The Development of the Frog 19 



small dark cells at the upper pole, and four 

 larger white cells at the lower pole ; that is to 

 say, the third plane is horizontal but nearer the 

 upper than the lower pole (Fig-. 5, B). 



The next division is by two vertical planes, 

 at right angles to each other and half-way 

 between the first two planes. Thus we have 



FIG. 5. VERTICAL SECTIONS OF SEGMENTING EGGS. 



A, 2-cell stage. , 8-cell stage. 



the egg made up of sixteen blastomeres (Fig. 

 i,E). 



The thirty-two-cell stage is formed by two 

 horizontal planes, one above and one below 

 the first horizontal plane (Fig. i, F). 



After the thirty-two-cell stage the segmenta- 

 tion proceeds so rapidly and so irregularly that 

 it cannot be followed with certainty, indeed 

 it is seldom that the processes above described 



