444 PKACTICAL DIRECTIONS. [APP. 



G. The following transverse sections will perhaps be 

 the most ifistmctive. 



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, mesoblaatic somites and commencing cleavage 

 of the mesoblast. 



6. 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 transverse sections. 



T fl. Examination of an Embryo of about 48—50 hours. 

 A. Opening the egg — as in II. A. 

 R. Examination of the blastoderm in situ. 

 Observe 



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

 tinct than at the earlier stage. 



2. The beating of the lieart. 



3. Tlie general features of the circuliUion. 



