230 PROF. W. K. PAEKER ON THE 



the tibia. Even now the main shaft-bones are present in the middle third of the long 

 rods — femur, tibia, fibula, and the 2nd, 3rd, and 4th metatarsals (Plate XXIV. /., t.,fb., 

 mt. 2 ~ i ). Even now, all the segments are not composed of hyaline cartilage, they will be 

 in one or two days more. 



Tbe rapidity with which the bird-type is developed in the embryo is very marvellous; 

 already the femur has its trochanteric condyle, as well as its round upper and double 

 lower condyle, clearly formed. The cnemial process of the tibia is well seen, and 

 the fibula, already falling short of the end of the leg, is at its widest part only half the 

 diameter, and one fourth the substance, of the tibia. 



So much for the " propodial " and "epipodial" elements of the hind limb; the 

 "mesopodials " are rapidly forming; the " metapodials" are formed, but are not united, 

 they are a straddling series, like those of a Lizard ; the " phalangeals " are all marked 

 off, but the distal segments are not quite chondrified. 



The mesopodials, or tarsal segments, are highly modified at their first appearance, and 

 the distal row does not become segmented vertically, but remains as one transverse 

 mass adapted to the three developed metatarsals — the 2nd, 3rd, and 4th (Plate XXII., 

 Plate XXIV. fig. 1, d.t.). But the proximal row of tarsals is for a time differentiated 

 into the two normal segments, the tibiale and fibulare *. 



Nevertheless, all observers are now agreed that the mass which was supposed to be 

 the astragalus, merely, or tibiale, represents both astragalus and calcaneum ; there is 

 a distinct fibulare on the outer side, determinable both in the early and scarcely 

 chondrified state, and afterwards in the ossification by two endosteal centres of the solid 

 bilobate cartilage that forms the double condyle in this type of ankle-joint. 



Miss Alice Johnson (Stud. Morph. Lab. Camb. vol. ii. plate 5. fig. 9, fbe., tbe.) 

 showed the double nature of this mass, but failed to find the intermedium (loc. cit. vol. ii. 

 p. 25, plate 5. fig. 9). Dr. G. Baur (" Der Tarsus der Vogel und Dinosaurier," Morph. 

 Jahrb. Bd. viii. plate 20) gives a large series of sections of the ankle of the Chick, but 

 these prove nothing against Dr. Morse's f views and mine as to the nature of the bony 

 shaft that afterwards appears in the ascending process of the tibiale or astragalus. 



In the stages worked out by Miss A. Johnson and Dr. Baur, the tibiale is only 

 partially chondrified ; the fibulare solidifies earlier. Thus the cortical part of the tibiale 

 and the ascending growth are in that stage, which is my first (Plate XXIV. fig. 1), still 

 composed of indifferent tissue. In a more advanced stage an ear-shaped process of 

 cartilage grows upwards from the outer and upper corner of the tibiale, and applies 

 itself to the upper and inner edge of the fibulare, mounting up above it (Plate XXIV. fig. 2, 

 i; 2nd stage). When this part is bent upwards against the pisiform fibulare, then the 

 band has a thin isthmus, and this is the true segmental part, which, however, keeps its 

 cartilaginous continuity with the tibiale ; thus these two normal segments are connate, 



* In my introductory paper " On the Morphology of Birds," Proe. Roy. Soe. 1887, p. 58, I have spoken of the 

 distinctness of three cartilaginous nuclei at this part : this is an error. 



T " On the Identity of the Process of the Astragalus with the Intermedium." — -Anniversary Memoirs, Bost. Soc. 

 Nat. Hist. 1880. 



