244? PRACTICAL DIRECTIONS. [APP. 



8. Indications of the omphalo-mesaraic arteries. 



9. The as yet barely formed tail-fold. 



10. The commencing blood-vessels in the pellucid and 

 vascular areas. 



E. Surface view of the transparent embryo from 



below. 



The coverslip must now be removed and the glass 

 slide again immersed in a vessel of clean salt solution. 

 By gently seizing the extreme edge of the opaque 

 area with a pair of forceps, no difficulty will be found 

 in so floating the blastoderm, as to turn it upside 

 down, and thus to replace it on the slide with the 

 under surface uppermost. 



The points which most deserve attention in this 

 view, are : 



1. The heart : its position, its union with the omphalo- 

 mesaraic veins, its arterial end. 



2. The fold of the splanchnopleure marking the hind 

 limit of the gut ; the omphalo-mesaraic veins run- 

 ning along its wings. 



3. The protovertebrce on each side of the neural canal 

 behind the heart ; farther back still, the vertebral 

 plates not divided into protovertebrse. 



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 become \ 

 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 '1 or '5 per cent, solution of chromic acid. 



