This is the second of a series describing normal stages in the development of Rana pipiens. I t s purpose is to afford a method of identifying from sectioned material the stages described in the first communication (Shumway, '40). Typical sections through carefully staged embryos are described briefly and illustrated by photomicrographs.
All embryos were fixed in Smith's ('15) frog fluid for 24 hours and washed in 5% formalin. The egg jelly was removed in a 10% solution of Clorox. a f t e r dehydration in successive concentrations of alcohol, the embryos were cleared in benzene and embedded in tissuemat. Serial sections were cut in each of the customary planes at a thickness of 20 microns.
Most sections were stained with Feulgen's nucleal reaction. The most satisfactory results were obtained by hydrolyzing 45 minutes and staining for 1 hour. After a thorough washing in sulfurous acid water and distilled water, they were counterstained with picro-indigocarmine, dehydrated, cleared and mounted in clarite. Figures 8, 9, 10, 11, 24, 25 are from slides stained in Delafield's hematoxylin and eosin.
Photographs were made (except 1 A) at an initial magnification of 14 X, using a Zeiss planachromat 3 X lens and a Phoku camera. Eastman Panatomic X cut film was employed With the technical assistance of George Svihla.