436 BONE, TEETH 



solution has progressed to the proper stage, nothing but the 

 bone will take up the stain. If the clearing has not been com- 

 plete enough the muscles and other tissues take the stain almost 

 as readily as the bone itself. Following the staining, the tissues 

 are placed in Mall's solution, water 79 parts, glycerin 20 parts, 

 and potash 1 part. 



When properly cleared they are passed up through increasing 

 concentrations of glycerin and stored in pure glycerin. 



926. Dawson's Method for the Extraction of Fat from Embryos 

 Prior to Clearing by the Potash Method. In specimens prepared 

 by the potash method the fat in the superficial and muscular 

 fasciae is partially saponified and appears in the cleared material 

 as opaque white masses which often prove a serious impediment 

 to accurate observation. 



This difficulty may be obviated by the extraction of the fat 

 prior to beginning the treatment with KOH. Of the several 

 common fat solvents tried, acetone was found to be most satis- 

 factory. It acts quickly and does not injure the tissue or affect 

 its clearing and staining qualities. 



After the material has been fixed in 95 per cent, alcohol it is 

 transferred directly to acetone and left there for several days, or 

 longer, depending on the bulk of tissue being treated. Following 

 this, the specimen is transferred directly back to 95 per cent, 

 alcohol for twenty-four hours. After washing in 95 per cent, 

 alcohol the clearing and staining are carried out as outlined 

 above. 



927. Osteoblasts in Vitro. J. C. Hill {Arch. f. exper. Zellf., 

 1936) used the methods described in Chapters XXVIII and XXX. 



