APP.] THE EMBRYO AS AN OPAQUE OBJECT. 245 



If the blastoderm be simply immersed by itself in 

 the chromic acid solution, the edges of the opaque 

 area will curl up and hide much of the embryo. The 

 method suggested above prevents these inconveni- 

 ences. 



The embr}'0 thus hardened and rendered opaque 

 by immersion in chromic acid, (a stay of 24 hours in 

 the solution will be sufficient) may be removed to a 

 watch-glass, containing either some of the solution, or 

 plain water, and examined with a simple lens, under 

 a strong direct light. The compound microscope will 

 be found not nearly so advantageous for this purpose 

 as the simple lens. A piece of black paper placed under 

 the watch-glass, will throw up the lights and shadows 

 of the embryo, with benefit. The watch-glass should 

 have a flat bottom; or a shallow flat glass cell should 

 be used instead. 



a. Looking at the embryo from above, observe : — 



1. The head-fold ; the head distinctly projecting from 

 the plane of the blastoderm, and formed chiefly by 

 the forebrain and optic vesicles. 



2. The elevation of the medullary canal, aud the 

 indications of the side walls of the embryo. 



3. The indications of the tail. 



4. The Amnion partly covering the head. Tear it open 

 with needles. Observe its two folds. 



b. Having turned the blastoderm upside down, 

 observe the following points, looking at the embryo 

 from below. 



1. The hinder limit of the splanchnopleure in the head- 

 fold, marking the hind limits of the fore-gut. The 

 opaque folds now conceal the head almost entirely 

 from view. 



2. The commencing tail-fold, and the shallow boat- 

 shaped cavity (of the alimentary canal) between it 

 and the head-fold. 



The student should not fail to make sketches of 



