NO. 3 



COMPARATIVE HISTOLOGY OF FEMUR FOOTE 



35 



round, and a few are indeterminate. In the elliptical sections the lateral 

 diameters are longest. The medullary canals are full of marrow, and in one 

 femur, Amblystoma tigrinum, the canal is occupied by cancellous bone. The 

 medullary index varies from zero to 129%, with an average of 36.6%. The type 

 of structure is principally first; the basic or undifferentiated bone substance is 

 found in Amblystoma tigrinum, the most primitive of amphibians, the lamel- 

 lated or differentiated bone in the majority of the remaining species; while 

 the II and III, la differentiations occur in the toads. The lacunae are round, 

 oval, or long, and the canaliculi are short and bushy or long and straight. The 

 structure may be uniform throughout the whole section, or it may present a 

 twofold division, as seen in Hyla arenicolor ; or a threefold division, as seen in 

 Necturus. 



On the whole, then, the amphibian femora show the first type bone, and 

 also mark the beginning of the second and third types. 



Detailed Examination 

 femora of rana catesbiana. bull frog 



The femora of four bull frogs were examined, the first unusually large, the 

 second of medium size, the third and fourth small. 



They showed different developments of the same type of bone (pi. 1, 

 figs. 1-4). 



RIGHT FEMUR OF RANA CATESBIANA (LARGE). FIRST BULL FROG. 

 CREIGHTON MEDICAL COLLEGE 



Pl. I, Fig. 1. Synoptic Table T 



Antero posterior diameter of bone, 3.5 mm.; lateral, 4.5 mm. 

 Antero-posterior diameter of medullary canal, 1 mm.; lateral, 2 mm. 

 The medullary canal is full. Medullary index, 16%. 



The section is surrounded by a narrow ring of external circumferential 

 lamella\ Their lacunas are round and oval, their canaliculi are short and bushy, 

 and all are poorly developed. The central ring, situated between the external 

 and internal lamellae, is interrupted by many large, bush-like, radiating canals. 

 The lamellae are indistinct, their lacunae are round and oval, and the canaliculi 

 communicate with the radiating canals. 



The canals are just visible to the naked eye. Some of them extend from 

 the internal to the external circumferential lamellae, some about two-thirds 

 of that distance, and some are interrupted at various points along the way. 

 The central ring forms about four-fifths of the thickness of the bone, is thicker 

 in the posterior half than in the anterior, and presents a low development. 



