1880.] Ossification of the Terminal Phalanges of the Digits. 67 



of it is discoverable in the specimen drawn in fig. 1, in which the 

 cartilaginons diaphysis has received its final development, and is 

 already in process of being destroyed. That this structure makes its 

 first appearance at a very late stage in the development of the phalanx, 

 is shown by the fact that it had not yet begun to be formed in another 

 cat embryo 8 centims. long. An examination of the process of 

 growth in the long tapering ungual phalanges of certain seals might 

 show that for the greater portion of their length they were formed 

 entirely from the ungual expansion of the subperiosteal cap, and had 

 never pre-existed as cartilage. Illustrations of the simple outline of the 

 primitive cartilaginous diaphysis, and of the variety of the forms that 

 may be assumed by its bony superstructure, are afforded by figs. 2, 3, 

 in neither of which has any absorption of the cartilage yet taken place. 

 In fig. 3, taken from a foetal pig, the young phalanx, which is destined 

 to support a hoof, is seen to have its subperiosteal cap greatly enlarged 

 on the palmar aspect. 



As an example of the process in birds, the terminal phalanges of the 

 second and third digits of the manus, and of all the digits of the pes, 

 were examined in a young sparrow. The calcification of the cartilage 

 and its attendant changes, and the formation of the subperiosteal cap, 

 were seen to be starting from the tip, and proceeding in exactly the 

 same manner as already described ; specimens, however, to show the 

 invasion of the cartilage by osteoblastic tissue could not at that time 

 be conveniently obtained. 



Among reptiles, a young alligator showed the same processes with 

 great clearness ; but it was again impossible to procure a specimen 

 sufficiently advanced to exhibit the course of the osteoblastic irruption. 



An interesting modification of the process is furnished by the am- 

 phibia. Fig. 4 represents a section in the sagittal plane through the 

 last two phalanges of the middle digit of the fore-foot of the Proteus. 

 In this specimen, which there was every reason to suppose adult, no 

 irruption whatever of osteoblastic tissue has taken place in the last 

 two phalanges, which accordingly consist each of a core of partly 

 calcified cartilage ; in the case of the penultimate phalanx surrounded 

 by the hollow cylinder or dice-box of true bone already alluded to, in 

 the case of the terminal phalanx plunged into a deep receptacle, also 

 of true bone, which is in fact the subperiosteal cap.* Thus a state of 

 things which in the higher Yertebrata belongs only to a temporary 

 stage in embryonic development, is in the Proteus persistent through- 

 out life. The growth and calcification of the cartilage and the for- 

 mation of the subperiosteal layer of bone proceed, it may be assumed, 

 in the regular manner ; but having advanced so far the process stops, 



* From the only specimens procurable, no conclusions could be arrived at with, 

 regard to the condition of the proximal phalanx of the digit, or indeed of any other 

 of the bones. 



