APP.] EMBRYO OF THE FOURTH DAY. 259 



V. Examination of an Embryo of the Fourth Day. 



A. Opening the egg, as in II. A. 



Great care will be required not to injure the blas- 

 toderm, which now lies close to the shell-membrane. 



B. Examination in situ. Observe : — 



1. The now conspicuous amnion. 



2. The allantois, a small, and as yet hardly vascular 

 vesicle, beginning to project from the embryo into 

 the space between the true and the false amnion. 



3. The rapidly narrowing somatic stalk. 



C. Removal of the embryo, as in II. C. and IV. C. 



The remarks made in the latter place apply with 

 still greater force to an embryo of the fourth and 

 succeeding days. 



D. Surface view of the transparent embryo. For 

 manipulation, vide IV. D. 



The points to be observed are : — 



1. The formation of the fifth, seventh, and ninth cranial 

 nerves. 



To observe these, a small amount of pressure is 

 advantageous. 



2. The formation of the fourth visceral cleft, and the 

 increase in size of the superior maxillary process. 



3. The formation of the nasal pits and grooves. 



4. The great relative growth of the cerebral lobes and 

 the formation of the pineal gland from the roof 

 of the vesicle of the third ventricle. 



5. The great increase in the investing mass. 



6. The formation and growth of the muscle-plates, 

 which can now be easily seen from the exterior. 



7. The allantois. Make out its position and mode of 

 opening into the alimentary canal. 



17—2 



