FIRST PERIOD OF DEVELOPMENT. 51 



of size between the cells of the lower and the upper 

 pole is further considerably emphasized. 



In the next place, from these eight large cells of 

 the lower pole eight smaller ones are separated, 

 which lie towards the upper pole, this again being 

 by equatorial division. These eight smaller cells 

 also divide through meridional division into a circle 

 of sixteen. \Ve have now a stage before us in which 

 we may count four upper tiers, eich of 16 cells, 

 and a lower one with eight large cells (Fig. 12). 



The lowest tier remains then without change 

 throughout a series of stages, composed of eight 

 large cells which considerably surpass all the other 

 cells of the embryo in size. 



FURTHER INCREASE OF THE TIERS OF CELLS THROUGH 

 A NUMBER OF EQUATORIAL DIVISIONS (Fias. 13, 14). 



There follows now at constantly shorter intervals 

 a series of equatorial fumnvs by which the number 

 of the tiers with sixteen cells is increased. 



We have, for example, represented in Fig. 13 a 

 stage of this period in which, besides the lower 

 tier with eight cells, five with sixteen are to be 

 counted. In one of the latter we see also all the 



