OCR Text |
Show 1885.] MISS B. LINDSAY ON THE AVIAN STERNUM. 699 other birds. There is every reason to expect that this tendency will occasionally be expressed (as apparently it is in the present instance) by the thick anterior part of the keel, as well as by the posterior part. (iv.) THE CHICK. (37 embryos, from 16 days' to early 5 days'; of which all but 7 were under the age of 12 days.) 1. Continuity of the inner part of the pectoralis with outer fibres of the rectus was observed in Chicks of 7 days. At this age the part indicated cannot be strictly identified, but it seems to occupy the position of the pectoralis minor of the adult. 2. At the age of 6 days the condition of the rectus is peculiar. Its anterior part (the equivalent of the sterno-hyoid and genio-hyoid, of which only a few fibres have as yet been caught up by the hyoid bones) passes between the open clavicles to end in a wedge-shaped piece between the sternal halves, at a stage when the approximation of the latter has already cut the continuity of the primitive rectus band. This state of the rectus and that described in the Guillemot warn us that embryonic conditions, when not comparable to those of any known adult form, cannot always be supposed to have a phylogenetic value. W e can scarcely imagine that there ever existed a type in which, as in the Guillemot embryo, the rectus was attached to the clavicles, while the halves of a highly developed sternum, provided with processes, failed to meet in the middle line; or in which, as in the embryo chick, the presternal part of the rectus passed through open clavicles, while the sternum was closed. 3. The primitive bands of the intercostales externi are seen from the end of the fifth to the end of the sixth day. Their supercostal position seems to be due to the curvature of the ribs, each of these having passed forward under the intercostal muscle of the next; for in the cervical region, where several, usually three, are present anterior to the coracoid, complete dissection shows that they are, as in the case of the Gull, alternate with the rudimentary ribs of this part, although in the sternal region they are supercostal as in the Ostrich and Guillemot. The reason why no such alteration can be traced in the sternal region seems to be that the proximal ends of the bands thin gradually away in the region where the intercostales externi of the adult find their dorsal limit; in the neck region, on the contrary, they can at first be traced up to the spinal column, although at a later stage their spinal ends disappear. No bands were seen in the cervical region of the types previously described. These bands on dissection are found to consist, at the end of the 6th day, of strong fibres at an angle with the direction of the band ; distally these fibres form a blunt mass, so that the band has a round end, abruptly marked off from the thin undifferentiated muscle in which the bands lie. The reasons for concluding that these bands represent (a) the |