1885.] MISS B. LINDSAY ON THE AVIAN STERNUM. 689 



The foregoing summary of theory with regard to tlie Avian 

 Sternum would be incomplete without reference to the recent 

 researches on the Mammalian Sternum ■wliich have been already 

 mentioned, namely those of Ruge, which established the costal origin 

 of the xiphisteravm of man ; and tliose 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 xij)histernum of 

 man ^ ; and in both it greatly weakens the argument for the inter- 

 clavicular 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, witii 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 his three typical 

 rows of ossific centres v^ith the confidence which at first siijht 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 here- 

 after. 



Secondly, with regard to Harting's theory, several objections 

 present themselves at first sight. Were the system of ligaments, 

 with the bony processes wliich 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 differen- 

 tiated, amounts to a reductio ad ahsurdum 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 of 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 hi comparing the results obtained with those of the 



^ It is possible, however, that they may be comparable to that of the Jerboa 

 or tfie Eangaroos, 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 Kuge's view of the latter be correct. 



