58 



On the Morphology of Birds. 



[Jan. 27, 



only one, and that has merely the function of a " sesamoid," and has 

 been mistaken continually for a bone of that sort ; that nucleus answers 

 to our naviculare, morphologically termed the " centrale." 



Notwithstanding the extreme diversity in the habits of existing 

 birds, and the great difference seen in their shank bone, this part is 

 always single, although composed of three metatarsals. As in reptiles, 

 the joint at this part is not between the astragalus and tibia, as in 

 mammals, but through the tarsal series ; no sign of this structure is 

 seen in the adult bird. That which appears to be the condyloid end 

 of the tibia is a row of tarsal bones, the tibiale, fibulare, and inter- 

 medium ; these have long been known as separate bones in young 

 birds, but their distinctness in the early embryo as cartilaginous 

 nuclei has only lately been made out. 



I have been able, however, to demonstrate this repeatedly in different 

 kinds of birds. The centrale also, although seen in the embryo as 

 one of the tarsal series, was not properly identified ; it is a constant 

 element, but becomes degraded. 



The distal series of tarsals exists as a single tract of cartilage, and 

 then as a single plate of bone. But it is related to three metatarsals, 

 and the middle or thick part is the first to chondrify in the embryo, 

 and to ossify in the chicken or young bird; there are here three 

 connate nuclei, with very slight signs of distinctness. The whole 

 mass answers to our middle and external " cuneiform bones," and to 

 the inner half of the " os magnum." Thus five tarsals can be always 

 made out clearly, and two more accounted for. 



The 1st metatarsal, which has been known, for some time, through 

 the valuable researches of Morse, to have occasionally a proximal as 

 well as a distal rudiment, has, I find, always a proximal rudiment as 

 well. 



Then, as Dr. G. Baur and Miss A. Johnson have shown, there is a 

 5th metatarsal ; it is a small pisiform cartilage, which soon coalesces 

 with the 4th, and with the great distal tarsal. I can only find a 

 " pre-hallux " by turning to Teratology, and this is not the lawful 

 method. 



There may, however, be some " reversion " or " atavism " in the 

 polydactyle foot of the Dorking fowl, which has a well developed 

 "pre-hallux" and a double "hallux;" the twin digits of that part 

 have a very ichthyosaurian appearance. 



