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 there- 

 fore to afford a clear interpretation of its origin. Now the keel, ac- 

 cording to Giitte'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 occu- 

 pied solely by muscular tissue, underlying the thiimest 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 hijpothesi, 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 rectus ' 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 diiferentiated from the tlioracic region of the rectus, their estab- 

 lishment 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 at- 

 tended with some diificulty ^. Moreover, there exists in the Penguins 



^ 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 Eeptiles {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. 



^ For a full discussion of these muscles in Reptiles, see the same paper ; 

 which states that in Ptyodacfi/lus the pars abdominalis pectoralis reaches as far 

 as the third inscription of the rectus ventralis ; but that in Monitor, Lacerta, 

 and Cncmidophorus it is often found coalesced with the rectus lateralis. 



45* 



