NO. 3 COMPARATIVE HISTOLOGY OF FEMUK FOOTE 15 



row external aud iuterual sheaths of circumfereutial lamellae often surround 

 the bone aud its medullary cavity, but the characteristic structural units are 

 the Haversian systems. The type presents two stages of differentiation— the 

 incomplete (I) (pi. A, figs. K, L, M), and tlie complete (C) (pi. A, figs. N or 3; 

 pi. B, figs. 3, 6). The incomplete differentiation is further divisible into three 

 sub-stages which are indicated in the synoptic tables and text as the la, lb, and 

 Ic stages (pi. A, figs. K, L, M). These stages can be readily distinguished micro- 

 scopically, and may all occur in a single bone. As a matter of fact, it is often 

 impossible to include all Haversian systems of a bone under any one stage of 

 differentiation. Term (la) signifies early, (lb) intermediate, and (Ic) the late 

 stage of incomplete differentiation. When tlie process of differentiation is 

 completed, the Haversian system is referred to as (C) (pi. A, fig. N). 



Ill, la. — This was first observed in a few of the amphibians. In the femora 

 of these animals the system was merely a minute, more or less irregularly 

 shaped, canal, extending in a direction parallel with the long axis of the bone. 

 In cross-section the canal was round, oval, or irregular in shape and surrounded 

 by a small, clear, more or less circular area of bone substance, across which 

 were seen a few caualiculi on tlieir way from neighboring lacunse. That is, 

 they were not concentrically arranged aud appeared to be independent of the 

 canal with the exception of a slight connection by means of the caualiculi. The 

 figTire presented merely a suggestion of an Haversian system. This, stage of 

 development was found in all classes of animals from amphibians to man. Its 

 characteristic structure may be seen by referring to plate A, figure K. 



As one studies the many sections in which this stage is found it is clearly 

 evident that it is associated with the earlier periods of development both phylo- 

 and ontogenetically, since the Haversian canal alone is present, and this occurs 

 in the lamellar areas of the bone. There seems to be no particular relation of 

 the canal to adjacent lacunae. 



in, Ih. — This stage represents a morphological advancement characterized 

 by a more definite relation of neighboring lacunae to the Haversian canal, and 

 by more extensive communication of the bone cells of the lacunae with the Ha- 

 versian circulation. The result is that the Haversian system is fairly well out- 

 lined. As this stage is but a step in advance of (la) it might be expected to 

 appear in any femur in which (la) is present, and to a certain extent this is 

 true. It was observed in all classes of animals excepting amphibians and birds 

 (pL A, fig. L). It w^as first seen in some of the reptiles, such as the turtles. 

 In these femora the canal was surrounded by a clear, circular area of bone 

 substance, and the lacunae were arranged concentrically around the circum- 

 ference of the area. There was a noticeable increase of the caualiculi passing 

 from the lacunae to the canal. The whole figure presented a circular form and 

 the Haversian system could be clearly distinguished. 



