50 THE AMPHIOXUS. 



of cells, so that those which are situated at the upper 

 pole are the smallest. 



The segmentation cavity becomes considerably 

 larger, owing to separation of the cells, while the 

 cells at the upper and the lower pole begin to close 

 together in such a manner that the segmentation 

 cavity which was formerly open is at once completely 

 closed, as can be seen from Fig. 11. 



CESSATION OF THE PKOCESS OF DIVISION AT THE 

 LOWER POLE (Fia. 12). 



The next division, which we have to consider, is 

 the first which does not concern all the cells of the 

 embryo but only a part of the same. What happens 

 is that all the cells of the three upper tiers are 

 simultaneously divided, separating as they do through 

 meridional planes of division, each of them into 

 two equal parts. Thereby the number of the cells 

 in each of the three upper tiers is increased from 

 eight to sixteen, while the fourth and lowest keeps 

 its former number of eight. 



Through this cessation in the division on the part 

 of the lowest tier of cells, the difference in respect 



