ArP.] EMBRYO AT THE END OF THE THIRD DAY. 257 



The ear-vesicle will be seen either just closing 

 or completely closed. 



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

 paid to: 



a. The visceral clefts. 



h. The investing-viass, 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 protovertehrce. 



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

 along the outer side of the hinder protovertebra3. 



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

 between the folds of the soraatopleure and 

 splanchnopleure at the hind end of the body, but 

 as yet hardly projects beyond the body cavity. 



F. T/ie 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 

 made out at this stage in the hardened specimens are 

 those connected with the visceral clefts and folds and 

 the mouth. 



If the amnion has not been removed it will be 

 necessary to pick it completely away with needles. 

 Without further preparation a view of the visceral 

 folds and clefts may be obtained from the side ; but a 

 far more instructive view is that from below, in order 

 to gain which the following method may be adopted. 



Pour a small quantity of melted black wax (made 

 by mixing together lampblack and melted wax) into 

 a watch-glass, using just enough to cover the bottom 

 of the glass. While still soft make a small depression 

 in the wax with the rounded end of a pen-holder or 

 handle of a paint-brush and allow the w^ax to cool. 



E. 17 



