ANIMAL BIOLOGY. [Part I. 



of the cells, which then tend to break down and atrophy. Mean- 

 while the tissue becomes vascularized by the introduction of 

 blood-vessels from the vascular perichondrium. The channels 

 occupied by these vessels, uniting with the enlarged and coales- 

 cent cell cavities, convert the tissue into a vascular sponge-work, 

 which becomes calcified, and thereby strengthened by the de- 

 position of granules of calcareous matter in the remaining 

 strands or trabeculse of cartilage. 



So far we have merely calcified cartilage, not true bone. 

 But along the edges of the primary marrow cavities, as they 

 are termed, of the sponge-work, a number of osteoblast cells 

 may now be seen. Each secretes, and may be seen to be sur- 

 rounded by, a droplet of osteogen, from which true bone is 

 formed ; and as this true bone replaces the cartilage, the tissue 

 is converted into " spongy endochondral bone." 



Meanwhile, around the blood-vessels of and beneath the peri- 

 chondrium, osteoblasts are developed, which here, too, form 

 spongy bone. The perichondrium thus becomes periosteum ; 

 and the spongy bone developed in this region is termed periosteal 

 spongy bone. 



If now we transfer our attention to the centre of the shaft, 

 we shall find that the spongy endochondral bone is there being 

 absorbed or eaten away through the instrumentality of special- 

 ized cells (osteoclasts), some of which are very large (giant cells). 

 The spongy bone in the centre of the shaft is thus completely 

 absorbed, and a marrow cavity produced. 



Around this cavity the two processes bone formation by 

 osteoblasts, bone absorption by osteoclasts may be seen going 

 on side by side. Large spaces or marrow cavities are excavated, 

 and are then gradually filled in with bone, or narrowed to the 

 dimensions of a small canal for the passage of a blood-vessel. 

 And thus are formed embryonic Haversian systems, which 

 differ from those of the completed bone in their greater irregu- 

 larity, and the much looser texture of the lamellae. 



Finally, we have to note that beneath the periosteum a 

 delicate tube of bone forms round the middle of the shaft, and 



