48 DEVELOPMENT OF BONE. 



stroy the parent membrane and attain a greater size (J^-Q of an inch) than 

 the parent cell, each cell being, as in the previous case, separated to a 

 slight extent from its neighbour by intercellular substance. By one other 

 repetition of the same process, each cell producing four, or five, or six 

 young cells, a cluster is formed, containing from thirty to fifty cells. These 

 clusters lie in immediate relation with the punctum ossificationis ; they are 

 oval in figure (about ^io m length by - 5 ^-<j in breadth), and placed in the 

 direction of the longitudinal axis of the bone. The cells composing the 

 cluster lie transversely with regard to its axis. In the first instance they 

 are closely compressed, but by degrees are parted by a thin layer of inter- 

 cellular substance, and each cluster is separated from neighbouring clus- 

 ters by a broader layer (3-0*5^ of an inch) of intercellular substance. Such 

 are the changes which occur in cartilage preparatory to the formation of 

 bone. 



Ossification is accomplished by the formation of very fine and delicate 

 fibres within the intercellular substance : this process commences at the 

 punctum ossificationis, and extends from that point through every part of 

 the bone, in a longitudinal direction in long, and in a radiated manner in 

 flat bones. Starting from the punctum ossificationis, the fibres embrace 

 each cluster of cells, and then send branches between the individual cells 

 of each group. In this manner the network, characteristic of bone, is 

 formed, while the cells by their conjunction constitute the permanent 

 areolae and Haversian canals. With a high magnifying power, the deli- 

 cate ossific fibres here alluded to are seen themselves to be composed of 

 minute cells having an elliptical form and central nuclei. These cells 

 attract into their interior the calcareous salts of the blood, and their nuclei 

 become developed, as I believe, into the future corpuscles of Purkinje. 

 It is possible also that some of the cartilage cells become corpuscles of 

 Purkinje in the fully developed bone. 



During the progress of the phenomena above described, the contents 

 of the cells undergo certain changes. At first, their contents are transpa- 

 rent, then they become granular, and still later opaque, from the presence 

 of amorphous matter mingled with nuclei, nucleoli, and the remains of 

 secondary cells. In the latter state they also contain an abundance of 

 minute oil-globules. These latter increase in size as the ossific changes 

 advance, and in the newly formed osseous areolee they are very numerous 

 and have attained the ordinary size of adipose cells. 



Cartilaginification is complete in the human embryo at about the sixth 

 week ; and the first point of ossification is observed in the clavicle at 

 about the seventh week. Ossification commences at the centre, and 

 thence proceeds towards the surface ; in flat bones the osseous tissue ra- 

 diates between two membranes from a central point towards the periphery, 

 in short bones from a centre towards the circumference, and in long bones 

 from a central portion, diaphysis, towards a secondary centre, epiphysis, 

 situated at each extremity. Large processes, as the trochanters, are pro- 

 vided with a distinct centre of development, which is named apophysis. 



breadth. To the right of the figure osseous fibres are seen occupying the intercellulai 

 spaces, at first bounding the clusters laterally, then splitting them longitudinally, and 

 encircling each separate cell. The greater opacity of the right-hand border is due to a 

 threefold cause, the increase of osseous fibres, the opacity of the contents of the cells, 

 and the multiplication of oil-globules. In the lower part of the figure some attemp 

 has been made to show the texture of the cells. 



