APP.] SURFACE VIEW. 443 



shadows of the embryo, with benefit. The watch- 

 glass should have a flat bottom; or a shallow flat 

 glass cell should be used instead. 



a. Looking at the embryo from above, observe : 



1. The head-fold ; the head distinctly projecting from 

 the plane of the blastoderm, and formed chiefly by 

 the forebrain and optic vesicles. 



2. The elevation of the medullary canal, and the 

 indications of the side walls of the embryo. 



3. The indications of the tail. 



4. The Amnion partly covering the head. Tear it 

 open with needles. Observe its two folds. 



b. Having turned the blastoderm upside down, 

 observe the following points, looking at the embryo 

 from below. 



1. The hinder limit of the splanchnopleure in the 

 head-fold, marking the hind limits of the fore- 

 gut. The opaque folds now conceal the head almost 

 entirely from view. 



2. The commencing tail-fold, and the shallow boat- 

 shaped cavity (of the alimentary canal) between it 

 and the head-fold. 



The student should not fail to make sketches 

 of the embryo, both as a transparent, and as an 

 opaque object, seen from below as well as from 

 above. These sketches will be of great service to 

 him when he comes to study the sections of the 

 same embryo. 



