94 



OSTEOLOGY 



chondrium and the cartilage, by the intramembranous mode of ossification just 

 described. There are then, in this first stage of ossification, two processes going 

 on simultaneously: in the center of the cartilage the formation of a number of 

 oblong spaces, formed of calcified matrix and containing the withered cartilage 

 cells, and on the surface of the cartilage the formation of a layer of true mem- 

 brane bone. The second stage consists in the prolongation into the cartilage of 

 processes of the deeper or osteogenetic layer of the perichondrium, which has 

 now become periosteum (Fig. 79, ir). The processes consist of bloodvessels and 

 cells osteoblasts, or bone-formers, and osteoclasts, or bone-destroyers. The latter 

 are similar to the giant cells (myeloplaxes) found in marrow, and they excavate 

 passages through the new-formed bony layer by absorption, and pass through 

 it into the calcified matrix (Fig. 80). Wherever these processes come in con- 

 tact with the calcified w r alls of the primary areolse they absorb them, and thus 

 cause a fusion of the original cavities and the formation of larger spaces, which 

 are termed the secondary areolse or medullary spaces. These secondary spaces 

 become filled with embryonic marrow, consisting of osteoblasts and vessels, derived, 



in the manner described above, from the 

 osteogenetic layer of the periosteum (Fig. 80). 

 Thus far there has been traced the forma- 

 tion of enlarged spaces (secondary areolse), 

 the perforated walls of which are still formed 

 by calcified cartilage matrix, containing an 

 embryonic marrow derived from the processes 

 sent in from the osteogenetic layer of the 

 periosteum, and consisting of bloodvessels and 

 osteoblasts. The walls of these secondary 

 areolse are at this time of only inconsiderable 

 thickness, but they become thickened by the 

 deposition of layers of true bone on their sur- 

 face. This process takes place in the follow- 

 ing manner: Some of the osteoblasts of the 

 embryonic marrow, after undergoing rapid 

 division, arrange themselves as an epithelioid 

 layer on the surface of the wall of the space 

 (Fig. 81). This layer of osteoblasts forms a 

 bony stratum, and thus the w r all of the space 

 becomes gradually covered with a layer of 



FIQ. 80. Part of a longitudinal section of 

 the developing femur of a rabbit, a. Flattened 

 cartilage cells, b. Enlarged cartilage cells, c, 

 d. Newly formed bone. e. Osteoblasts. /. 

 Giant cells or osteoclasts. g, h. Shrunken 

 cartilage cells. (From "Atlas of Histology," 

 Klein and Noble Smith.) 



Osteoblasts 



FIG. 81. Osteoblasts and osteoclasts on trabecula of lower jaw of 

 calf embryo. (Kolliker.) 



true osseous substance in which some of the bone-forming cells are included as 

 bone corpuscles. The next stage in the process consists in the removal of these 

 primary bone spicules by the osteoclasts. One of these giant cells may be found 

 lying in a Howship's foveola at the free end of each spicule. The removal of the 



