324 LESLIE B. AREY 



continuous at both ends, but especially at the right, with 

 osteoblasts. 



According to these observations, therefore, the osteoclast 

 arises from depleted osteoblasts which have first formed a syn- 

 cytium before being transformed into the oxyphilic osteoclast. 

 As bone resorption continues, osteoblasts progressively lose 

 their former relation to the bone, come into association with the 

 advancing osteoclasts and are incorporated into them. If the 

 spicule in figure 11 were resorbed it is believed that the simul- 

 taneously advancing (thigmotactic?) osteoclast would take up 

 the osteoblasts in its path. In a similar manner, smaller osteo- 

 clasts may, by fusion, merge into larger ones. 



But the osteoblasts, as such, are not the only source from 

 which the nuclear and cytoplasmic contents of the osteoclasts 

 are recruited. Bone cells, embedded in the matrix, are laid 

 bare by the resorptive processes and are ingested by the 

 oncoming osteoclasts. All intermediates may be found between 

 the initial and final stages of inclusion. At the left of figure 4 a 

 cytoplasmic process of an osteoclast is in contact with the capsule 

 of a bone cell which is otherwise embedded in the bone matrix. 

 Two succeeding stages appear in figure 5; on the right the area 

 of contact is extensive; at the left the bone cell is half within 

 and half without the osteoclast. Figure 11, from a human 

 fetus, and figure 10 show similar steps, as do text figures A 

 and B. A last stage appears in figure 12, at the right. 



Furthermore, bone cells are enclosed normally within a cap- 

 sule which is known to be resistant, for example, to the action 

 of strong hydrochloric acid. Encapsulated and distinctly 

 stellate cells, which resemble bone cells identically, are 

 occasionally found, embedded in the osteoclastic cytoplasm 

 (fig. 13; compare also figs. 2, 6, and 12). Such cells are inter- 

 preted as bone cells whose capsules have resisted cytoplasmic 

 digestion. From the relative infrequency with which such 

 persistent capsules are seen, it is probable that the enclosed 

 bone cells are eventually liberated. Ingested bone cells must 

 contribute in substantial numbers to the formation of osteoclasts. 

 This is perhaps especially true on flat resorption surfaces/ 



