| OCR Text |
Show 692 MISS B. LINDSAY ON THE AVIAN STERNUM. [June lb, 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 Ratitae 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 Gotte'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 the early embryos of Rhea or Casuarius, or above all on those of Dromaus, whose rudimentary clavicles and single sternal plate mark a type singularly different from the other Ratitae. 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 (of. Plate XLII. fig. 9 ; Bronn figures a similar specimen). In the absence of further 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 |
