NO. 3 COMPARATIVE HISTOLOGY OF FEMUK 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, Aniblystoma tigrinmn, the canal is occupied by cancellous bone. The 

 medullary index varies from zero to 1297<7, with an average of 36.6%. The type 

 of structure is principally i3rst ; the basic or undifferentiated bone substance is 

 found in Aniblystoma 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 lacunar 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 Eyla 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 

 femoha 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 



Tl. 1, Fig. 1. Synoptic Table I 



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



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



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



The section is surrounded by a narrow ring of external circumferential 

 lamella;. Their lacunaj are round and oval, their canaliculi are short and bushy, 

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

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

 The lamella? are indistinct, their lacunse 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 lamella^, 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 in-esents a low development. 



