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Show 1885.] MISS B. LINDSAY ON THE AVIAN STERNUM. 695 gave ample opportunity for investigating the formation of the keel, which attains a maximum development in this bird, and ought therefore to afford a clear interpretation of its origin. Now the keel, according to Gotte's theory, is formed by the posterior growth of the clavicles in the median line; this growth ought, then, to be traced in stages subsequent to the one described, in which the clavicles are already united, but in which the median line posterior to them is occupied solely by muscular tissue, underlying the thinnest possible tract of skin. Such, however, is not the case; the median part of the furcula never exhibits any further posterior growth than in the six days' embryo, where it has the same relative extent as in the adult. The sterno-clavicular ligament, too, which in the course of the process of growth supposed should be formed before the more posterior keel, is entirely absent in the 6 days' embryo, but appears in an embryo of about 8-9 days, where the keel is fully formed; moreover, far from exhibiting the retrogression with advancing age necessary ex hypothesi, since it is supposed to have taken place in the course of phylogenetic development, this ligament is very fine at first, and augments in strength as it approaches the adult stage. Stages were not obtained which showed the gradual development o the keel as in the Chick. In consequence of the early closure of the clavicles, which, as stated above, takes place before the sternal halves are fused, the rectus is attached, at this stage, to the clavicle, a condition to be contrasted with that which occurs in the Chick (v. infra). 2. The rectus, in stages considerably later, presents a remarkable complexity. In the embryo of some 12-16 days it is divided into three parts, described below :- A. Outer sheet, very thiu and continuous with the posterior part of the pectoralis major. This condition of the muscles named was observed in several of the Carinate types examined, while in the case of the Chick something similar occurs at a much earlier stage ; the outer part of the rectus1 described becomes gradually atrophied until the pectoralis major acquires its usual independent character. The above facts suggest that the pectoral muscles have been phylogeneti-cally differentiated from the thoracic region of the rectus, their establishment taking place in connection with that of the sternum, which breaks up the uniformity of the latter muscle, primitively continuous from pubis to jaw. This conclusion is to some extent borne out by the condition observed in certain reptiles, where the distinction between the pars abdominalis pectoralis and the rectus lateralis has been attended with some difficulty2. Moreover, there exists in the Penguins 1 This part of the muscle thins away posteriorly, and cannot be traced to the pubis ; but we are not therefore entitled to deny it the character of rectus; for cf. the condition in some Reptiles {Monitor and Lacerta), where in the subcu-taneous part of the muscle some of its fibres become attached to scales. Vide account of the Abdominal Muscles of Reptiles, by H. Gadow, Ph.D. 2 For a full discussion of these muscles in Reptiles, see the same paper; which states that in Ptyodactylus the pars abdominalis pectoralis reaches as far as the third inscription of the rectus ventralis; but that in Monitor, Lacerta, and Cnemidophorus it is often found coalesced with the rectus lateralis. 45* |
