442 PRACTICAL DIRECTIONS. [APP. 



2. The fold of the splanchnopleure marking the hind 

 limit of the gut ; the vitelline veins running along 

 its wings. 



3. The mesoblastic somites on each side of the neural 



canal behind the heart; farther back still, the ver- 

 tebral plates not divided into somites. 



F. The examination of the embryo as an opaque 

 object. 



This should never be omitted. Many points in 

 the transparent embryo only become intelligible after 

 the examination of it as an opaque object. 



Having removed the putty ring and coverslip, if 

 previously used, allow the blastoderm so far to be- 

 come dry, that its edge adheres to the glass slide. 

 Care must of course be taken that the embryo itself 

 does not become at all dry. Place the glass slide 

 with the blastoderm extended flat on it, in a shallow 

 vessel containing a solution of picric acid (I. B.). 



If the blastoderm be simply immersed by itself in 

 the picric 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 embryo thus hardened and rendered opaque 

 by immersion in the acid (a stay of 2 to 3 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, imder 

 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 



