PLATE 1 



EXPLANATION OF FIGURES 



7 Photograph of the median plane in the floor of the brain at the plica enceph- 

 ali ventral s. The section is nearly medial throughout. The floor plate is on 

 the left, extending to the fovea isthmi. Compare with figure 3. Calf, 23 mm. 

 X 37i. 



8 Photograph of the floor of the brain of the shark, Squalus acanthias, 40 mm. 

 length, at the fovea isthmi. Only the region included between the two lines is 

 median. Note the sudden cessation of the floor-plate, its characteirstic structure 

 in comparison with the floor of the midbrain. Compare with figure 1 and also 

 with figure 7. X 67|. 



9 Photograph of the floor of the brain of the chick, seven and one-half days' 

 incubation, at the fovea isthmi. Only the region included between the two lines 

 is median. The cephalic side, as in figure 2, is toward the left. X 67§. 



10 Photograph of a section cutting horizontally the floor of the brain at the 

 plica encephali ventralis through the summit of the included mesenchyme (i.e., 

 the fossa interpeduncularis). Calf, 20 mm. (Cornell Univ. series C.21). X 371. 



11 Photograph as in figure 10, but seventeen sections (170/t) farther dorsal, 

 where 'C marks the upper end of the floor plate. In figures 10 and 11 the ceph- 

 alic side is up. X 37|. 



134 



