114 



EMBRYOLOGY 



25 



3+ 



46 



ANIMAL HEMISPHERE 



VEGETAL HEMISPHERE 



GRAY CRESCENT 



/f\ 



FIRST CLEAVAGE 

 2 CELLS 



SECOND CLEAVAGE 

 4 CELLS 



THIRD CLEAVAGE 

 CELLS 



FOURTH CLEAVAGE IN 

 PROGRESS 

 2 TO 16 CELLS 



Fig. 55. The developing egg of Rana 

 sylvatica from the unfertilized egg to the 

 fourth cleavage. The fourth cleavage is com- 

 plete in the animal cells but not in the vegetal 

 cells. The animal pole is uppermost in all 

 drawings, and each drawing is identified by 

 stage number (st. no.) and the time in hours 

 at 18° C. elapsed from fertilization. Size 

 does not increase significantly during these 

 early stages. The black animal hemisphere is 

 represented by heavy stippling, the light- 

 colored yolk region is very lightly stippled. 

 At stage 2, one hour after fertilization, the 

 gray crescent is shown as an intermediate re- 

 gion between the animal and vegetal hemi- 

 sphere at the left. (From Pollister and 

 Moore, Anatomical Record (1937), 68, 

 489.) 



opposite side of the egg. At about 2.5 hours after fertilization the first cleav- 

 age is completed and the plane of this cleavage passes through the animal- 

 vegetal axis. A few hours after fertilization a furrow or groove appears on 

 the upper surface of the egg,. The furrow cuts across the animal pole of the 

 egg and becomes deeper and deeper. At the same time the furrow extends 

 from the animal pole along a meridian of the egg until it is completed at the 



