122 THE AMPHIOXUS. 



ullary plate, which is already deepened like a channel, 

 shrinks together, so that we have the closing of the 

 channel which was originally open underneath the 

 epiblast. 



We will now follow this process in a single meso- 

 blastic somite. 



On comparing with one another sections from the 

 same region of the embryo, but of various stages, 

 we see that the neural plate grows considerably 

 smaller during the deeper incurvation. This diminu- 

 tion is partly attributable to the fact that the single 

 cells alter their form, while becoming columnar and 

 smaller ; otherwise this is effected through the elonga- 

 tion of the embryo. During this latter process a 

 movement of the material of the cells takes place ; for 

 we see that a smaller number of cells composes the 

 transverse section of the neural in the earlier, than 

 in the later stages. 



When the incurvation has reached to such a degree 

 by the edge cells of the neural plates advancing 

 more strongly towards the middle line, the closing 

 is at last completed by these cells elongating towards 

 the middle line, and there growing together (Figs. 

 116, 119). 



