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Show 690 MISS B. LINDSAY ON THE AVIAN STERNUM . [June 16, above-named observers, owing to some discrepancies in their statements as to the age of embryos. Gotte describes the establishment of coracoid, clavicle, and sternum in chick embryos of the age of 4-5 days, and yet considers that his embryos were slightly older than Fig. I. A. Sternum of'adult G-uillemot (TJria troilc). B. Sternum of Ostrich embryo, consisting, before fusion of the lateral halves, of costal sternum only. C. Sternum of Chick embryo, consisting, before fusion, of both costal sternum and metasternum. c.s, costal sternum, extending from the attachment of the coracoid to that of the last sternal rib; m.s, metasternum, extending posteriorly from the last sternal rib ' ; m.f, median furcular apophysis ; m.a, median anterior sternal apophysis; a.l.p, anterior lateral process of the sternum; p.l.p, posterior lateral process of the sternum. Rathke's. These descriptions do not correspond with the usually received opinion that the histological differentiation of the mesoblast takes place during the fifth day of incubation in the chick 2; still less 1 This region does not exactly correspond with L'Herminier s metasternum, for that bears some of the posterior ribs, and therefore includes part of the costal sternum. It corresponds, however, with the region occupied by Parker's; Lophosteon -f- Metosteon + Urosteon, in other words its extent is not only posterior but also median. In short, the term metasternum is used here to express rather its development in time than its position in place. 2 ' Elements of Embryology,' Foster and Balfour, 2nd ed. p. 270. |