Osteoclasts of normal and ia rats were subjected to morphological and histochemical studies. The three-dimensional aspects of the morphology of these cells were studied by means of camera-lucida tracings of serial sections of osteoclasts from the region of the sockets of developing teeth. The nucleic acids in the osteoclasts were studied in sections cut through the alveolar region of five-day-old normal and five-day-old ia rats. The sections were stained by the Feulgen technique and with thionin.

1. No differences were observed in size and shape between the osteoclasts of normal and of ia animals.

2. The osteoclasts were found to be much larger and their cytoplasmic extensions much wider than has been previously suspected. In some areas, the osteoclasts formed a syncytium and large areas of their cytoplasm were found to be nucleus-free. Osteoclasts possess long processes which may or may not contain nuclei. Thus, in non-serial histological sections passing through the processes of these cells, the osteoclastic cells may be overlooked or mistaken for osteocytes or osteoblasts lying in areas of resorption.

3. The cytoplasm of the normal osteoclasts stained more intensely with thionin than did the cytoplasm of the ia cells. The possibility of decreased protein synthesis by ia osteoclasts is considered.

4. Nucleoli were present in the nuclei of both normal and ia osteoclasts with no apparent difference between the two.