294 Studies of the Dcvclopjiiciit of tlie liiuiuui fSkeleton 



embryos of about 30 mm. the joint-cavities of the foot are filled with a 

 loose fibrous tissue, in embryos of 50 mm. definite cavities are to be made 

 out. The sesamoid bones develop later than the period to which this 

 investigation extends. 



During the prbgress of form difl:erentiation above described the shape 

 of the foot is markedly altered. At the beginning of the development of 

 the foot the tarsal and metatarsal bones lie nearly, though not quite, in 

 the same plane as the bones of the leg, Figs. 7, 8 and 59. They are so 

 arranged, however, that the foot is convex on its dorsal surface and con- 

 cave on the plantar, and the projections of the calcaneus and astragalus 

 serve to deepen the plantar fossa. The metacarpals spread widely apart. 

 As differentiation proceeds the metacarpals come to lie more nearly 

 parallel to one another and the tarsal elements become compacted in such 

 a way as to give rise to the tarsal arch. The foot at the same time is 

 flexed at the ankle and turned slightly outwards. The toes are flexed. 

 Fig. G8 shows the extent of the tarsal arch in an embryo of 23 mm. 



In the further development of the skeleton of the foot the various 

 constituent structures are elaborated and the foot gradually becomes 

 more flexed and turned toward the fibular side. Yet even in the infant 

 the head of the astragalus is directed more inwards than in the adult. 

 Leboucq, 82, pointed out that the first metatarsal is relatively short in 

 the foetus and points more toward the tibial side than later. 



Ossification. — This begins in the metatarsals and phalanges during 

 the third month and is perichondral in nature. The tarsals begin to be 

 ossified considerably later. The center for the calcaneus appears in the 

 sixth month, that for tlie astragalus in the seventh month of fcetal life. 

 The ossification of the other bones begins during the first five years of 

 life. Authorities differ as to the exact time at which the process begins 

 in the various bones. In Quain's Anatomy the following dates are given : 

 cuboid, at birth, external cuneiform, 1st year; internal cuneiform, 

 3d year; middle cuneiform, 4th year; navicular, 5th year. 



I have studied the ossification in the third and fourth months of embry- 

 onic life. In an embryo about 4 cm. long, cleared according to the 

 Schultze method, I have found centers of ossification in the 2d, 3d and 

 4th metatarsals, and in the terminal phalanx of the big toe of each foot. 

 In Embryo XCVI, length 4-^ mm., there is a very thin layer of bone 

 being laid down about the center of the shaft of the 2d, 3d and 4th meta- 

 tarsals. I have been unable definitely to determine whether or not bone 

 has been deposited in the terminal phalanx of the big toe. In Embryo 

 XCY, length JfG mm., ossification has begun in the 2d, 3d and 4th meta- 

 tarsals and in the terminal phalanges of the 1st and 2d toes; in Embryos 



