EXPLANATION OF PLATE II. 



F'ig. 3. Photomicrograph of cephalic extremity of another section of the five 



millimetei- embryo from which Figures 1 and 2 were made. 

 Fig. 4. Photomicrograph of the region under consideration from still another 

 section of the five millimeter embryo. This section is in the med- 

 ian plane. 



N. Nest of mesoblastic cells. 

 S. P. Seesel's pockel. 

 H. C. Hypophysis cerebri. 

 I. Infundibulum. 

 P. I. Processus infundibuli. 

 Not. Notochord. 



