G94 MISS B. LINDSAY ON THE AVIAN STERNUM. [Juiie 16, 



8. The differentiation of the lateral processes of the metasternum, 

 whicli also are uot ossified even in the adult, is traced by their 

 connection with the muscles. They are prolongations, established 

 between the ages of seven and ten days in the embryo, of the ends 

 of the primitive halves of the costal sternum, to which the lateral 

 parts of the rectus are attached. In later stages this lateral part of 

 the rectus becomes very thin, and the processes are found to afford 

 attachment in addition to j^art of the obliquus externus, as is usually 

 the case with the posterior lateral processes of the sternum in the 

 Carinatse. Coincideutly with this change of their muscular relations, 

 the ends of the processes develop between the 15th and 21st days 

 a double outgrowth, resembling the end of the xiphoid process in 

 the Chick, and apparently due in both types to the somewhat lateral 

 direction of the strain exercised by the obliquus externus. The 

 attachment of this muscle becomes rather obscure in late stages, 

 because in the adult its major part is, as stated by Bronn, aponeurotic ; 

 I find that it may, however, be still traced to the process in a bird 

 newly hatched. 



It appears, then, that in the Ostrich at any rate the posterior 

 lateral processes 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. Their change of use is due to the reduction that 

 takes place, during the establishment of the Avian type, of the 

 rectus, which muscle finds its maximum development in Reptiles, 

 The primitive costal origin of these processes finds a parallel among 

 the Carinatse in the case of the Gannet : it is interesting, too, to 

 compare them with the costal processes of the sternum in Iguana, 

 formed by the fusion of the sternal portions of posterior ribs. 



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

 of the metasternum, since the primitive part bears but a small 

 proportion to the later additions. 



(ii.) The Guillemot. 

 (13 specimens, from about 1/ days' to 6 days' incubation.) 



This type exhibits two marked peculiarities : {a) the great 

 development of the keel and median furcular apophysis; (6) the 

 elongated condition of the sternum, which in the adult ossifies seven 

 ribs and is very narrow llie former character, since the bird has 

 small wings and flies but little, must be attributed to the necessity for 

 strengthening the fore limbs in swimming ; the second seems to be 

 correlated with the general elongation and narrowing of the trunk 

 established in connection with the habit of diving. The embryos 

 dissected exhibited the following j)oints whicli call for remark : — 



1 . The union of the clavicles takes place very early ; in a 6-7 days' 

 embryo, when the sternal halves are uot yet united, they are already 

 closed, and the median furcular apophysis is mapped out in cartilage 

 continuous with the clavicles. The embryos examined were hence 

 unsuitable for inquiry into the ciigin cf the latter structure, but they 



