zsro. 3 



COMPARATIVE HISTOLOGY OF FEMUR FOOTE 



19 



the separation of the uniformly lamellar wall into two and three concentric 

 divisions, the presence of lamina? in Bufo americaua and Hyla gratiosa, and the 

 appearance of Haversian canals of the (la) differentiation in the Surinam and 

 other toads (pis. 2-3, figs. 6-39). 



REPTILES 



The reptiles seem to be divided in the matter of bone differentiation into 

 two groups — the division occurring between the lizards and the turtles. The 

 first group, which includes the lizards, is characterized by the first type bone 

 structure and the second, including the turtles, by the first and third. Belonging 

 to the lizard group in their type of bone were the femora of the Python regius, 

 which were composed of crude first type structure with a twofold division. 

 The femur of the alligator represents a later differentiation than that of the 

 lizards and snakes, inasmuch as the second and third types have appeared. 

 Both of these units have reached a later degree of development than they have 

 in any femur preceding them in the zoological scale, while they are not as far 

 advanced as they are in the turtles which follow them. Comparing- the class 

 of reptiles with that of amphibians the chief evidences of type advancement 

 are seen in the more complete differentiations of the third type units (pis. 3, 4, 

 figs. 40-73). 



BIRDS 



The study of the femora of birds is, to some extent, unsatisfactory. While 

 the three types of bone structure are present, either alone or in combination, 

 they generally bear more or less of a transitional character. The several units 

 — lamella 1 , lamina 1 , and Haversian systems — show, usually, incompleteness of 

 development. In some instances, it is difficult to recognize any distinct type. On 

 the whole, however, evidences of advancing differentiation are prominent in 

 birds, since all three units have appeared and generally in a more complete form 

 than in the femora of reptiles or amphibians. 



The first type of bone (lamellar) is present in some species, as in the robin, 

 and is of a very simple form. The second (laminar) appears in a larger num- 

 ber of species, as in the turkey, grouse, and ostrich, and is, perhaps, the most 

 representative type among birds. The laminae, generally, show an incomplete 

 differentiation. The third type (Haversian systems) is found in several species, 

 and presents distinct characters by which they can be distinguished from the 

 Haversian systems of other animals. 



In some cases Haversian systems are present in the posterior ridges, in 

 others, of larger size, in the posterior ridges and anterior walls, and in still 

 others they form the entire central ring (pis. 5, 6, 7, figs. 74-112). 



