346 The Developn\ent of the Connective Tissues 



endoplasm increases greatly in quantity, and the bone substance is 

 either transformed exoplasm or is deposited into it. This latter pro- 

 cess is first marked by the fibrils of the exoplasm becoming sharper and 

 staining more intensely blue than before. Soon, however, the sub- 

 stance between the fibrils of the exoplasm takes up the blue stain, mak- 

 ing it appear as if the tissue were injected with a blue color. In fact I 

 thought for a long time that in this region there was an extravasation 

 which stained blue, until I recognized the osteoblasts in older stages. 

 Furthermore, the extravasation proved to be constant and took on the 

 characteristic bone stain when treated with hfematoxylin and eongo red. 

 The gradations in color from hferaatoxylin to congo red are in the order, 

 nuclei, endoplasm, exoplasm, and bone substance. 



The frontal and mandibular bones have increased in size in embryos 3 

 cm. long, the bone deposit is beginning to radiate into the surrounding 



^ ^ 2 ^-^ '^ ^ 







? 



Fig. 10. Fig. 11. 



Fig. 10. Section through the occipital cartilage of au embryo pig 20 mm. long 

 ( X 250 diameters). The ground substance is deposited in the exoplasm of the 

 syncytium. 



Fig. 11. Section through the frontal bone of a pig 30 mm. long ( x 250 diameters). 



exoplasm, and the osteoblasts are larger and more numerous along these 

 radiations. The bone radiations are still more pronounced in embryos 

 5 cm. long and are lost in the prefibrous tissue which is now arising 

 from the surrounding exoplasm. 



Most instructive specimens showing the - extension of membranous 

 bone as well as of the beginning of periosteal ossification are obtained 

 from embryos 5 cm. long, stained by Mallory's method. In such speci- 

 mens it is seen that the frontal bone is well started, with bone corpus- 

 cles imbedded within it, and osteoblasts encircling it. The bone sub- 

 stance with its radiations stains intensely blue, and it is seen that the 

 radiations, with the accompanying osteoblasts, extend into the mem- 

 branous skull and are gradually lost in the exoplasm and prefibrous 

 tissue. At the extreme tips of the bone radiations the scattering 

 fibrils are surrounded with osteoblasts. At the outer border of the 



