16 SMITHSONIAN CONTRIBUTIONS TO KNOWLEDGE VOL. 35 



III, Ic. — This stage of Haversian development may possibly occupy a transi- 

 tional position between the early and late differentiations, although it does not 

 resemble either one very closely. It differs from botli in the intricate character 

 of the canaliculi, and in some respects suggests as early a develoijment as that 

 seen in (la) or (lb). The Haversian canal is regular in shape and small in 

 size. The lacunae have assumed a definite concentric arrangement around the 

 canal, and, with their bushy, branching canaliculi forming a delicate network, 

 occupy the clear area of bone substance referred to under (la) and (lb). The 

 structure has taken on the definite form of an Haversian system and presents 

 a peculiar dull character by which it may be identified anywhere (pi. A, fig. M). 

 This stage was observed in birds only and is easily recognized. 



in, C. — This is the completely differentiated Haversian system and belongs 

 to the higher mammals and man. In this stage the dim characters of the in- 

 completely differentiated systems have disappeared. The outlines and struc- 

 tural units of the Haversian systems are clearly distinct. The canaliculi are 

 slightly wavy and parallel (pi. A, fig. N). 



Such systems were not found below the mammals nor in the lower mam- 

 mals, such as the monotremes, marsupials, and some of the edentates and 

 Chiroptera, but were always present in the higher mammals and adult man. 

 Therefore it seems safe to regard tliis form as the Haversian system of the 

 latest differentiation. 



The Haversian system in early or late stages of differentiation, appears 

 also in all classes of animals ; but there are wide and often characteristic dif- 

 ferences in the variety and extent of such bone. In the amphibians it is merely 

 an outline, but in its extension through the reptiles, birds, and mammals to man, 

 it presents phases of an advancing differentiation. The lower the class the 

 earlier the stage, and the higher the class of animals the more complete the 

 stage of development of the Haversian systems. The development seems to 

 be one of a transformation of the first bone type into a third by some ijlau of 

 arrangement, since a complete Haversian system is composed of concentric 

 lamellas enclosing an Haversian canal. The earlier stages are generally found 

 in all classes of animals in which the first type bone is present as a character- 

 istic structure, and the later stages are found more especially in those classes 

 in which the first type is disappearing or has disappeared.' 



" In addition to the ordinary method of Haversian system development one or two other peculiar 

 forms were observed. In a fractured femur of a frog the upper and lower fragments were united by 

 new bone formed around them. The new bone was cancellous or channeled in character and In some 

 of the meshes lamellEe were concentrically deposited until the meshes were filled with the exception 

 of small canals in their centers. The results, at least in appearance, were Haversian systems tpl. 

 A, fig. O; pi. I, figs. 5, C, E, F). Such results were rather unexpected since there was no structure 

 in the original femur which in the least resembled an Haversian system or even cancellous bone. 



Another peculiarity was observed in some second type (laminar) femora, as those of the domestic 

 pig and wild boar. In these bones small, circular enlargements of the concentric canals between the 



