702 MISS B. LINDSAY ON THE AVIAN STERNUM. [JuilC 16, 



formation of the Chick's keel is, however, as follows : — Stage 1: at an 

 age when both sternum and clavicles are cartilaginous, they are 

 united by a tissue, which at the age of 5 days is comparatively thin 

 and dim, but between 4 and 5 days is dark. Stage 2 : subsequently, 

 but still prior to the closure of the sternal halves, this tissue looks 

 like a projection continuous with the sternum. Stage 3 : the cartila- 

 ginous borders of the episterual region of the clavicles meet to form 

 the keel : at what date this takes place he does not make clear, but it 

 must be either during, or prior to, the latest stage he describes, viz. 

 the 8 days' Chick, in which he considers the median broadening 

 of the tract of commencing ossification in the furcula to represent 

 the ossifying interclavicle. 



Now in the specimen examined the said dim and thin tissue be- 

 tween the clavicles and sternum was never observed to present such 

 an appearance as that described in Stage 2 ; but it was differentiated, 

 immediately on the complete differentiation of the mesoblast, into 

 muscular fibres, forming a tract that gradually narrowed as the cla- 

 vicles closed. During the closure of the sternum it becomes reduced 

 to a thin string ; and all muscular fibres having now disappeared 

 from it, it assumes its character as the sterno-clavicular ligament of 

 the adult. No indication was observed tending to show that its 

 origin differed from that of other ligaments contemporaneously 

 established, such as those of the wing ; nor was there seen any stage 

 in which the cartilaginous borders of the so-called episterual region 

 of the furcula approached the keel ; they were always separated 

 from it by this tract, first consisting of embryonic muscular fibres, 

 and afterwards of thin ligament. 



8. So much with regard to the development of the keel ; if we 

 accept the view that it is an apophysis of the sternum, established 

 for the attachment of the pectoral muscles, we have none the less, 

 however, to examine the evidence as to the existence of an inter- 

 clavicle. That the furcula presents a median prolongation, is 

 undeniable ; but the following statements on two points will show 

 reasons for supposing it to be an outgrowth derived from the cla- 

 vicles only at a late date. 



(1) Date of the formation of the so-called Interclavicle. — It is 

 somewhat diflicult to ascertain the exact age of Chicks later than 

 the 5th day of incubation, because the changes they undergo are 

 comparatively slight ; but we may recall the fact that the two anterior 

 ribs lose tiieir attachment to the sternum about the end of the 6tb 

 day, while the occasional free rudiment of a rib eighth from the first 

 long one of the adult is never seen later than the 8th day. In other 

 words, the ribs at any rate have estabhshed their generic characters 

 at this date, which renders it probable that the broadeninj;- ossifica- 

 tion of the median region ot the clavicles, described by Giitte as 

 established during the 8th day, is an outgrowth of the Avian furcula 

 rather than a pre-Avian interclavicle — a view which is expressed by 

 giving to it (as has been done throughout this paper) the name of 

 median furcular apophysis rather than i^ii interclavicle. 



(2) Position of parts in the Shoulder-girdle of the .5 daijs' Chick. — 



