Early Development of Appendicular Skeleton of the Ostrich. 357 



Unfortunately she does not state whether she regards the elements 

 as chondrified. It is possible, however, that they are not, as she 

 appears to be able to dissect out the elements of the shoulder girdle 

 in the chick at a stage when sections " show comparatively little 

 differentiation in the cells." Personally, I feel considerable hesita- 

 tion in believing in structures which can be got by dissection but 

 cannot be seen in microscopic sections. At any rate my results by 

 microtome methods are very different from those obtained by 

 Miss Lindsay. 



In the 10-day ostrich embyro which I have examined, the whole 

 shoulder girdle can be distinctly made out, but it is not yet fully 

 chondrified. The coracoid and scapula proper are much better 

 differentiated than is the descending process of the scapula. There 

 is farther no trace of a division between the coracoid and scapula : 

 both are merely parts of a single bar. The coracoid is a short, broad 

 structure, with a well-developed base for attachment to the sternum, 

 but it does not yet meet the sternum. In the middle it is somewhat 

 constricted and above it broadens out where it meets the scapula. 

 The axis of the scapula makes with that of the coracoid an angle of 

 about 100°. The upper part of the scapula is long and narrow and 

 lies nearly parallel to the axis of the vertebral column. The lower 

 end is broad and turned downwards to meet the coracoid. Anteriorly 

 it is continued into a short, badly differentiated prescapular process, 

 and on its outer side is a well-marked short process apparently 

 corresponding to the " acromion process " figured by Jeffery Parker 

 in the Apteryx. There is no clavicle present. 



The sternum is very imperfectly differentiated, but can be fairly 

 well traced. It is joined by 5 ribs, but none of the structures are as 

 yet chondrified. 



In the 11-day ostrich embryo the girdle is well chondrified except 

 the prescapular process which is still precartilage. The coracoid is 

 more elongated than in the earlier stage and now articulates with 

 the sternum. The scapula makes with the coracoid a more obtuse 

 angle than before and has its posterior end expanded somewhat. 

 Anteriorly it curves downwards to the prescapular process, which 

 is well developed but not chondrified. The process passes down- 

 wards and then slightly forwards and nearly reaches the base of 

 the coracoid. As in the younger embryo there is no trace of a 

 clavicle. 



The most probable explanation of the phylogeny of the ostrich's 

 shoulder girdle seems to be that it is descended from a reptilian type 

 such as is seen in some Phytosaurs, where there is an elongated 

 scapula and a short coracoid which is without a coracoid foramen. 



