middle, and dissection shows that the spinal axis is continued backward and upward as a cartilaginous rod,[1] terminating at the upper border, just under the hinder pair of fulcra, and at the point where the filament was attached. The rays are all attached to the lower border of the spine; and there is only a lower lobe of the tail.
A similar structure exists in the tail of Amia, which Prof. Huxley gives as an example of heterocercal tail. It seems better, however, to discriminate between it and the previous stage, where the upper lobe (filament) exists, and it may, therefore, provisionally be called the masked heterocercal, or perhaps the pseudo-homocercal.
Prof. Huxley has more recently given figures and descriptions of the tail of embryo Teleosts (Gasterosteus), in which the structure is nearly identical with that of the adult Amia and Lepidosteus.[2]
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| A. Protocercal. First stage of Lepidosteus. Permanent in Amphioxus. Petromyzon. Lepidosiren, Polypterus. Also in some ancient Ganoids, as Glyptolœmus. |
| B and C. Heterocercal. In the sturgeons, and most sharks, and many mesozoic fossils. |
| D. Not represented, so far as I know, among recent or fossil foms. |
| E. Masked heterocercal. In adult Amia and Lepidosteus. In the embryo of many Teleosts. In Megalurus and some other fossils of Mesozoic and more recent epochs. |
Fig. 10.—Diagrams intended to illustrate the Correspondence of the Successive Stages of Transformation of the Tail of Lepidosteus, with the Tails of Certain Living Forms more and less generalized, and of Certain Fossils more and less ancient.
A, the first or protoceral stage, where the end of the vertebral column (Vc) is horizontal and divides the tail into upper and lower lobes nearly equal in size. B and C, the heterocercal stage, where the original tail is more or less elevated by the lower or infra-caudal lobe (IC), and becomes the filament (Fi), usually called the "upper lobe." In D the infracaudal lobe is longer than the filament, and in E the latter has wholly disappeared, and the tail assumes the last or "masked heterocercal" condition.
The same author concludes that in many adult Teleosts the posterior end of the spine is more or less strongly bent up, although the tail is outwardly nearly or quite symmetrical.
- ↑ This rod consists of the notochord, and a slender prolongation of the spinal cord, surrounded by a cartilaginous sheath.
- ↑ The writer has found the same condition in newly-hatched catfishes (Amiurus), and it has been observed in the embryo of a species of Cottus, by Mr. S. H. Gage, a student of natural history at Cornell University.
