LIMBS. 109 



The carpus and tarsus correspond essentially with those of 

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

 may become fused together. Thus the intermedium and tibiale 

 as a rule unite to form an astragalus, 1 while the fourth and 

 fifth carpals become fused to form the so-called unciform bone, 

 and the corresponding tarsals give rise to the cuboid. A 

 centrale is always present at an early stage in all five-fingered 

 Mammals, but as a rule it becomes fused later with the radiale, 

 as in the case of the Gorilla, the Chimpanzee, and Man, though it 

 may persist (in 4 cases per cent.) in the human subject throughout 

 life. In the tarsus the centrale (navicular) remains distinct, and 

 usually lies on the inner border of the foot. (Compare p. 106 and 

 small type below for mention of the pisiform bone.) 



A considerable modification of the homologies of the carpal and tarsal 

 elements described above must be expected shortly. The results of recent 

 researches (Bardeleben, Baur, Albreeht) on this subject are briefly as 

 follows : 



The astragalus corresponds to an intermedium as well as to another element 

 which remains independent in Marsupials, but in other forms (e.g. Man) exists 

 only in the embryo, and unites later with the intermedium. This second element 

 is either to be looked upon as a first centrale, or perhaps as a second inter- 

 medium, and is represented in the carpus by the cuneiform (ulnare). The 

 navicular corresponds to the scaphoid of the Mammalian carpus, that is, 

 to a navicular proper plus a second centrale. The pisiform corresponds to 

 the whole calcaneum. In human embryos of the second month a distinct 

 cartilage is present on the tibial side of the tarsus, and this probably answers 

 to a small bone on the tibial border of the foot of Monotremea, American 

 Marsupials, Edentates, Carnivores, Eodents, Insectivores, and Monkeys. This 

 most likely corresponds to an extra (first) toe (" prehallux," Bardeleben). In 

 the animals mentioned above, with the exception of Monotremes and the 

 addition of Cheiroptera, a " prepollex " is also present in the manus, consist- 

 ing of a carpal and a rudimentary metacarpal. The distal rows of the carpus 

 and tarsus correspond as regards their individual elements. That the unciform 

 and cuboid originally each represent two elements 2 (Bardeleben) is shown 

 by the fact that two digits are attached distally to each, and that in Marsupials, 

 Rodents, and Hyperoodon, an indication of a division into two parts persists 

 throughout life. Centetes madagascariensis alone has a double centrale in the 

 carpus. 3 



It is interesting to note the reduction which has taken place in the feet 

 of Ungulates in the course of time. In Fig. 95 sketches of the stages in the 

 phylogenetic development of the fore-foot of the Horse are given, showing 

 how it has been gradually derived from a tetra- or pentadactyle ancestor. 

 While in this case the third digit becomes greatly enlarged relatively (per- 

 issodactvle form), and eventually is the only one remaining, in cloven-footed 

 Ungulates the third and fourth digits are both functional, and equally strongly 

 developed (artiodactyle form), and may be united together to form a 



1 In Marsupials only does the intermedium remain as an independent bone. In 

 the human embryo it exists as an independent cartilage, but later almost always 

 becomes fused with the tibiale. 



2 According to Baiir, the cuboid and unciform arise each as a single mass, 

 their double condition being secondary. 



3 Baur states that a double centrale is never present in any Mammalian 

 embryo. 



