52 THE AMPHIOXVS. 



cells in equatorial division. In this example can be 

 noticed the remarkable regularity with which all 

 the cells of a tier are simultaneously affected by 

 the division. We see all the cells of this tier 

 taking a biscuit-like form : 



During the equatorial division of a tier, the 

 whole embryo takes a form elongated in the chief 

 axis. The purpose of this is that it shall return to 

 the spherical form in which, after completion of the 

 division, the products again lie closer to each other. 



In Fig. 14, in which we see represented the same 

 stage in optical section, we may observe the segmen- 

 tation cavity already considerably increased. 



In this stage I was still generally able to see the 

 polar body at the upper pole. 



FURTHER INCREASE OF THE CELLS, AND DISAPPEAR- 

 ANCE OF THE REGULAR TIERS (Fias. 15-18). 



With the appearance of new meridional divisions 

 there is lost the hitherto regular arrangement of the 

 cells in tiers. I could still, indeed, in individual 

 cases observe stages with more than ten tiers, 

 of which the lowest still counted eight cells, while 

 most of the others, at any rate the lower and middle 



