64 PROF. W. K. PAEKEE ON THE 



nineteenth are 35 millim. long, and those on the first dorsal 37 millims. These two 

 pairs of ribs have evident uncinate processes, whilst those on the second and third dorsals 

 are rudimentary, only widening the ribs somewhat. In the first stage the free cervicals 

 have no appendages (PI. VII.), but these are seen on the four dorsals and the first 

 dorso-sacral. The ribs gradually nan-ow backwards ; each rib also narrows downwards 

 and becomes the width of the sternal piece ; these lower segments are short and strong. 

 Even the costal appendages show that this is a generalized bird, both in their form 

 and in their variability. 



V. The Sternum and Shoulder-girdle of Opisthocomus. 



From a very early period of my study of the Anatomy of Birds, I have been in the 

 habit of comparing these intensely-specialized forms with the Imago Insect ; the 

 Reptiles proper being looked at as a sort of active Pupa, and the lower forms of Fishes 

 as representatives of the larval stage of those noble Invertebrates. 



Nothing shows this relation better than the shoulder-girdle ; for in this part the 

 intense life of these hot-blooded creatures has transformed the old elements, creating 

 them anew, as it were, " into something rich and strange," making of those simple 

 limb -roots the proximal part of the highest type of an organ of flight. The tooth- 

 bearing extinct forms [Archceopteryx, Hesperornis, Ichthyornis) throw a much fainter 

 light on this great change than this isolated archaic Fowl. The lowest Mammals, the 

 oviparous Monotremes, still retain the simple or unfused condition of the splint-bones 

 of the shoulder-girdle. The existing Reptiles, especially Lizards, show these splints in 

 their free state, and also a large generalized shoulder-plate belonging to the endo- 

 skeleton, although its most superficial part ; for it is formed between the ribs and the 

 skin. Both the ribs and shoulder-plates are formed in the outer layer of the body of 

 the embryo, namely in the somatopleure \ 



The sternum in the 1st stage (PI. IX. figs. 1, 2) is already perfectly formed in 

 hyaline cartilage ; even in the 3rd stage no ossification has appeared. The breadth of 

 this short sternum is five-sixths of its length in the fore half; it narrows in behind the 

 last sternal rib, and then widens out again so as to be 1 millim. wider than in front. 

 The form now attained in this half-ripe chick of Opisthocomus is fairly comparable to 

 that which is seen in the sternum of the Common Chick on the sixth or seventh day 

 (Lindsay, pi. xlv. figs. 1-3), and is relatively shorter than in the adult (Huxley, op. cit. 

 p. 306, fig. 8). Otherwise, except in its histological condition and its size, the sternum 

 undergoes but little further change. The rostral process {r.st.) is small, rounded, and 

 superior ; it grows forwards between the upper lips of the coracoid grooves. 



The copious illustrations generously allowed me bj' the Council of the Raj' Society in my memoir on the 

 Shoulder-girdle and the Sternum, published in 1868, come in very useful now, to illustrate this, my newer 

 work. I am also greatly indebted to Miss Beatrice Lindsay for her excellent paper " On the Avian Sternum " 

 (P.Z.S. June 16, 1885, pp. 684-716, pis. slii.-slv.). 



