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



no trace was found either of interclavicle, clavicle, or keel. 

 Too much stress cannot be laid on the absence of the latter, since 

 the character of" the fore limb as a true although degraded wing has 

 frequently led, from a point of view exactly opposite to that of Gotte, 

 to the suggestion (founded on no other evidence than the analogy 

 indicated) that the sternum of the Ratitse has lost a formerly 

 existing keel : this statement must now be directly negatived. The 

 absence of a keel might indeed be inferred, by all who do not share 

 Gbtte's view as to its origin, from the smallness of the pectoral 

 muscles, the earlier stages of which afford no ground for supposing 

 that they have assumed their present condition by degradation from 

 a carinate type. 



It is unfortunate that no observations have been made on tlie 

 early embryos of Hhea or Casitarius, or above all on those of 

 BromcEus, whose rudimentary clavicles and single sternal plate mark 

 a type singularly different from the other Ratitse. The Ostrich 

 embryos, however, presented numerous points of interest, given below. 



1. In the 4 days' embryo, scapula and coracoid are not united; 

 this points to the conclusion that their subsequent union is a 

 secondary rather than an ancestral reptilian condition. Furthermore, 

 the coracoid and precoracoid are separate. Gotte and Hoffmann 

 both came to the conclusion that the distinction between the two 

 parts arose by the establishment of a foramen in a cartilage 

 originally uniform ; the 4 days' embryo shows, however, that the 

 division between them is of different origin. The precoracoid thus 

 exhibits its maximum development in the earliest stage ; in the adult 

 it sometimes shows a tendency to atrophy {cf. Plate XLII. fig. 9 ; 

 Bronn figures a similar specimen). In the absence of furtlier evidence 

 this fact would have gone far to justify a belief that the precoracoid 

 was wanting in the shoulder-girdle of other birds, but, as will be 

 seen hereafter, there are reasons for a contrary opinion. 



2. The rectus abdominis muscle is in the 7 days' embryo 

 attached to the unfused sternal halves, passing up between them in 

 the median line, about to the level of the third sternal rib, where 

 they meet one another ; the muscle is broad, and its lateral portions 

 exhibit two thick bands which are attached to the sternal halves. 

 In the adult the median portion is attached to the borders of the 

 cartilaginous metasternum, which is apparently developed for its 

 support, while the lateral portions become very thin. 



3. In the 4 days' embryo are seen eight thick transverse muscular 

 bands, overlying as many ribs, i. e. all except the two anterior free 

 ribs : in the case of the two posterior ribs, these muscles are longer 

 than the ribs ; and in the case of the posterior sternal ribs, they do 

 not follow the sharp curve which these make towards the sternum, 

 but pass nearly straight towards the median line, so that they only 

 overlie the ribs during part of their course. Towards the spine 

 they become lost in undifferentiated muscular tissue. These 

 muscles are on no account to be confounded with the muscle-plates 

 as apparent in a four days' chick ; bands similar to them appear in a 

 six days' chick : their position also, as stated, corresponds with that 



