132 COMPARATIVE ANATOMY 



Sirenia, Cheiroptera, and certain Marsupialia, a sesamoid bone is 

 developed in the distal tendons of the great extensor musqles of 

 the shank, and is known as the knee-cap or patella. This is 

 already present in certain Lizards and in Birds. 



The carpus and tarsus most nearly correspond with those of 

 Urodeles and Chelonians, and, as in them, certain of the elements 



1-2 earn. tceatr. 

 ^ — ^^^ 



Fig. 112. — A. Fore-arm and MAstrs of Embryo Penguin (Eudyptes chryso- 

 come). (Fourteenth day of incubation.) (After Th. Studer.) (SB is a sesa- 

 moid developed in the tendon of the triceps in this Bird. ) B. Shank and 

 Foot or Embryo Penguin. (At the same stage.) 



may become fused together. Thus the intermedium and tibiale 

 as a rule unite to form an astragalus, while the fourth and fifth 

 carpals become fused to form the so-called unciform hone, and the 

 corresponding tarsals give rise to the cuboid. A centrale, varying 

 much in form and size, is usually present at an early stage in all 

 five-fingered Mammals, but as a rule it becomes fused later with 

 one, or with two, of the neighbouring carpals — generally the 



