THE OSSEOUS SYSTEM. 



331 



of earthy salts and the metamorphosis of its cells. The primary point 

 of ossification having thus appeared, for instance in the parietal bone, 

 its growth advances simultaneously with the horizontal extension of the 

 membraniform blastema, in such a way that a delicate lamina composed 

 of reticulated osseous spicules is shortly produced, from which, slender 

 rays stretch out into the still unossified blastema (Fig. 135). If this 

 formation be examined more closely, it will be observed, that the indi- 

 vidual bone-spicules originate in the membranous blastema, by the 

 ossification of its elements, and, that to a certain extent, the latter is 

 absorbed in the spaces occupied by the spicules, remains of it being left in 

 the interstices ; and moreover, that the formation of the osseous elements 

 proceeds exactly in the same way, that it does in the periosteal layers ; the 



Fig. 135. 



Fig. 136. 



A 



..t 



rays of bone as they extend further into the soft blastema becoming 

 softer and paler, and containing less earthy matter, whilst their cells 

 become more and more like the soft formative cells, till, at last, the spi- 

 cules lose all distinct limitary outline, and are lost in the blastema. At 

 first, the growth of these bones proceeds in a superficial plane only, the 

 rays, as they extend and become connected by transverse branches con- 

 Fig. 135. Parietal bone of a fourteen-weeks' old foetus; magnified 18 diameters. 

 Fig. 136. From the inner surface of the parietal bone of a new-born child ; magnified 300 

 diameters: a, bone with lacunae, still pale-colored and soft; 6, border of the same; c, ossi- 

 fying blastema with its fibres and cells. jB, three of these cells, magnified 350 diameters. 



