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Show 1885.] MISS B. LINDSAY ON THE AVIAN STERNUM. 689 The foregoing summary of theory with regard to the Avian Sternum would be incomplete without reference to the recent researches on the Mammalian Sternum which have been already mentioned, namely those of Ruge, which established the costal origin of the xiphisternum of man ; and those of Albrecht, which have lately added more force to Ruge's conclusions as to the costal origin of the manubrium sterni. The result in the former case precludes us from admitting that the posterior processes of the avian sternum are, as suggested by Huxley, homologous with the xiphisternum of man l ; and in both it greatly weakens the argument for the interclavicular homology of the keel, while at the same time it points out a line of research with regard to the anterior region of the avian sternum, which has been followed during the investigation hereafter described. (ii.) Certain of the theories named call for some comment. Firstly, with regard to that of L'Herminier, it is obvious that at the • date of his researches the phylogenetic value of anatomical features was but little understood ; we are therefore not bound to accept L'Herminier's own estimation of the equal value of bis three typical rows of ossific centres with the confidence which at first sight might seem due to his numerous facts and clear description of them. Nevertheless his record of the respective dates at which the various centres appear affords a valuable clue to the order of development of the respective parts, the details of which will be discussed hereafter. Secondly, with regard to Harting's theory, several objections present themselves at first sight. Were the system of ligaments, with the bony processes which afford them attachment, to be regarded as an " episternum," it would afford a unique case of a bone undergoing degradation at the centre, yet ossifying at odd points in its original periphery. Again, there is no reason to place any especial emphasis on the occurrence of the ossifications indicated, since all ligaments tend to present occasional ossifications. Further, the fact that the ligament system of the Ostrich is claimed as an episternal apparatus in which the clavicles have not been differentiated, amounts to a reductio ad absurdum of the theory ; for these ligaments, exceedingly thin in the adult, are scarcely possible to find in the embryo, in which, on Harting's hypothesis, we should expect to find them better developed. It appears, then, that the evidence for the interclavicular homology of the keel must rest on the embryological researches uf Gotte and Hoffmann. For this reason, further evidence upon the disputed point of the keel has been sought in a series of embryos of five types. All points of interest observed in these embryos are described in the ensuing section, Part II. It is to be noticed, however, that some difficulty occurs in comparing the results obtained with those of the 1 It is possible, however, that they may be comparable to that of the Jerboa or the Kangaroos, in which animals this process is probably a secondary addition to the costal sternum, or even to the xiphisternum of the Frog, which also is an appendage to a costal sternum, if Ruge's view of the latter be correct. |