THE STRUCTURE OF THE ELEMENTARY TISSUES. 



63' 



The 'cartilaginous rod which forms the foetal femur is sheathed in a 

 membrane termed the perichondrium, which so far resembles the peri- 

 osteum described above, as to consist of two layers, in the deeper one of 

 which spheroidal cells predominate and blood-vessels abound, while the 

 outer layer consists mainly of fusiform cells which are in the mature 

 tissue gradually transformed into fibres. Thus, the differences between 

 ihe foetal perichondrium and the periosteum of the adult are such as 



usually exist between the embry- 

 onic and mature forms of connec- 

 tive tissue. 



Between the hyaline cartilage 

 of which the foetal femur consists 

 and the bony tissue forming the 

 adult femur, there are two chief 

 intermediate stages viz. (1) of 





Jf&la 



r 



=>G3 



Fig. 66. 



Fig. 67. 



Fig. 66. Ossifying cartilage showing loops of blood-vessels. 



Fig. 67. Longitudinal section of ossifying cartilage from the humerus of a foetal sheep. Cal- 

 cified trabeculae are seen extending between the columns of cartilage cells, c, Cartilage cells. 

 X 140. (Sharpey.) 



calcified cartilage, and (2) of embryonic spongy bone. These ma- 

 terials, which successively occupy the place of the foetal cartilage, are 

 in succession entirely absorbed, and their place is taken by true bone. 

 The process by which the cartilaginous is transformed into the 



