180 



STRUCTURE AND ENDOWMENTS OF ANIMAL TISSUES. 



distinct centres of ossification, but which early unite in Man, remain 

 permanently distinct in the lower Fishes ; the division of the frontal 

 bone, just adverted to, is constant amongst Fishes and Reptiles ; and in 

 these classes we meet with a permanent separation of the parts of the 

 occipital and temporal bones, which, being formed from distinct centres 

 of ossification, are at first distinct in the higher animals. 



301. During the formation of the punctum ossificationis, and the 

 spread of the vessels into the cartilaginous matrix, certain changes are 

 taking place in the substance of the latter, preparatory to its conversion 

 into bone. Instead of single isolated cells, or groups of two, three, or 

 four, such as we have seen to be characteristic of ordinary Cartilage 

 ( 267), we find, as we approach the ossifying centre, clusters made tip 

 of a larger number, which appear to be formed by a continuance of the 

 same multiplying process as that already described (Fig. 50). And 



Fig. 50. 



Section of Cartilage, near the seat of ossification ; each single cell haying given birth to four, five, or six 

 cells, which form clusters. These clusters become larger towards the right of the figure, and their cells 

 more numerous and larger ; their long diameter being l-1500th of an inch. 



when we pass still nearer, we see that these clusters are composed of a 

 yet greater number of cells, which are arranged in long rows, whose 

 direction corresponds with the longitudinal axis of the bone ; these clus- 

 ters are still separated by intercellular substance, and it is in this, that 

 the ossific matter is first deposited (Fig. 51). Thus if we separate the 



Fig. 51. 



The same cartilage at the seat of ossification: the clusters of cells are arranged in columns; the intercel- 

 lular spaces between the columns being l-3250th of an inch in breadth. To the right of the figure, osseous 

 e seen occupying the intercellular 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 multipli- 

 cation of oil-globules. 



cartilaginous and the osseous substance at this period, we find that the 

 ends of the rows of cartilage-cells are received into deep narrow cups of 



