224 ME. W. P. PYCEAET ON THE MOEPHOLOGY AND 



external lateral process at its base. The pre-coracoid is wanting. The scapula is free, 

 as in the young of the flightless members of this group. 

 The furcula is slender and U-shaped. 



The Development of the Sternum and Pectoral Arch. 



a. Sternum. 



Miss Lindsay's [48] researches have shown that during development the sternum 

 undergoes a process of shortening both anteriorly and posteriorly, at least in so far as 

 its costal elements are concerned; the posterior shortening is, however, more than 

 compensated by the addition of a long metasternum. That the shortening takes place 

 both anteriorly and posteriorly is proved by the existence of ribs, which in the embryo 

 are connected with the sternum, but which in the adult become divided therefrom and 

 atrophied. The cause of the shortening is " consequent on that posterior translation 

 of the shoulder-girdle which is at once expressed by the lengthening of the neck and 

 the shortening of the trunk in the avian as compared with the reptilian type." This 

 sternal abbreviation is common to both Talmo- and Neognathm. 



In Struthio " the 7 days' embryo shows an anterior part which may probably be 

 compared to the manubrium stemi of mammals." A study of a series of embryos shows 

 that the anterior lateral processes are an outgrowth of the costal sternum ; in the Neo- 

 gnathm these processes are to be regarded as vestiges of a former extension of the 

 costal sternum anteriorly. 



The posterior lateral processes in the very early chick, according to Miss Lindsay, 

 " consist of the ends of the primitive costal bands, preserved and prolonged for the 

 attachment of the lateral part of the rectus, and subsequently modified ... to afford 

 attachment to the obliquus externus." 



These processes, as existing in the adult, may be considered a part of the meta- 

 sternum, since the primitive part bears but a small proportion to the later additions. 



The metasternum, both in Struthio and Rhea, remains for the most cartilaginous 

 throughout life. The cartilaginous metasternum of Dromceus and Casuarius is 

 relatively somewhat smaller. 



Posterior lateral processes are wanting in the sterna of Rhea, Casuarius, and 

 Dromceus. 



In Apteryw, according to Parker [7 1], " it appears certain that . . . each half of the 

 sternum is not formed by the antero-posterior union of the whole of the sternal ribs." 

 In one stage of the series which he examined the sternum apparently increased in 

 length from before backwards to the level of the fourth sternal rib, which extended 

 mesiad to meet it. 



The metasternum did not appear until after the posterior lateral processes had 

 attained nearly their full length. In one specimen the metasternum was double, and 



