TYPES OF CLEAVAGE 



293 



of the dark pigment, while in the lower portion of each blastomere the yellow- 

 white yolk is concentrated. As a rule, the substance of the gray crescent is 

 found in two of the blastomeres; the four blastomeres under the circumstances 

 are not qualitatively equal. 



The third or latitudinal cleavage plane is at right angles to both of the 

 foregoing and somewhat above the equator, dividing each of the four blas- 

 tomeres into an animal pole micromere and a larger vegetal pole macromere 



-ANIMAL POLE 



Fig. 141. Normal development of Rana sylvatica. (A) Egg at fertilization. (B) 

 Formation of gray crescent, sharply defined at one hour after sperm entrance. (C) First 

 cleavage furrow meridional. (D) Second cleavage furrow meridional. (E) Third cleav- 

 age furrows, latitudinal in position. Four micromeres above and four macromeres below. 

 (F) Fourth set of cleavage furrows, meridional in position, although some variation 

 may exist and vertical furrows may occur. (G-1) Later cleavage stages. Pigmented 

 pole cells become very small, and pigmented cells creep downward over vegetative pole 

 area. (J) Appearance of dorsal blastoporal lip. (K) Blastoporal lips spread laterally, 

 forming a broad. V-shaped structure. Pigmented cells proceed toward blastoporal lips. 

 (L) Yolk-plug stage of gastrulation. (After Pollister and Moore, '37.) 



