410 ALICE JOHNSON. 



matrix has yet appeared, but the '^ prochondral elements" are 

 visible in the femur, tibia, and fibula. 



Soon after — when the limb is 0'17 in. long — separate ele- 

 ments begin to appear in the tarsus. Of these there are three, 

 two in the proximal row and one in the distal. The tarsus is 

 still continuous throughout and continuous also with the tibia, 

 fibula^ and metatarsals. But in these three centres, as well as 

 in the tibia, fibula, and metatarsals, the differentiation of tissue 

 has gone further. The outlines of the various parts are indis- 

 tinct. They all pass gradually into one another by means of 

 the general groundwork of condensed tissue formed by the 

 tarsus. The knee-joint is, however, developed at this stage. 



A little after this stage, the first metatarsal, which does not 

 keep step with the others in histological development, begins 

 to split off from the tarsus and soon lies at some little distance 

 from it. Baur describes the first metatarsal as originating 

 quite independently and never coming into any connection 

 with the tarsus. 



The phalanges next begin to appear. When the limb is 

 about 0"2 in. long, they are marked off by constrictions from 

 the metatarsals, but are cartilaginously continuous with them. 

 Later, when the limb is 0"3 in. long, the phalanges are marked 

 off by intervening tracts of condensed tissue with no matrix in 

 it. The tip of each toe at this period and for some time to 

 come consists of a mass of condensed tissue such as always 

 precedes cartilage (see fig. 9). This appears to be the grow- 

 ing point of the cartilage. From these facts it seems that the 

 phalanges are produced by a lengthening and subsequent seg- 

 mentation of the original distal cartilages of the limb, so that 

 these cartilages represent the skeleton of the digits as well as 

 the metatarsals. 



On the eighth day (length of hind limb 0"27 in. — see fig. 9) 

 all the elements of the tarsus are at their most distinct and in- 

 dependent stage, though they are still united Avith one another, 

 with the tibia and fibula, and with the metatarsals by the con- 

 densed tissue of the groundwork of the tarsus. 



Later, the distal and proximal parts of the tarsus become 



