AP APP.] OPAQUE EMBRYO. 449 



The hind-brain consists of a front portion, the 

 cerebellum with a thickened roof; and a hinder 

 portion, the fourth ventricle with a very thin and 

 delicate roof. 



3. Organs of sense. 



The eye especially is now in a very good state 

 to observe. The student may refer to Fig. 51, 

 and the description there given. 



The ear-vesicle will be seen either just closing 

 or completely closed. 



4. lu the region of the heart attention must also be 

 paid to : 



a. The visceral clefts. 



b. The investing-mass, i.e. the growth of mesoblast 

 taking place around the end of the notochord. 



c. The condition of the heart. 



5. In the region of the body the chief points to be 

 observed are : 



a. The increase in the number of the somites. 



b. The Wolffian duct, which can be seen as a streak 

 along the outer side of the hinder somites. 



c. The allantois, which is now a small vesicle lying 

 between the folds of the somatopleure and 

 splanchnopleure at the hind end of the body, but 

 as yet hardly projects beyond the body cavity. 



E. Zlfe embryo as an opaque object. 

 Preparation as in II. F. 



The general form of the embryo can be very satis- 

 factorily seen when it is hardened and examined as an 

 opaque object; but the most important points to be 



P. & B. 29 



