64 THE ESSENTIALS OF HISTOLOGY. 



LESSON XIV. 



DEVELOPMENT OF BONE. 



1. MOUNT in Canada balsam a section of a foetal lower jaw which has been 

 stained in bulk with magenta or hsematoxylin and embedded in paraffin. 1 

 Find the part where the lower jaw-bone is becoming ossified, and carefully 

 study the appearance which it presents. The bone is prolonged in the form 

 of osteogenic fibres which are covered with osteoblasts. 



2. Intramembranous ossification may also be studied in the parietal bone 

 of a foetus which has been preserved in Miiller's fluid. A piece of the growing 

 edge is scraped or brushed free from its investing membranes, and from most 

 of the cells which cover and conceal it, and is mounted in glycerine with or 

 without previous staining with carmine. 



3. Mount in Canada balsam sections of a foetal limb which has been stained 

 with magenta. The bones will be found in different stages of ossification, 

 those of the digits being least developed. Make sketches illustrating the 

 three chief stages of endochondral ossification. Notice the peculiar ter- 

 minal ossification of the third phalanx. 



4. Make with a sharp scalpel a longitudinal section at the line of ossifica- 

 tion in a more advanced bone which has not been decalcified. Other sections 

 may be carried across a bone near its plane of ossification, and others through 

 an epiphysis. These sections will show the mode of progress of the calcification. 

 The sections can be mounted in glycerine. 



True bone is essentially formed in all cases by an ossification of 

 connective tissue. Sometimes the bone is preceded by cartilage, which 

 first becomes calcified, and this is then invaded, and for the most part 

 removed, by an embryonic tissue which re-deposits bony matter in 

 the interior of the cartilage, whilst at the same time layers of bone are 

 being formed outside underneath the periosteum. This is intracar- 

 tilaginous or . endochondral ossification. Sometimes the bone is not pre- 

 ceded by cartilage, and then the only process which occurs is one 

 corresponding to the subperiosteal ossification of the former variety ; 

 the ossification is then known as intramembranous. 



Ossification of cartilage. This may be described as occurring in 

 three stages. In the first stage the cells in the middle of the cartilage 

 become enlarged and arranged in rows radiating from the centre 



1 For the methods of staining and embedding 1 and 3, see Appendix, ' Embedding 

 in Paraffin.' 



