144 THE AMPHIOXUS. 



dorsal, a ventral and a middle, the latter containing 

 the large flattened vacuoles (cp. Figs. 131-134). 



The vacuoles extend so much that there remain 

 between them only thin perpendicular walls of separ- 

 ation. These are the notochord plates, which through 

 the thickening of their outer layer appear with a 

 sharp contour. 



On the dorsal and ventral cell series the cell 

 boundaries and granules are to be plainly distin- 

 guished. 



In the middle cell series, penetrated by large 

 vacuoles, which represents the peculiar and charac- 

 teristic notochord tissue, the cell boundaries are no 

 more to be seen. The cell granules too become there 

 less distinguishable, still they may be seen by means 

 of proper reagents. 



The histological differentiation of the notochord 

 may quite well be followed on complete stained pre- 

 parations of the stages following one upon the other. 

 For the thorough comprehension of the formations 

 before us, transverse sections also are necessary. A 

 further examination too is of great importance for the 

 control of results so gained, namely, the examination 

 of the posterior end of the notochord of older embryos 



