466 PRACTICAL DIRECTIONS. [APP. 



3. The commencement of the area vasculosa around 

 the hind end of the area. This is seen better 

 after treatment with picric acid. 



B. Treatment and Examination of the embryo. 



a. Preserve the vesicle in picric see I. B. 1. 

 Stain in haematoxylin, cut out the embryonic 

 area, leaving a considerable margin, imbed and 

 cut into sections. 



b. In transverse sections observe : 



1. At the anterior end of the area the single row of 

 columnar epiblast and the single row of flattened 

 hypoblast cells. 



2. Immediately in front of the primitive streak be- 

 tween these two layers a few irregularly shaped 

 mesoblast cells. 



3. Through the middle of the primitive streak, 



a. Several ] ay ers of rounded mesoblast cells attached 

 to, and continuous with, the epiblast in the 

 middle line, and stretching out laterally beyond 

 the edge of the area. 



b. A single layer of flattened hypoblast. 



4. The epiblast outside the embryonic area in the 

 form of flattened cells and, except in the region 

 around the primitive streak, overlying a layer of 

 flattened hypoblast. 



XV. Examination of an eight days 7 embryo. 



A. To obtain the embryo. 



The uterus will be found here and there to be 

 swollen. In these swellings the embryos lie; and 



