116 THE FROG CHAP. 



binary fission, so that two cells are formed in one cell- 

 space : the two then gradually separate from one 

 another anl intercellular substance is formed between 

 them. In the embryo, this tissue consists entirely of 

 closely packed cells which gradually separate and form 

 a structureless matrix which is firm and elastic, and 

 which in some parts (pp. 46 and 48) may become calcified. 

 Bone. As we have already seen (p. 52) bone is formed 

 of two constituents, a basis of animal matter, in which 



FIG. 35. Transverse section of dry femur of Frog, (x about 125.) 

 c. canaliculi ; Ic. lacunas ; Int. lamella ; m. marrow-cavity. 



mineral matter calcium phosphate and carbonate is 

 deposited. In microscopic examination we may there- 

 fore investigate either the mineral matter by examining 

 dried bone, or the animal matter by examining decalci- 

 fied bone. 



A thin section of a dried long bone, such as the femur, 

 shows that it is formed of very numerous thin layers or 

 lamella (Fig. 35, Im), surrounding and concentric with 

 the marrow-cavity. The lamellae contain numerous 



