DEVELOPMENT OF BONE. 5 



the ovum itself originally presented this simple form, and the embryo 

 at an early period is wholly composed of such nucleated cells. In their 

 relation to each other, cells may be isolated and independent, as is 

 exemplified in the corpuscles of the blood, chyle and lymph ; secondly, 

 they may cohere by their surfaces and borders, as in the epidermis 

 and epithelium ; thirdly, they may be connected by an intermediate 

 substance which is then termed intercellular, as in cartilage and bone ; 

 and fourthly, they may unite with each other in rows, and upon the 

 removal by liquefaction of the adherent surfaces be converted into 

 hollow tubuli. In the latter mode capillary vessels are formed, as 

 also are the tubuli of nerve and muscular fibre. One of the properties 

 of cells may also be adverted to in this place ; it is that of reproducing 

 similar cells in their interior. In this case the nucleoli become the 

 nuclei of the secondary cells, and as the latter increase in size, the 

 membrane of the primary or parent cell is lost. 



Bone, in its earliest state, is composed of an assemblage of these 

 minute cells, which are soft and transparent, and are disposed within 

 the embryo in the site of the future skeleton. From the resemblance 

 which the soft tissue bears to jelly, this has been termed the gelatinous 

 stage of osteo-genesis. As development advances, the cells, heretofore 

 loosely collected together, become separated by the interposition of a 

 transparent intercellular substance, which is at first fluid, but gradually 

 becomes hard and condensed. The cartilaginous stage of osteogenesis 

 is now established, and cartilage is shown to consist of a transparent 

 matrix, having minute cells disseminated at pretty equal distances and 

 without order through its structure. Coincident with the formation of 

 cartilage is the development of vascular canals in its substance, the 

 canals being formed by the union of the cells in rows, and the subse- 

 quent liquefaction of the adhering surfaces. The change which next 

 ensues is the concentration of the vascular canals towards some one 

 point ; for example, the centre of the shaft in a long, or the mid-point 

 of a flat bone, and here the punctum ossificationis or centre of ossifica- 

 tion is established. What determines the vascular concentration now 

 alluded to, is a question not easily solved, but that it takes place is 

 certain, and the vascular punctum is the most easily demonstrable of 

 all the phenomena of ossification. 



During the formation of the punctum ossificationis, changes begin to 

 be apparent in the cartilage cells. Originally they are simple nucleated 

 cells ( 3 O 'o 6 t 20*00 f an i ncn i n diameter), having a rounded form. 

 As growth proceeds, they become elongated in their figure, and it is 

 then perceived that each cell contains two and often three nucleoli 

 around which smaller cells are in progress of formation. If we ex- 

 amine them nearer to the punctum ossificationis we find that the young 

 or secondary cells have each attained the size of the parent cell (-^oW 

 of an inch), the membrane of the parent cell has disappeared, and the 

 young cells are separated to a short distance by freshly effused inter- 

 cellular substance. Nearer still to the punctum ossificationis a more 

 remarkable change has ensued, the energy of cellule reproduction has 



