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



the sternum fully formed, lying in a position corresponding with 

 that of the masses just described, and fused with the rib. The keel 

 exists as a broad and thick longitudinal ridge, divided from the body 

 of the sternum by a deep groove on the dorsal side. The coracoid 

 has now acquired attachment to the sternum ; the clavicle is still 

 shut ofip from the sternum by the pectoral muscle. The scapula 

 does not join the coracoid to form a coraco-scapular plate, either at 

 this or any other stage. A rudiment is apparent of the ligament 

 joining the top of the coracoid with the top of the keel ; and a very 

 strong rudimentary ligament, reduced in later stages, unites the top 

 of the scapula with the top of the keel. The anterior-lateral pro- 

 cess is very thin and trilobed, which perhaps indicates an origin from 

 three anterior ribs belonging to the three anterior muscle-bands 

 indicated in the last stage. There are 7 sternal ribs. 



The outline of the fore limb at this stage indicates a division into 

 three digits ; it is shown in Plate XLIII. fig. 4 a, for couiparison 

 with the fore limb of the 7 days' Ostrich. 



Atli Stage, corresponding loith a Chick of about 12 days^ incu- 

 bation. — The lateral halves of the sternum are closed, the keel is 

 fully formed, and the pectoral muscle has acquired the adult struc- 

 ture, namely, of three interconnected sheets. The clavicle is united 

 to the keel, but dorsally a thia crack still indicates their junction 

 (fig. 16, Plate XLV.). The anterior-lateral process of the last stage 

 has been atrophied ; this fact may be compared with that of the 

 loss of the two anterior ribs at a subsequent stage. On the right 

 side is seen the pecuhar insertion of the coracoid characteristic of the 

 adult. 



The posterior-lateral process of the metasternum has been added. 

 No posterior reduction of the ribs is to be traced. In fact the 

 posterior part of the sternum seems to retain what we may suppose 

 to be a primitive type, as will be seen on comparison of the succes- 

 sive stages (figs. 15, 17, 18, Plate XLV.) with those of the Ostrich 

 (figs. 3, 4, and 5, Plate XLII.). We may contrast these with the 

 condition of the metasternum of the Chick, doubtless a form highly 

 specialized in this respect as in many others ; in it, as we have seen, 

 the processes of the metasternum are established from the first. 



bth Stage, corresponding to a \A days' Chick. — This shows the 

 addition of the median j)rocess of the metasternum. The double 

 apophysis of the median and anterior region is not added till a later 

 stage. In the Chick, the median anterior apophysis is added be- 

 tween the end of the 9th and the beginning of the 14th day of 

 incubation. 



Part III. Conclusions drawn from the above data, 



in connection with facts previously known. 



1. Conclusion drawn from the existence in the embryo of anterior 

 ribs atrophied in the adult, of which some are seen attached to the 

 sternum, and of muscle-bands apparently representing the intercostales 



