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. 



^, 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 



