444 PRACTICAL DIRECTIONS. [APP. 



G. The following transverse sections will perhaps be 

 the most instrnctive. 



Manipulation as in I. B. 3. 



1. Through the optic vesicles, shewing the optic 

 stalks. 



2. Through the hind-brain, shewing the auditory 

 sacs. 



3. Through the middle of the heart, shewing its re- 



lations to the splanchnopleure and alimentary canal. 



4. Through the point of divergence of the splanch- 

 nopleure folds, shewing the venous roots of the 

 heart. 



5. Through the dorsal region, shewing the medullary 

 canal, mesoblastic somites and commencing cleavage 

 of the mesoblast. 



G. Through a point where the medullary canal is still 

 open, shewing the mode in which its closing takes 

 place. 



Longitudinal sections should also be made and 

 compared with the titans verse sections. 



III. Examination of an Embryo of about 48 50 hours. 



A. Opening the egg as in II. A. 



B. Examination of the blastoderm in situ. 

 Observe 



1. The ybrm of the embryo, which is much more dis- 

 tinct than at the earlier stage. 



2. The heating of the heart. 



3. The general features of the circulation. 



