176 THE FIFTH DAY. [CHAP' 



first, second, and third are the usually recognized elements, and to these 

 Dr Rosenberg's investigations (loc. cit.) have added a fourth. The first, second 

 and third persist in the adult, though they become anchylosed in all recent 

 birds. They also fuse, as we have said above, with the distal row of the carpals. 

 Phalanges belonging to the first, second, and third metacarpals are present. 



There thus seem in the avian manus to be no representatives of the centrale, 

 the fifth carpal, the fifth metacarpal and the phalanges of the fourth and fifth 

 digits. 



Of the elements we have spoken of in the avian hand, the only ones which 

 require further notice are the carpaP-", carpal"'"'^, and the fourth meta- 

 carpal. 



The united first and second carpal first appears as a small mass of cartilage 

 close to the proximal end of the second metacarpal. In this condition it 

 persists for some time but commences finally to fuse with the first metacarpal; 

 and at a slightly subsequent period with the second metacarpal. Tliese rela- 

 tions with the first and second metacarpals shew without doubt that this 

 little mass of cartilage is the representative of the first and second bones of 

 the distal row of the carpus. In a still later ptage carpal '~" fuses also with 

 carpal"'"''^. Its distinct nature as a separate element in the bird's manus is 

 again shewn during ossification, when there appears for it a separate centre of 

 ossification. 



Carpal "'"'^ appears about the same time as carpal'"'' but is at its first appear- 

 ance united with metacarpals three and four ; it soon becomes separated from 

 metacarpal three, and afterwards also from metacarpal four. It subsequently 

 undergoes considerable changes of shape, and rather later fuses with carpal'"". 

 Its true nature is again, as with carpal'"" shewn during ossification by the 

 appearance, of a separate centre of ossification for it. 



The fourth metacarpal is, as we have described, at first united with carpal'""'^, 

 but subsequently the neck connecting the two becomes constricted, and finally 

 they become completely separated from each other. The small independent 

 mass of cartilage thus formed represents the fourth metacarpal ; it applies itself 

 closely to the side of the third metacarpal, though without becoming united with 

 it. It ossifies very late— some time after the hatching of the chick, and after 

 ossification fuses with the third metacarpal — and then in most cases disappears 

 completely. 



The pes of a fowl in its early embryonic condition consists of 



(i) a mass of cartilage close to the distal end of the tibia. It represents 

 (Gegenbaur) the proximal row of tarsal bones, viz. the Hihiale,' the 'inter- 

 medium,' the 'fihidarc,' and the 'centrale.' Ttxis cartilage fuses in the adult 

 with the distal end of the tibia. 



(2) a mass of cartilage representing the five bones of the distal row of 

 the tarsus. In the adult this unites with the metatarsus, forming a tarso- 

 metatarsus. 



(3) the metatarsus. There are usually stated to be four metatarsal bones 

 present in the metatarsus of a fowl, which are anchylosed in the adult, but are 

 represented by separate rods of cartilage in the embryo. They are the distal 

 extremity of a first metatarsal, and complete second, third and fourth metatarsal 

 bones. In addition to these Dr Rosenberg {loc. cit.) has found a small oval mass 

 of cartilage representing a fifth metatarsal. Soon after its appearance this 

 becomes fused with the end of the tarsal mass of cartilage representing the 

 fifth tarsal, but later entirely atrophies. 



(4) There are four phalanges present both in the embryo and the adult, a 

 number which is never exceeded in birds (except amongst some abnormal breeds 

 of fowls, e.g. the Dorking fowls) ; though one or more of the four are frequently 

 deficient. 



