4 8 THE AMPHIOXUS. 



into a smaller part situated at the upper pole, and 

 a greater at the lower. 



Simultaneously with this first equatorial furrow 

 there takes place a difference of size in the segmen- 

 tation sphere, and a clearer distinction between the 

 upper and lower poles results. The segmentation 

 cavity still remains wide open at the upper and 

 lower pole, as is to be seen in Fig. 8. 



SIXTEEN-CELLED STAGE (Fia. 9). 



The next division which again takes place with- 

 in a quarter of an hour concerns all the eight cells 

 simultaneously ; each one of them divides by means 

 of a meridional furrow into two equal parts, so that 

 we see an upper tier of eight smaller cells and a 

 lower of eight larger ones. 



This stage arises, as we see, through the simul- 

 taneous appearance of four new meridional segmen- 

 tation planes. 



So long as the cells immediately after the division 

 still possess a more or less spherical form, it should 

 be noticed that the stages are considerably in- 

 creased in breadth and are depressed from the 

 upper to the lower pole. 



