differ in their 

 mode of development from all other groups of vertebrates and fish-like 

 animals, except the sturgeons, in having a cavity, the segmentation cav- 

 ity, which persists and ultimately extends around the entire yelk as a 

 paravitelline space between the epiblast and hypoblast layers of the 

 embryo ; this cavity in these fishes not being evanescent, as it appears 

 to be in the embryos of the other sub-classes. The paravitelline space 

 does not wholly disappear in the young fish until as much as two weeks 

 after it leaves the egg. The segmentation cavity probably does not 

 l)ersist as long in fish embryos with a vitelline system of vessels as in 

 the pike and stickleback for example. 



An annulus or thickened ring of cells all round the edge of the blas- 

 toderm, continuous at one point with the tail of. the embryo, limits the 

 cavity alluded to above ; the ring of cells the speaker called the peri- 

 blastodermic annulus, and is characteristic of the embryos of true fishes 

 and sturgeons. This feature characterizes these types, as far as is now 

 known, as sharply from their relatives as does the anatomy of the 

 adults. 



The cleavage of the germ disk in bony fishes and sturgeons is regu- 

 lar, which further distinguishes them from other types, but they resem- 

 ble the sharks in that the germ of the young fish is developed at the 

 edge of the disk, and not in its center as in birds and reptiles. 



A vesicle appears at the tail of the embryo when the blastoderm has 

 covered rather more than half of the vitellus. This structure, which 

 has been called Kuj)fier's vesicle, has not yet been proved to be an allan- 

 tois, as was at first supposed, but is almost certainly a result of the in- 

 vagination of the gastrula mouth or blastopore at the tail. The canal 

 passing from it may be called Kupffer's canal, and opens on the dorsal 

 face of the embryo. It may be continuous with the medullary canal. 



The true nature of the gastruLi was pointed out on homological 

 grounds. The true gastrula of Teleosts appears to originate as an in- 

 vagination at the tail of the embryo, represented by Kupfier's canal, 

 essentially the same as in AmpMoxus, and is not homologous with the 

 gastrula regarded as such by Haeckel. 



The paired fins originate from lateral folds, and the first skeletal ele- 

 ments of the breast tins in the cod are a pair of curved cartilaginous 



