5G 



Prof. W. K. Parker. 



[Jan. 27, 



change. The atlas is still composed of four distinct pieces of car- 

 tilages, but the ribs have become fused above and below with the 

 transverse processes, and the notochord is now most constricted at the 



intercentra. 



Besides this, in the pre-sacrals, it is constricted in two places within 

 each centrum ; so that each centrum in the modern bird corresponds 

 to three subdivisions of this axial chord. 



Por two or three days there is evidence of an archaic subdivision of 

 the notochord into three times as many vertebral divisions as are made 

 now in the modern bird. 



In the sacral the constrictions are fewer ; they are only at the inter- 

 centra, and in the middle of the centrum. 



The only absolutely necessary part of the sternum is that where the 

 sternal ribs are attached ; that is a very small part, and the rest is 

 for the attachment of the huge muscles that act upon the wings, and 

 for the obliqui and recti abdominis. 



The limb-girdles are each in three pairs of distinct cartilages. In 

 front, the scapula, the minute pre-coracoid, the coracoid ; behind the 

 ilium, pubis and ischium ; the pre-pubis is part of the ilium, and that 

 has two regions, the pre-ilium and the post-ilium. 



These parts in the bird are not continuous tracts of cartilages, 

 ossified by several centres, but are distinct, first as cartilages, then 

 as bony tracts ; those of the shoulder keep distinct ; those of the hip 

 soon coalesce. 



The wings at the end of the 7th day are three-toed webbed paws, 

 with all the digits turned inwards. The rods that compose the main 

 part of it are composed of solid cartilage ; the humerus, radius, ulna, 

 and 1st and 2nd metacarpals have a bony sheath round their middle 

 part ; the ends of the digits and the carpals are but partly chondrified. 

 Five carpal nuclei, however, can be made out, and the two proximal 

 nuclei are known to be further subdivided, each into two, in other 

 types ; hence we can already account for seven carpals in the bird, 

 which has only two in the adult, in a free state. 



Moreover, the 1st digit has two, and the 2nd three phalanges, the 

 normal number, as in lizards ; the 3rd, which should have four, but 

 in birds has as a rule only one, has now two, as in the ostrich, and a 

 few other birds ; there is no sign at the end of the 7th or even of the 

 8th day of incubation of any more than three digits, but we have in 

 the wrist an intermedio-radiale, a centralo-ulnare, and three distal 

 carpals, answering to the three developed metacarpals. The digits 

 up to the end of the 8th day are rounded and flattish, and are quite 

 like those of a young newt or frog. But in two days more, at the 

 end of the 10th day, the wing has almost acquired the adult form ; 

 and one more bony centre, that of the 1st metacarpal, has appeared. 

 The overgrowth of the 2nd distal carpal and the 2nd metacarpal, 



