If, however, Götte's account of the formation of the amphibian vertebræ is correct, there are serious objections to a comparison between the vertebræ of Lepidosteus and Amphibia on developmental grounds. The essential point of similarity supposed to exist between them consists in the fact that in both there is a great development of intervertebral cartilage which constricts the notochord intervertebrally, and forms the articular faces of contiguous vertebræ.
In Lepidosteus this cartilage is, as we have seen, derived from the bases of the arches; but in Amphibia it is held by Götte to be formed by a special thickening of a cellular sheath round the notochord which is probably homologous with the cartilaginous sheath of the notochord of Elasmobranchii, and therefore with part of the notochordal sheath placed within the membrana elastica externa.
If the above statements with reference to the origin of the intervertebral cartilage in the two types are true, it is clear that no homology can exist between structures so differently developed. Provisionally, therefore, we must look elsewhere than in Lepidosteus for the origin of the amphibian type of vertebræ.
The researches which we have recorded demonstrate, however, in a very conclusive manner that the vertebræ of Lepidosteus have very close affinities with those of Teleostei.
In support of this statement we may point: (1) To the structure of the sheath of the notochord; (2) to the formation of the greater part of the bodies of the vertebræ from ossification in membrane around the notochord; (3) to the early biconcave form of the vertebræ, only masked at a later period by the development of intervertebral cartilages; (4) to the character of the neural arches.
This latter feature will be made very clear if the reader will compare our figures of the sections of later vertebræ (Plate 42, fig. 78) with Götte's[523] figure of the section of the vertebra of a Pike (Plate 7, fig. 1). In Götte's figure there are shewn similar intercalated pieces of cartilage to those which we have found, and similar cartilaginous dorsal processes of the vertebræ. Thus we are justified in holding that whether or no the opisthocœlous form of the vertebræ of Lepidosteus is a commencement of a type of vertebræ inherited by the higher forms, yet in any case the vertebræ are essentially built on the type which has become inherited by the Teleostei from the bony Ganoids.
Part III.—The ribs of Fishes.
The nature and homologies of the ribs of Fishes have long been a matter of controversy; but the subject has recently been brought forward in the important memoirs of Götte[524] on the Vertebrate skeleton. The alternatives usually adopted are, roughly speaking, these:—Either the hæmal arches of the tail are homologous throughout the piscine series, while the ribs of Ganoids and Teleostei are not homologous with those of Elasmobranchii; or the ribs are homologous in all the piscine groups, and the hæmal arches in the tail are differently formed in the different types. Götte has brought forward a great body of evidence in favour of the first view; while Gegenbaur[525] may be regarded as more especially the champion of the second view.
One of us held in a recent publication[526] that the question was not yet settled, though the view that the ribs are homologous throughout the series was provisionally accepted.
It is admitted by both Gegenbaur and Götte that in Lepidosteus the ribs, in the transition from the trunk to the tail, bend inwards, and finally unite in the region of the tail to form the ventral parts of the hæmal arches, and our researches have abundantly confirmed this conclusion.