’Swirski concludes that the so-called præcoracoid bar is to some extent a secondary element, and that the coracoid bar corresponds to the whole of the ventral part of the girdle of Elasmobranchii, but his investigations do not appear to me to be as complete as is desirable.
Amphibia and Amniota. The pectoral girdle contains a more or less constant series of elements throughout the Amphibia and Amniota; and the differences in structure between the shoulder girdle of these groups and that of Fishes are so great that it is only possible to make certain general statements respecting the homologies of the parts in the two sets of types.
The generally accepted view, founded on the researches of Parker, Huxley, and Gegenbaur, is to the effect that there is a primitively cartilaginous coraco-scapular plate, homologous with that in Fishes, and that the membrane bones in Fishes are represented by the clavicle and interclavicle in the Sauropsida and Mammalia, which are however usually admitted to be absent in Amphibia. These views have recently been challenged by Götte (No. [466]) and Hoffmann (No. [467]), on the ground of a series of careful embryological observations; and until the whole subject has been worked over by other observers it does not seem possible to decide satisfactorily between the conflicting views. It is on all hands admitted that the scapulo-coracoid elements of the shoulder girdle are formed as a pair of cartilaginous plates, one on each side of the body. The dorsal half of each plate becomes the scapula, which may subsequently become divided into a supra-scapula and scapula proper; while the ventral half forms the coracoid, which is not always separated from the scapula, and is usually divided into a coracoid proper, a præcoracoid, and an epicoracoid. By the conversion of parts of the primitive cartilaginous plates into membranous tissue various fenestræ may be formed in the cartilage, and the bars bounding these fenestræ both in the scapula and coracoid regions have received special names; the anterior bar of the coracoid region, forming the præcoracoid, being especially important. At the boundary between the scapula and the coracoid, on the hinder border of the plate, is placed the glenoid articular cavity to carry the head of the humerus.
The grounds of difference between Götte and Hoffmann and other anatomists concern especially the clavicle and interclavicle. The clavicle is usually regarded as a membrane bone which may become to some extent cartilaginous. By the above anatomists, and by Rathke also, it is held to be at first united with the coraco-scapular plate, of which it forms the anterior limb, free ventrally, but united dorsally with the main part of the plate; and Götte and Hoffmann hold that it is essentially a cartilage bone, which however in the majority of the Reptilia ossifies directly without passing through the condition of cartilage.
The interclavicle (episternum) is held by Götte to be developed from a paired formation at the free ventral ends of the clavicles, but he holds views which are in many respects original as to its homologies in Mammalia and Amphibia. Even if Götte’s facts are admitted, it does not appear to me necessarily to follow that his deductions are correct. The most important of these is to the effect that the dermal clavicle of Pisces has no homologue in the higher types. Granting that the clavicle in these groups is in its first stage continuous with the coraco-scapular plate, and that it may become in some forms cartilaginous before ossifying, yet it seems to me all the same quite possible that it is genetically derived from the clavicle of Pisces, but that it has to a great extent lost even in development its primitive characters, though these characters are still partially indicated in the fact that it usually ossifies very early and partially at least as a membrane bone[210].
In treating the development of the pectoral girdle systematically it will be convenient to begin with the Amniota, which may be considered to fix the nomenclature of the elements of the shoulder girdle.
Lacertilia. The shoulder girdle is formed as two membranous plates, from the dorsal part of the anterior border of each of which a bar projects (Rathke, Götte), which is free at its ventral end. This bar, which is usually (Gegenbaur, Parker) held to be independent of the remaining part of the shoulder girdle, gives rise to the clavicle and interclavicle. The scapulo-coracoid plate soon becomes cartilaginous, while at the same time the clavicular bar ossifies directly from the membranous state. The ventral ends of the two clavicular bars enlarge to form two longitudinally placed plates, which unite together and ossify as the interclavicle.
Parker gives a very different account of the interclavicle in Anguis. He states that it is formed of two pairs of bones ‘strapped on to the antero-inferior part of the præsternum,’ which subsequently unite into one.
Chelonia. The shoulder girdle of the Chelonia is formed (Rathke) of a triradiate cartilage on each side, with one dorsal and two ventral limbs. It is admitted on all hands that the dorsal limb is the scapular element, and the posterior ventral limb the coracoid; but, while the anterior ventral limb is usually held to be the præcoracoid, Götte and Hoffmann maintain that, in spite of its being formed of cartilage, it is homologous with the anterior bar of the primitive shoulder-plates of Lacertilia, and therefore the homologue of the clavicle.
Parker and Huxley (doubtfully) hold that the three anterior elements of the ventral plastron (entoplastron and epiplastra) are homologous with the interclavicle and clavicles, but considering that these plates appear to belong to a secondary system of dermal ossifications peculiar to the Chelonia, this homology does not appear to me probable.