690 



MISS B. LINDSAY ON THE AVIAN STtRNUM . [June 16, 



above-named observers, owing to some discrepancies in their state- 

 ments as to the age of embryos. Gotte describes tlie establishment 

 of coracoid, clavicle, and sternum in chick embryos of the age of 

 4-0 days, and yet considers that his embryos were slightly older than 



Fig. I. 



A. Sternum of'adult Guillemot ( Uria froilc). B. Sternum of Ostrich embryo, 

 consistiug, before fusion of the lateral halves, of costal sternum only. 

 C. Sternum of Ohick embryo, consisting, before fusion, of both costal 

 sternum and metasternum. 



c.s, costal sternum, extending from the attachment of the coracoid to that of 

 the last sternal rib ; m.s, metasternum, extending posteriorly from the 

 last sternal rib ' ; m.f, median furcular apophysis ; m.a, median anterior 

 sternal apophysis ; a.l.p, anterior lateral process of the sternum ; p.l.p, 

 posterior lateral process of the sternum. 



Rathke's. These descriptions do not correspond with the usually 

 received opinion that the histological diflfereutiation of the mesoblast 

 takes place during the fifth day of incubation in the chick "; still less 

 ^ This region does not exactly correspond witli L'Herminiers metasternum, 

 for that bears some of the posterior ribs, and therefore includes part of the 

 costal sternum. 



It corresponds, however, with the region occupied by Parker a Lophosteon 

 + Metosteon + Urosteon, in other words its extent is not only posterior 

 but also median. In short,! he term metasternum is used here to express rather 

 its development in time than its position in place. 



^ ' Elements of Embryology,' Foster and Balfour, 2ud ed. p. 270. 



