Charles \l. Bnrdccii 279 



C. 



THE DEVELOPMENT OF THE SKELETON OF THE POSTERIOR LIMB. 



One of the most studied subjects in morphology has been the develop- 

 ment of the vertebrate limbs. Fortunately critical summaries of its 

 immense literature have recently been given by several keen investigators, 

 among whom may be mentioned Wiedersheim, 92, Mollier, 93, 95, 97, 

 Gegenbaur, 98, 01, Klaatsch, 00, Eabl, 01, Fiirbringcr, 02, Kuge, 02, and 

 Braus, 04. Therefore no attempt will here be made to review previous 

 Avork except in so far as it deals directly with the development of the 

 human limb. 



During the third week of embryonic life the limb-buds become filled 

 with a vascular mesenchyme (Bardeen and Lewis, 01, p. 17, Figs. 18 and 

 19). The source of this tissue is uncertain. In part it may come from 

 the primitive body-segments, but it seems probable that in the main it 

 comes from the parietal layer of the unsegmented mesoderm. 



During the fourth week the mesenchyme increases in amount and the 

 limb-bud begins to protrude further from the body-wall. Observed 

 structural differentiation does not, however, begin until the early part of 

 the fifth week, at the time when the lumbo-sacral spinal nerves are 

 beginning to form a plexus. At this period the tissue at the center of 

 the base of the limb becomes greatly condensed. Embryo CLXIII, length 

 9 mm.. Fig. 45, Plate XI. The boundaries of the mass are not perfectly 

 definite, but a wax-plate reconstruction based upon drawings made as 

 definite as possible gives rise to the structure shown in Fig. 2, Plate II. 

 The relations of this tissue mass to other structures are shown in Plate 

 II, Fig. B, and Plate III, Fig. C, of the article by Bardeen and Lewis, 01. 

 The condensation represents the acetabulum and the proximal end of the 

 femur. This is indicated by its relations to the nerve plexus. 



Once begun skeletal differentiation proceeds rapidly. In Embryo CIX, 

 length 11 mm.. Figs. 3 and 4, it may be seen that from the original center 

 of skeletal formation the condensation of tissue has extended both dis- 

 tally and proximally, but much more rapidly in the former than in the 

 latter direction. Distally the sclero-blastema shows femur, tibia, fibula, 

 and a foot-plate; proximally, an iliac, a public and an ischial process. 

 A series of sections through the skeletal mass (Figs. 46 to 53) shows that 

 in the femur, tibia and fibula chondrofication has begun. At the centers 

 of the blastema of the ilium, ischium and pubis a still earlier stage of 

 chondrofication has made its appearance. The leg of this embryo, there- 

 fore, represents a stage of transition from the blastemal to the chondro- 

 genous stage of development. Fig. 55, Plate XII, shows a longitudinal 



