474 K. W. YOUNG 



Supraoilal Staiiiitig 



Calvaria only were used. Immediately upon sacrifice, the calvaria 

 were placed in a solution of 0.01 per cent neutral red in 0.9 per cent 

 saline at 37 °C for 50 minutes, then were fixed in 1 per cent mercuric 

 chloride for at least 1 hour. The preparations were examined with 

 the dissecting microscope, intact and unmounted. 



Microradiography with Uhrasoft X-Ratjs 



Bones were fixed in neutral formalin and embedded in paraffin. 

 Sections were floated onto Kodak spectroscopic plates (649-0) ac- 

 cording to the technique of Greulich ( 1960 ) . Exposure to the ultra- 

 soft (wavelength > 10 A) polychromatic x-rays varied from 45 to 

 50 minutes at 1.5 kv and 1 ma (copper target). The plates were 

 developed in Dektol (Eastman Kodak) for 5 minutes at 20°C. Sec- 

 tions were floated off the plates before development, remounted on 

 glass slides, and stained with hematoxylin and eosin, or with PAS- 

 hematoxylin. 



Microradiography with Soft X-Rays 



Bones were fixed in absolute ethanol and embedded without de- 

 calcification in methyl methacrylate. Sections were cut on a rotary 

 saw at 250 microns, and ground by hand on abrasive paper to about 

 15 microns. The sections were then placed on Kodak spectroscopic 

 plates (649-0) and exposed to soft (wavelength < 10 A) poly- 

 chromatic x-rays for 10 to 12 minutes at 12 kv, 19 ma ( copper target 

 and beryllium filter) at a tube-to-film distance of 15 cm. The ex- 

 posed plates were developed in Dektol for 5 minutes at room tem- 

 perature. Sections were examined separately in the phase contrast 

 microscope without removal of the embedding medium. 



Autoradiography 



Animals were injected intraperitoneally at 6 days of age with 5 

 fic/gm body weight of glycine-H'^ ( New England Nuclear Corpora- 

 tion, sp. act. 194 mc/mmole) in 0.1 ml of isotonic saline, and sac- 

 rificed at various intervals after injection. Autoradiograms of sections 



