234 PEOr. W. K. PAEKEE, ON THE 



only ossified in two jolaces, both ectosteally; namely, the tibial shaft (if.), which now 

 almost reaches the tendon-bridge, and the intermedium (i.) in front of, and above, the 

 calcaneal lobe, the nnossified region of the fibulare (fbe.). Clear of that lobe externally, 

 the internal and lower angle of the intermedium is continuous with the astragalar region, 

 that of the tibialc (tb.) by a thin isthmus of cartilage ; this band forms the lower 

 border of the passage for the extensor tendon. More than the upper two-thirds of the 

 flat triangular intermedium is now an ectosteal bone — a distal remnant of an intercalary 

 "epipodial" bar. The fibula (see fig. 2, /6.) is now far away, proximally; it is the 

 aborted remains of an outer " epipodial " bar. Thus we see the mysterious transforma- 

 tion of the Reptilian hind limb into the single-shafted leg of the Bird — a transformation 

 that culminates in the leg of the Flamingo and Stilt- Plover (Phcenicoptems and 

 Jlimanto^nis). 



This unification — not mere fagoting or binding closely together — of the elements of the 

 leg (epipodial, mesopodial, and metapodial) necessitates the localization of the ankle- 

 joint through the middle of the metapodial elements, and its specialization, as a hinge- 

 joint, to a degree of perfection that exists in no other Vertebrate. The locking together 

 of the upper and lower condyloid faces seems to be, at first sight, very loose and free, 

 yet the convexities of the upper face of the joint have corresponding concavities in the 

 lower, and between them there is a very perfect structure of fibro-cartilage belonging 

 to the region of the ceutralia, and actually developing on its hinder part one good-sized 

 centrale (fig. 9, c), sometimes a second lesser nvicleus (T. J. Parker, in Apteryx). 



But the prepotency of the 3rd distal tarsal has to do with the perfection of this 

 most admirable joint. At first, alone, this part not only sent out its right and left plano- 

 concave lobes — the morphological equivalents of the 2nd and ith distal tarsals, — but in 

 its copious growth it sends downwards, behind, the mass for the tendon-sheaths and 

 groove, and upwards in front a well-formed intercondyloid lobe, which becomes 

 most developed in the Grallatorial tribes, and is the peg which fits so neatly into the 

 special round fossa on the intercondyloid face of the upper part of the ankle-joint 

 (Plate XXIV. figs. 8, 9). The inner ridge on the posterior outgrowth (fig. 9) is the 

 larger of the two main thickenings ; these have not any cartilaginous bridge uniting 

 them ; that is formed, afterwards, by a periosteal growth ; in the figure (fig. 9) the 

 centrale (c.) is shown semi-detached, with its inner face upwards. 



In the 6th stage (Plate XXIV. figs. 10-12) the ankle-joint of a Chick two or three 

 days after incubation is shown. Here we see that in Chicks double the age, from the 

 beginning of incubation, of those in which the intermedium had begun to ossify (figs. 6, 7, 

 12 days), the supposed parent of that element is itself still unossified. The solid carti- 

 laginous mass representing both tibiale and fibulare is being tunnelled, within, by capil- 

 laries, but I can see no osseous deposit at present. 



In the 7th stage, a chicken 33 days old (Plate XXIV. figs. 13, 11, tb.,fbe.), there is a 

 considerable internal nodule of bone forming the two proximal tarsals ; these deposits 

 come very near the surface of the cartilage, but to have formed the "ascending jirocess" 

 they must have been wholly ossified, and the osseous deposit must have run riot into 

 the periosteum and adjoining fascia above these elements. This 7tli stage is profitable, 



