[210] The fact of the clavicle going out of its way, so to speak, to become cartilaginous before being ossified, may perhaps be explained by supposing that its close connection with the other parts of the shoulder girdle has caused, by a kind of infection, a change in its histological characters.
[211] This process, known as the coracoid process, is held by Sabatier to be the præcoracoid; while this author also holds that the upper third of the glenoid cavity, which ossifies by a special nucleus, is the true coracoid. The absence of a præcoracoid in the Ornithodelphia is to my mind a serious difficulty in the way of Sabatier’s view.
[212] F. M. Balfour. Monograph on Elasmobranch Fishes, pp. 101-2.
[213] Both Maclise and Humphry (Journal of Anat. and Phys., Vol. V.) had previously suggested that the paired fins were related to the unpaired fins.
[214] Davidoff in a Memoir (No. [477]) which forms an important contribution to our knowledge of the structure of the pelvic fins has attempted from his observations to deduce certain arguments against the lateral fin theory of the limbs. His main argument is based on the fact that a variable but often considerable number of the spinal nerves in front of the pelvic fin are united, by a longitudinal commissure, with the true plexus of the nerves supplying the fin. From this he concludes that the pelvic fin has shifted its position, and that it may once therefore have been situated close behind the visceral arches. If this is the strongest argument which can be brought against the theory advocated in the text, there is I trust a considerable chance of its being generally accepted. For even granting that Davidoff’s deduction from the character of the pelvic plexus is correct, there is, so far as I see, no reason in the nature of the lateral fin theory why the pelvic fins should not have shifted, and on the other hand the longitudinal cord connecting some of the spinal nerves in front of the pelvic fin may have another explanation. It might for instance be a remnant of the time when the pelvic fin had a more elongated form than at present, and accordingly extended further forwards.
In any case our knowledge of the nature and origin of nervous plexuses is far too imperfect to found upon their character such conclusions as those of Davidoff.
[215] Thacker more especially founds his view on the adult form of the pelvic fins in the cartilaginous Ganoids; Polyodon, in which the part which constitutes the basal plate in other forms is divided into separate segments, being mainly relied on. It is possible that the segmentation of this plate, as maintained by Gegenbaur and Davidoff, is secondary, but Thacker’s view that the segmentation is a primitive character seems to me, in the absence of definite evidence to the reverse, the more natural one.
[216] If, which I very much doubt, Gegenbaur is right in regarding certain rays found in some Elasmobranch pectoral fins as rudiments of a second set of rays on the posterior side of the metapterygium, these rays will have to be regarded as structures in the act of being evolved, and not as persisting traces of a biserial fin.
[217] Some arguments in favour of Gegenbaur’s theory adduced by Wiedersheim as a result of his researches on Protopterus are interesting. The attachment which he describes between the external gills and the pectoral girdle is no doubt remarkable, but I would suggest that the observations we have on the vascular supply of these gills demonstrate that this attachment is secondary.
[218] This seems to me clearly to follow from Götte and Strasser’s observations.