700 MIS3 B. LINDSA.Y ON THE AVIAN STERNUM. [JuneI6, 



intercostales externi and (5) a primitive arrangement are the fol- 

 lowing: — 



(a) (1). Their correspondence in number with the ribs, and inter- 

 costal position in the case of the Gull and Chick. 

 (2). Their subsequent fusion into a supercostal sheet, dif- 

 ferentiated finally into the intercostales externi. 

 (J) (1). Their curvature, slight as compared with that of the ribs. 

 (2). Their extent beyond corresponding ribs, in the case of 

 the cervical ribs of the Chick, and posterior ribs of the 

 Chick and Ostrich. 



It is to be hoped that further research may ehicidate the origin 

 and exact relations of these bands ; the inquiry must be attended 

 with some difficulty, however, since the mesoblast has but recently 

 acquired histological differentiation when they present the appearances 

 described. 



4. The evidences of the shortening of the sternum are the fol- 

 lowing :• — the adult condition presenting five ribs with uncinate pro- 

 cesses and two long unattached ribs without them, one anterior and 

 one posterior. 



(a) Anterior shortening. 



(i.) Chicks of 7 days showed in a number of cases the elongation 

 of 1-3 ribs anterior to the first of those named above ; none of these 

 reach the sternum. 



(ii.) Chicks early on the 6th day invariably showed two at least of 

 these ribs fully attached to the sternum ; they are very small and 

 close together. 



(iii.) The anterior muscle-bands, previously described, which 

 atrophy from their spinal ends onwards, suggest that their corre- 

 sponding ribs have passed into tlie sternum and disappeared by a 

 process of atrophy like that already noticed in the Ostrich. This view 

 is supported by the condition of a 5 days' Chick, which shows four 

 of the muscular rudiments named overlying four minute masses of 

 cartilage near the median line ; this specimen is in many respects 

 abnormal, but certainly these rudiments suggest the former existence 

 of a jjre-sternum. 



(h) Posterior shorteniuy. 



In about one out of every four Chicks from 6 to 8 days incubated, 

 there is a rudiment of a posterior 8th rib, atrophied at both ends as 

 in the Guillemot; in earlier stages this rib is seen, but not so fre- 

 quently, attached to the spinal end, or even at both ends. Before 

 this rudiment disappears it approaches very close to the next rib, 

 apparently because the intercostal muscle uniting them does not grow 

 so fast as those elsewhere, and in one case the rudiment is seen 

 attached to the next rib, forming a process {cf. Fig. III. diagr. a, 

 p. 708). Tlie 7th rib itself is often attached to the sternum up to 

 the end of the 9th day. 



The condition of the ribs in the ancestor of the Fowl, thus shown, 

 may be compared v.ifh that of allied forms, the Waterhcn and the 



