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Show 694 MISS B. LINDSAY ON THE AVIAN STERNUM. [J^e 16, 8. The differentiation of the lateral processes of the •^fj™* which also are not ossified even in the adult, « teff.JblS connection with the muscles. They are prolongations,f*™*™* between the ages of seven and ten days in the <embryo, ot th e m a m of the primitive halves of the costal sternum, to which_ the lateral parts of the rectus are attached. In later stages this lateral part ot the rectus becomes very thin, and the processes are found to attorn attachment in addition to part of the obliquus externus, as is usually the case with the posterior lateral processes of the sternum in tne Carinata.. Coincidently with this change of their muscular relations, the ends of the processes develop between the 15th and 21st days a double outgrowth, resembling the end of the xiphoid process in the Chick, and apparently due in both types to the somewhat lateral direction of the strain exercised by the obliquus externus. ine attachment of this muscle becomes rather obscure m late stages, because in the adult its major part is, as stated by Bronn, aponeurotic; I find that it may, however, be still traced to the process in a bird newly hatched. . It appears, then, that in the Ostrich at any rate the posterior lateral processes consist of the ends of the primitive costal bands, preserved and prolonged for the attachment of the lateral part ol the rectus, and subsequently modified to afford attachment to the obliquus externus. Their change of use is due to the reduction that takes place, during the establishment of the Avian type, ot the rectus, which muscle finds its maximum development in Reptiles. The primitive costal origin of these processes finds a parallel among the Carinata} in the case of the Gannet: it is interesting, too, to compare them with the costal processes of the sternum in Iguana, formed by the fusion of the sternal portions of posterior ribs. These processes, as existing in the adult, may be considered a part of the metasternum, since the primitive part bears but a small proportion to the later additions. (ii.) THE GUILLEMOT. (13 specimens, from about 17 days' to 6 days' incubation.) This type exhibits two marked peculiarities: (a) the great development of the keel and median furcular apophysis; (b) the elongated condition of the sternum, which in the adult ossifies seven ribs and is very narrow. The former character, since the bird has small wings and flies but little, must be attributed to the necessity for strengthening the fore limbs in swimming; the second seems to be correlated with the general elongation and narrowing of the trunk established in connection with the habit of diving. The embryos dissected exhibited the following points which call for remark :- 1. The union of the clavicles takes place very early ; in a 6-7 days' embryo, when the sternal halves are not yet united, they are already closed, and the median furcular apophysis is mapped out in cartilage continuous with the clavicles. The embryos examined were hence unsuitable for inquiry into the origin of the latter structure, but they |