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Show 702 MISS B. LINDSAY ON THE AVIAN STERNUM. [June 16, formation of the Chick's keel is, however, as follows -.--Stage 1: age when both sternum and clavicles are cartilaginous, they are united by a tissue, which at the age of 5 days is comparatively thin and dim, but between 4 and 5 days is dark. Stage 2 : subsequently, but still prior to the closure of the sternal halves, this tissue looks like a projection continuous with the sternum. Stage 3 : the cartilaginous borders of the episternal region of the clavicles meet to form the keel: at what date this takes place he does not make clear, but it must be either during, or prior to, the latest stage he describes, viz. the 8 days' Chick, in which he considers the median broadening of the tract of commencing ossification in the furcula to represent the ossifying interclavicle. Now in the specimen examined the said dim and thin tissue between the clavicles and sternum was never observed to present such an appearance as that described in Stage 2 ; but it was differentiated, immediately on the complete differentiation of the mesoblast, into muscular fibres, forming a tract that gradually narrowed as the clavicles closed. During the closure of the sternum it becomes reduced to a thin string; and all muscular fibres having now disappeared from it, it assumes its character as the sterno-clavicular ligament of the adult. No indication was observed tending to show that its origin differed from that of other ligaments contemporaneously established, such as those of the wing ; nor was there seen any stage in which the cartilaginous borders of the so-called episternal region of the furcula approached the keel; they were always separated from it by this tract, first consisting of embryonic muscular fibres, and afterwards of thin ligament. 8. So much with regard to the development of the keel; if we accept the view that it is an apophysis of the sternum, established for the attachment of the pectoral muscles, we have none the less, however, to examine the evidence as to the existence of an interclavicle. That the furcula presents a median prolongation, is undeniable; but the following statements on two points will show reasons for supposing it to be an outgrowth derived from the clavicles only at a late date. (1) Date of the formation of the so-called Interclavicle.-It is somewhat difficult to ascertain the exact age of Chicks later than the 5th day of incubation, because the changes they undergo are comparatively slight; but we may recall the fact that the two anterior ribs lose their attachment to the sternum about the end of the 6th day, while the occasional free rudiment of a rib eighth from the first long one of the adult is never seen later than the 8th day. In other words, the ribs at any rate have established their generic characters at this date, which renders it probable that the broadening ossification of the median region of the clavicles, described by"Gotte as established during the 8th day, is an outgrowth of the Avian furcula rather than a pre-Avian interclavicle-a view which is expressed by giving to it (as has been done throughout this paper) the name of median furcular apophysis rather than of interclavicle. (2) Position of parts in the Shoulder-girdle of the 5 days' Chick.- |