1887.J 



On the Morphology of Birds. 



57 



with its large and dilated digit, has arrested the distal carpal of the 

 1st or short digit, the " pollex." This is the last nucleus to chondrify. 

 It is still a very small, limpet-like disk of cartilage, and is now only 

 to be seen on the flexor face of the manns, inside the top of: the 2nd 

 metacarpal ; the distal carpal of the 3rd ray is also small as compared 

 with the large crescentic 2nd distal nucleus. It is thrown on to the 

 ulnar or outer side of the manns, by the overgrowth of the middle 

 rod and its carpal. The curve of the digits at their end is now, not 

 inwards, or to the radial side, but outwards ; and the two developed 

 distal segments form now the core of two claws, that of the first, or 

 pollex, being of considerable length. 



Thus, by the end of the 10th day, the reptilian type of fore-foot 

 has been attained, and the amphibian type lost ; whilst the limb as a 

 whole is now a fore-leg no longer, but a wing, thoroughly specialised 

 by evolutional transformation. 



The fore-limb has not simply become modified into a wing by the 

 shortening of the pollex and 3rd ray, the enlargement of the 2nd, 

 and the abortion of the 4th and 5th of a fore-paw, like that of the 

 lizard ; but we have now the historical representatives of three more 

 rays which have cropped up since the end of the 8th day. 



I have repeatedly noticed that aborted parts, like overshadowed 

 plants, are late to appear, and soon wither, or are arrested in their 

 growth. This is the case here, for the new rays are late, small, and 

 scarcely functional in the fullest development. They are not lost, 

 however, but, like certain larval structures to be found in the skulls 

 of the highest types of birds, they are built up into the finished wing, 

 although they form an unimportant part of it as far as function goes. 



The first of these additional rays is the " pre-pollex ;" this is a 

 lunate tract of fibro- cartilage attached to the inner face of the 1st 

 metacarpal. The other two are composed of true hyaline cartilage, 

 and appear, one on the ulnar side of the 2nd, and the other on the 

 ulnar side of the 3rd developed metacarpal. 



I have described them as intercalary metacarpals, for they seem to 

 be the starved twins of the 2nd and 3rd large rays : each distal carpal, 

 very probably, in the archaic forms carried two rays. Thus there is 

 supposed, for such a fore-limb, a digit inside the pollex of the modern 

 bird, and then two pairs of rays, of which only the inner in each case 

 has been retained. 



The paddle of Ichthyosaurus shows this kind of primitive cheiroptery- 

 gium, admirably. 



Thus we can account for seven carpals and six digits in the wing of 

 the modern bird ; in the legs the specialisation is not so intense, but 

 is very great ; the study of the embryonic stages shows in it many 

 parts that the adult bird gives no signs of whatever. 



Instead of there being even two tarsals, free and functional, there is 



