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SMITHSONIAN CONTRIBUTIONS TO KNOWLEDGE 



VOL. 35 



preceding it was noticed that a structural variation had already appeared in 

 the latter bone. The irregular meshes seen in the former were considerably 

 elongated. They had assumed canal shapes and were arranged concentrically. 

 Basic bone substance, with round lacunas and bushy canaliculi, formed the 

 reticulum between the elongated meshes or canals. Furthermore, a difference 

 in structure was noticed between the posterior and remaining wall. In the pos- 

 terior wall the concentric arrangement of the canals, described above, was 

 absent, and a general direction of the canals from the medullary canal toward 

 the external surface of the posterior wall was assumed. Here and there in the 

 bone substance between the canals an Haversian canal of the (la) differentia- 

 tion appeared. This whole posterior structure was the beginning of the linea 

 aspera, and seemed to be a distinct bone formation. 



In still later femora (four or five months) the diameters of the shaft were 

 3.5 mm. X 2.5 mm., and those of the medullary canal were 0.6 mm. X 0.5 mm. 

 The medullary canal was situated eccentrically. The section was composed of 

 basic bone substance, enclosing wide, branching, concentric canals, giving the 

 appearance of a very primitive second type formation. The posterior wall 

 was more prominent, and composed of bone substance with round and oval 

 lacunas, enclosing long, wide, branching canals, directed toward the external 

 surface and at right angles to the structures of the lateral wall. The distinc- 

 tion between the posterior and lateral walls was more pronounced than in the 

 specimens of earlier femora (pi. 21, fig. 301). 



In still older femora (five to seven months) a distinct difference between 

 the posterior wall, which now appears as a ridge, and the remaining walls was 

 observed. The diameters of the bone were 3.5 mm. X 3 mm., and those of the 

 medullary canal 0.5 mm. X 0.5 mm. The canal was situated eccentrically. The 

 bone was composed of concentric lamime arranged in the shape of a horseshoe, 

 enclosing, in part, the medullary canal. The toe of the shoe formed the anterior 

 wall and the heel embraced the posterior ridge. The posterior ridge was com- 

 posed of elongated Haversian systems, as they appeared in cross-section, with 

 large, wide Haversian canals extending outward toward the external posterior 

 surface and at right angles to the laniinas of the remaining wall. In cross- 

 section the posterior ridge was wedge-shaped and clearly distinct. The laminas 

 of the remaining wall were incompletely differentiated. They were wide, and 

 composed of bone substance with round and oval lacunas and bushy canaliculi 

 (pi. 21, fig. 302). 



In the femora of the final stages of fetal development (eight to nine months) 

 the diameters of the bone were 4.5 mm. X 5 mm., of the canal 1 mm. X 1 mm. In 

 these bones the lateral diameter was longest. The medullary canal was irregu- 

 lar in shape, larger, and situated eccentrically. The bone was composed of 

 elongated, concentric Haversian systems (in cross-section) arranged in horse- 



