EMBRYOLOGY OF THE SEA LAMPREY 



119 



Figure 11. — Polar views of stage 8, full blastula, arranged to illustrate changes in cell size. 



just prior to this stage were consistently about 1.0 

 millimeter across as viewed and measured in 

 optical section perpendicular to the animal- 

 vegetal axis as well as along that axis. Subse- 

 quent measurements prior to the appearance of the 

 dorsal lip of the blastopore, the end-point of 

 stage 8, ranged from 1.2 to 1.4 millimeters. The 

 increase in volume is accompanied by translucency 

 of the animal-pole cells so that the outline of the 

 blastocoel becomes visible through approximately 

 one-third the surface of the embryo. The size of 

 the animal cells does not change during the 

 expansion process; the increase of the blastocoel 

 indicates that the expansion is due to blastocoel 

 enlargement. The blastocoel is at its greatest 

 volume at the end of stage 8. The animal cells 

 are still about one-half the size of the vegetal cells. 

 This size relation is maintained as long as the 

 epiboliznig animal cells can be compared externally 

 with the underlj'hig vegetal cells. 



The end-point of stage 8 is the appearance of the 

 blastopore. 



Stage 9: Gastrula (fig. 12) hours 64-104 



Size: 1.2 to 1.4 niillinieters decreasing to 1 .f) 

 millimeter. 



Animal hemisphere: Translucent to transparent 



Figure 12. — Posterior view of stage 9, gastrula, illus- 

 trating the blastopore. 



to opaque. Opacity extends progressively forward 

 from blastopore. Begins to flatten, forming 

 neural plate. 



