58 THE AMPHIOXUS. 



a surface of somewhat dark cells, which is easily dis- 

 tinguishable, and occupies about a third of the whole 

 extent. These cells are darker, as they contain a 

 greater number of yolk-granules, and therefore allow 

 the cell nuclei to show through less distinctly. This 

 surface now begins at once to flatten itself out (Fig. 

 21) and then to be indented, the purpose being that 

 the segmentation cavity should be obliterated and that 

 it should gradually become attached to the upper layer 

 formed of the smaller and clearer epiblast cells (Figs. 

 22, 23). The result of this process is a flat cap-like 

 two-layered stage in which no segmentation cavity 

 is noticeable, but rather only a sharp border line be- 

 tween epiblast and hypoblast (Fig. 24). 



If we compare with one another the stages from the 

 blastula up to this two-laj^ered, cap-shaped gastrula, 

 and especially if we regard the number and propor- 

 tionate sizes of the cells, we shall see that the lower 

 layer of cells, the true hypoblast, corresponds to but 

 little more than a third of the blastula. These cells, 

 however, have increased in size during the invagina- 

 tion process and simultaneously with the disappear- 

 ance of the segmentation cavity. This is only to be 

 explained by the fact that the hypoblast cells have 



