PLATE I. 



All figures magnified 100 diameters, and oriented on the plate with the chief 

 axis vertical and the anterior pole towards the top. 



Figure 1 A portion of the blastodermic vesicle containing the germinal disk. 

 The posterior overgrowth is marked, and the marginal elevations 

 and median depression are apparent. 



" 2. A later stage than the preceding, with the posterior and the lateral 



overgrowths fused at two points. 

 " 3. One continuous overgrowth or bridge, with no trace remaining of the 



fusion of parts. 



" 4. A later stage showing a more extensive bridge with a lateral opening 

 at the right, where the lateral and the posterior overgrowths have 

 not completely fused. 



" 5. A germinal disk with ovate outline and highly developed bridge. 

 The broad end of the disk is anterior. 



" 6. Enlarged disk, ovate, with no separate bridge. Sections show that 

 the bridge has fused with the ectoderm of the disk. 



