258 PKACTICAL DIKECTIONS. [APP. 



In the meantime cut off the head of the hardened 

 embryo by a sharp clean transverse incision carried 

 just behind the visceral clefts, transfer it to the watch- 

 glass and cover it with water or spirit. By a little 

 manipulation the head of the embryo may now be 

 shifted into the small depression in the wax, and thus 

 be made to assume any required position. It should 

 then be examined with a simple lens under a strong 

 reflected light, and a drawing made of it. 



When the head is placed in the proper position, 

 the following points may easily be seen. 



1. The opening of the mouth bounded below by the 

 first pair of visceral folds, and commencing to be 

 enclosed above by the now very small buds which 

 are the rudiments of the superior maxillary pro- 

 cesses. Compare Fig. 38. 



2. The second and third visceral arches and clefts. 



3. The nasal pits. 



G. Sections. Manipulation as in II. G. 



The embryo can still be stained as a whole. 

 The most important sections are : — 



1. Through the eyes in the three planes, vide Fig. 

 30, D. E. F. 



2. Through the auditory sac. 



3. Through the dorsal region, shewing the general 

 changes which, have taken place. 



Amongst these, notice 



a. The changes of the protovertebrm: the commencing 

 formation of the muscle-plates. 



b. The position of the Wolffian duct and the forma- 

 tion of the germinal epithelium. 



c. The aortce and the cardinal veins. 



d. The great increase in depth and relative diminu- 

 tion in breadth of the section. 



