FIRST PERIOD OF DEVELOPMENT. 47 



is repeated after each division, so that they lie 

 against one another with the planes of division pushing 

 against each other in a cruciform manner at right 

 angles (Fig. 6). In the middle between the four 

 segmentation spheres there remains however, above 

 and below, at the upper and lower pole, an open hollow 

 space which forms the first indication of the segmen- 

 tation cavity. 1 The opposite position of the four cells 

 can be best seen in Fig. G, where this stage is seen 

 from the upper pole, and in Fig. 7, where it was repre- 

 sented from the side, and in such a way that one of 

 the cells is turned towards the spectator. The divi- 

 sion into four cells is concluded approximately after 

 the lapse of the second hour from the liberation of 

 the eggs. 



EIGHT-CELLED STAGE (FiG. 8). 



The segmentation process begins, from now on, to 

 advance with much greater speed, for after the 

 lapse of another quarter of an hour we find all the 

 cells again divided. 



The division of the four cells follows at the same 

 time, and in a common, in fact, equatorial plane of 

 division. Every one of the four spheres is divided 



1 Blastocoel. 



