290 Studies of the Develoj)nient of the Human Skeleton 



ends of the bones (Fig. 13, Plate V). The development of the distal 

 extremities of the tibia and fibula may best be taken up in connection 

 with the development of the foot. 



C. ANKLE AND FOOT. 



Of the papers dealing with the early development of the skeleton of 

 the human foot the more important are those of Henke and Eeyher, 74, 

 Leboucq, 82, v. Bardeleben, 83, 85, Lazarus, 96, and Schomburg, 00. 



Since the work of Schomburg is the most recent of these and is based 

 on a considerable number of well-prepared embryos, I shall discuss his 

 results somewhat at length in connection with the results which I have 

 obtained. He recognizes four periods in the development of skeletal 

 structures, a mesench^mial, a prochondral, a cartilagenous and an osseous. 

 For the sake of ready comparison I shall take up each of these periods in 

 turn. The fourth period falls within the scope of this paper only in 

 so far as it overlaps the third. 



Mesenchymal (hlastemal) period. — This commences during the fifth 

 week of embryonic development. The free extremity of the limb-bud 

 becomes flattened and differentiated into the anlage of the foot and its 

 axial blastema becomes' differentiated into a foot-plate, from which later 

 the bones of the foot are derived. Schomburg states that the axial blas- 

 tema becomes distinct at the end of the fourth week. In Embryos 

 CCXLI, length 6 mm.: II, length 7 mm.; CLXIII, length 9 mm.; and 

 CCXXI, length 13 mm.^ I find no distinct signs of a foot-plate. In 

 each of the following embryos I find a foot-plate which has not distinctly 

 undergone further differentiation: CIX, length 11 mm.; CLXXV, 

 length 13 mm.: and CYI, length 11 mm. The last is a somewhat path- 

 ological specimen. In Fig. 3 a reconstruction of the foot-plate of CIX 

 is shown, in Figs. 51 and 52 transverse sections through this are repre- 

 sented, in Fig. 66 is pictured a longitudinal section through the foot- 

 plate of CLXXV. 



Toward the end of the fifth week, in embryos usually 14 to 16 mm. 

 long, the first differentiation of definite bones is manifested by a conden- 

 sation of tissue in specific areas. Within these areas of condensed tissue 

 precartilage soon makes its appearance. Schomburg says that the first 

 metatarsal is differentiated distinctly later than the other metatarsals. 

 This I find to be the case in none of Prof. Mall's embryos. I do, how- 

 ever, agree with Schomburg that the metatarsal bones become well 

 differentiated before the tarsals. When the metatarsals and phalanges 



' See note 1, Table A. p. 277. 



