NO. 3 COMPARATIVE HISTOLOGY OF FEMUK FOOTE 151 



Haversiau systems of the ridge are somewhat better developed than in the three 

 months' fetus. 



Type II-III, lb. 



EIGHT FEMUR OF A WHITE FETUS, FIVE TO SEVEN MONTHS. NO. 248045, U. S. NAT. MUS. 



Pl. si, Fig. 302. Syn. Tab. VIII 



Antero-posterior diameter of bone, 3.5 mm.; lateral, 3 mm. 



Antero-posterior diameter of medullary canal, 0.5 mm.; lateral, 0.5 mm. 



The medullary canal is very small and occupies a very eccentric position. 

 Medullary index, 2%. 



Structure. — The section is composed of a wide horseshoe baud of long and 

 short laminfe arranged around the medullary canal. The toe forms the very 

 narrow anterior wall and the heel embraces the very wide posterior ridge. The 

 laminae have central canals and are composed of lamellae with oval lacunas 

 and bushy canaliculi. They widen and shorten as they approach the posterior 

 ridge and on either side of it they have become Haversian systems of the (lb) 

 differentiation. The posterior ridge is composed of long and short spaces sur- 

 rounded by lamellae extending from the external to the medullary surface. The 

 section exhibits the early formation of laminae, the horseshoe arrangement, the 

 formation of Haversian systems, and the later formation of the posterior ridge. 

 Two laminae with round and oval lacunae and short bushy canaliculi surround 

 the medullary canal. 



Type II-III, lb. 



RIGHT FEMUR OF A WHITE FETUS, EIGHT TO NINE MONTHS. NO. 228842, 



U. S. NAT. MUS. 



Pl. 21, Fig. 303. Syn. Tab. VIII 



Antero-posterior diameter of bone, 4.5 mm. ; lateral, 5 mm. 



Antero-posterior diameter of medullary canal, 1 mm. ; lateral, 1 mm. 



The medullary canal is irregular in shape and is situated in the anterior 

 half of the microsection. Medullary index, 4%. 



Structure. — Around the outside of the section — posterior ridge excepted— 

 is a narrow band of incompletely developed lamellae with oval lacunae and short, 

 bushy canaliculi. The remainder of the anterior and lateral wall is composed 

 of a wide, horseshoe-shaped band enclosing the medullary canal. It is composed 

 of long lamina with central canals gradually shortening and widening as they 

 pass around the lateral wall until they are transformed into oval Haversian 

 systems close to the posterior ridge. The posterior ridge is composed of 

 elongated, crude Haversian systems of the (lb) differentiation, and short la- 

 mina extending from the external to the medullary surface and at right angles 

 to the laminae of the lateral wall. In the center of the ridge is a narrow space 

 n 



