116 THE FROG CHAP. 



from one another and 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 mineral 

 matter — calcium phosphate and carbonate — is deposited. 

 In microscopic examination we may therefore investigate 





V II., 35. — Transverse section of dry femnr i.f frog. 

 . c.annliculi ; Ic. lacunar ; /;;/. lamella; ; in. marrow cavity. 



either the mineral matter by examining dried bone, or the 

 animal matter by examining decalcified bone. 



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

 shows that it is formed of very numerous thin layers or 

 lamellM (Fig. 35, Im), surrounding and concentric with the 

 marrow cavity. The lamelke contain numerous cavities, 

 the lacuna {Ic), with delicate, branching lubes, the ainalkiili 



