EARLY CLEAVAGES 



115 



Fig. 56. A salamander egg, showing the first cleavage furrow. The egg at the 

 left is viewed from the animal hemisphere. The white spot on the surface of the 

 egg is very near to the animal pole. The vegetal hemisphere is not visible. At the 

 right the two-cell stage is shown in the same relative position. The furrow passes 

 exactly through the animal pole. (Courtesy of Professor John A. Moore, Co- 

 lumbia University.) 



vegetal pole. This cleavage furrow, by constricting the egg, finally meets in 

 the center, and two complete cells of equal size result. The two cells do not 

 separate, but remain in close contact. Figure 56 shows the uncleaved egg of 

 the salamander, together with a two-cell stage. 



The first cleavage is technically described as (l) holoblastic — i.e., there 

 is complete division of the egg, as opposed to partial division, or meroblastic 

 cleavage; (2) equal — i.e., the resulting cells are of equal size, as contrasted 

 with unequal cleavage, in which one cell is larger than the other; (3) 

 meridional — i.e., the cleavage furrow forms along a meridian of the egg, as 

 opposed to horizontal cleavage, where the furrow forms along the equator or 

 a parallel of the egg. 



The second cleavage occurs about one half hour after the first cleavage, 

 and the first two blastomeres divide simultaneously. The second cleavage 

 furrow forms at right angles to the first and also passes through the animal 

 pole. Thus the second cleavage may be described as holoblastic, equal, and 

 meridional. Looking down upon the animal pole, we note that the first and 

 second cleavage furrows form a cross (Fig. 57). The point at which the 

 furrows intersect is the animal pole. Figure 55 shows a side view of the 

 four-cell stage of the frog egg. 



