78 PEOF. W. K. PAEKER ON THE 



" calcaneal ossicle " of the older writers, belongs to a lower stratum ; it is a centrale. 

 The intermedium, the so-called " ascending process " of the astragalus, evidently 

 belongs to a higher series — namely, to those of the leg. I classify it with the tibia 

 and fibula. In the Chick, after seven days' incubation, my 1st stage in this research, 

 in the 2nd stage, after eight days, and in the .3rd stage, after ten days, I have examined 

 these parts with great care. In all these stages the ascending process of the astragalus 

 is an oblique flange of hyaline cartilage, which runs from the top of the tibiale towards 

 its outer side to the top of tlie fibulare on its inner, and then grows upwards. It 

 cannot, therefore, in this early condition be proved to be the intermedium. Miss 

 Johnson's preparation (Stud. Morph. Lab. Camb. vol. ii. 1886, pi. v. fig. 9) and Dr. 

 Baur's sections {op. cit.) do not, in embryos answering to my 1st stage, show the 

 ascending process. At this date the globular fibulare is much more solid than the 

 wedge-shaped tibiale ; the front surface of the latter, and especially the part which 

 forms the ascending process, is still composed of indifferent tissue. In a day or two, 

 however, the chondrification is completed, and then the ascending process partly 

 overlaps the fibulare and the end of the fibula. In the Chick the process is short, but 

 in Opisthocomus, as in the Ostrich, it is long. In my 1st stage (PI. X. fig. 10) the 

 tibiale and the fibulare are confluent ; the outer condyle is formed by the latter. The 

 rest of the ascending process, which is almost four times as long as the interspace 

 between the two condyles, has become partly confluent with the antero-superior face 

 of the fibulare ; a notch marks its line of junction. A little above the notch a small 

 ectosteal sheath has already appeared — the " os intermedium ; " above this the carti- 

 lage first enlarges and then becomes a narrow style, which lies in front of the tibia 

 towards its outer side. The remaining cartilage, at the lower end of the tibia, which 

 has the same depth as the distal tarsal mass, is still sharply separated from that mass. 

 Postero-internally there is a crescentic cartilage (Pis. VII. & X. fig. 10, c), the centrale 

 or scaphoid, which is formed out of the interarticular plate ; this forms the bone which 

 was supposed to be the os calcis. Below the joint the distal tarsal mass (d.t.^~*) is 

 cupped right and left to receive the condyles, but sends upwards in the middle an 

 intercondyloid knob ; its lower surface fits to the fiattened tops of the second, third, 

 and fourth metatarsals, which are not ossified above. A rudiment of a fifth metatarsal 

 did exist on its outer side ; the aborted first metatarsal is placed distally ; it has also 

 begun to ossify. In the relation of the distal tarsal mass to the three large and one 

 abortive metatarsal segments, a single cartilage or " chondrite " may be the connate 

 representative of several chondrites. In the third stage (PI. X. fig. 11) the ankle- 

 joint is considerably altered ; the tibia has much less cartilage below ; that is still, 

 however, distinct from the proximal tarsal mass. Over the interspace between the 

 two condyles there is now a cartilaginous tendon bridge (PI. X. fig. 11, t.br.). The 

 ascending process is relatively narrower in its lower half; its upper three fourths 

 is entirely ossified by an ectostosis, like the tibia and fibula. The larger tibiale and 



