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. We 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 bauds 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 



