328 LESLIE B. AREY 



do not arise in situ from the endothelium is supported by their 

 usual retrograde appearance; vacuolization, granulation and loss 

 of stainability of the cytoplasm, and pyknosis of the nuclei 

 occur. No stages have been observed which could be interpreted 

 as illustrative of an origin from the blood-vessel itself; on the 

 contrary, the condition appears to indicate a method of final 

 removal. This admission into embryonic vessels does not of 

 itself prove or imply an amoeboid activity by the osteoclast; 

 Meyer ('18) is wrong in assuming I hold such a belief. 



DISCUSSION 



The conclusions of Kolliker regarding the history and signifi- 

 cance of the osteoclasts have gained great prominence. It 

 should be kept clearly in mind, however, that his opinions were 

 almost wholly inferential. He neither offered direct proof of 

 the origin of osteoclasts nor of their fate, as has been pointed 

 out in the historical section. The apparent reasonableness of 

 these deductions, and the prestige of their originator, doubtless 

 account for their acceptance by numerous later investigators 

 and for their widespread inclusion in texts. 



Concerning the validity of many of the claims advanced by 

 other workers the writer can offer little except the negative 

 evidence of not having seen corroborative stages. Unfortu- 

 nately, in the past, too many dogmatic statements have been 

 made wholly unsupported by appropriate evidence. Only a 

 few have presented their claims adequately described and illus- 

 trated. Under these conditions it is not always easy to dis- 

 tinguish between surmises and conclusions drawn from actual 

 observations. 



Moreover, it seems reasonable to believe that several mor- 

 phologically similar but developmentally distinct elements 

 masquerade under the generic term osteoclast, so that, his- 

 torically, the controversy has not always been over identical 

 structures. The careful studies of Jackson ('04), Danchakoff 

 ('09, and Maximow ('10) appear to show convincingly that, at 

 least in the early stages of bone development, the osteoclasts 

 arise from the primitive connective tissue of the marrow. At 



