254 PRACTICAL DIRECTIONS. [APP. 



2. The now co-shaped heart; for further particulars 

 vide Chap. iv. § 18. 



3. The commencing 1st and 2nd visceral clefts and 

 the aortic arches. 



4. The circulation of the yolk sac, vide Fig. 23. Make 

 out all the points there shewn and ascertain 

 by examination that what have been called the 

 veins and arteries in that figure, are truly such. 



F. The embryo as an opaque object. 

 Treatment as in II. F. 



From above : 



Observe the amnion, which is a very conspicuous 

 object, and remove it with needles if not done pre- 

 viously. The external form of the brain and the 

 auditory sac appear very distinctly. 



From below: 



Observe the nature of the head- and tail-folds, 

 which are much more easily understood from the 

 opaque than from the transparent embryos. 



Observe also the alimentary canal, the widely open 

 hind end of the fore-gut, and the front end of the as 

 yet very short hind-gut. 



G. Sections. 



Manipulation as in II. G. The embryo can still 

 be stained as a whole. 



The more important sections to be preserved, are 

 1. Through optic lobes, shewing : 



a. The formation of the lens. 



b. The involution of the primary optic vesicle. 



c. The constriction, especially from above, of the 

 optic stalk. 



