ONTOGENY 



247 



pole, cutting the planes of the first two at right angles and for 

 that reason is called equatorial. The 8 blastomeres formed at the 

 end of the third cleavage consist then of an upper quartet of cells 

 about the animal pole smaller in size than the four cells of the 

 lower half of the egg. From this point on the rate of cleavage is 

 retarded in cells located in the vegetative region of the egg. 



Fig. 152. — Cleavage in the frog's egg. 1,2-cell stage, 1st cleavage plane bisect- 

 ing gray crescent, c. 2. 8-cell stage. 3. 8-cell passing into 16-cell stage. 4. 

 32-cell stage. 5-6. Later cleavage stages. 7. Section of egg in early stage of 

 blastocoel, b, or segmentation cavity; approximate position of gray crescent 

 material shown by stippling. 8. Section of later blastula; i, point where invag- 

 ination takes place. 



Since the dark pigmented cells formed of animal-pole material 

 divide more rapidly, they soon become smaller in size than 

 those containing yolk. The cleavage thus becomes unequally 

 holoblastic. 



Blastula. — A section of a developing egg at the end of the third 

 cleavage shows that the inner surfaces of the 8 blastomeres are 

 rounded off so as to produce a space or cavity between them. 

 This space is called the segmentation cavity or blastocoel and marks 



