468 PRACTICAL DIRECTIONS. [APP. 



C. Examination of sections. 



Prepare and cut into transverse sections as advised 

 in XIV. B. 



Notice 



1. In the sections of the anterior region, 



a. The lateral epiblast composed of several layers 



of columnar cells. 

 h. The epiblast in the median line one layer thick 



and in the form of a groove (medullary groove). 



c. The lateral plates of mesoblast. 



d. The flattened lateral hypoblast, and columnar 

 hypoblast underlying medullary groove (noto- 

 chord). 



2. In sections through the anterior end of the primi- 

 tive streak. 



Note the continuation of the epiblast, mesoblast 

 and hypoblast in the middle line. 



3. In sections through the posterior end of the area 

 the same points to be seen as in XIV. B. h. 3. 



XVI. Examination of an embryo about 8 days 12 hours. 



A. Manipulation as in XV. A. 



B. In surface view observe (cf. Fig. 106) : 



1. Area pellucida surrounding embryo, outside which 

 is the well marked area vasculosa. 



2. Widely open neural canal, at anterior end dilated, 

 and partially divided into the three primary vesi- 

 cles of the brain : note the optic vesicles. At the 

 posterior end, the sinus rhomboidalis. 



3. The mesoblastic somites, 4 to 8. 



