260 PRACTICAL DIRECTIONS. [APP 



F. The embryo as an opaque object. Manipulation 

 as II. F. For mode of examination vide 

 IV. F. 



The view of the mouth from underneath, shewing 

 the nasal pit and grooves, the superior and inferior 

 maxillary processes and the other visceral folds and 

 clefts, is very instructive at this stage. Compare 

 Fig. 48. 



G. Sections. Manipulation as in II. G. 



A slightly stronger solution of chromic acid may 

 be used than for the younger embryos. 



The student will most probably find that he can 

 still stain the embryo as a whole, especially if he 

 employs hematoxylin. If this cannot be done, the 

 sections must be stained individually after being cut. 

 The most important sections are, 



1. Through the eyes. 



2. Transverse section immediately behind the visceral 

 arches, shewing the origin of the lungs. 



o. Transverse section just in front of the umbilical 

 stalk, shewing the origin of the liver. 



4. Transverse section at about the centre of the dorsal 

 region, to shew the general features of the fourth 

 day. Compare Fig. 47. 



Amongst the points to be noticed in this section, are 



a. Muscle-plates. 



b. Spinal nerves and ganglia. 



c. Wolffian duct and bodies. 



d. Mliller's duct. 



e. Mesentery. 



f. Commencing changes in the spinal cord. 



5. Section passing through the opening of the allantois 

 into the alimentary canal. 



For the points to be observed in embryos of the 

 fifth and sixth days, the student must consult the 

 chapters devoted to those days. 



