92 



CLEAVAGE 



The third, fourth, and fifth cleavages from the animal pole and lateral views. 



lar pattern in the undisturbed egg, and may appear even as a well- 

 defined furrow before the third (horizontal) cleavage has reached its 

 maximum depth. The cleavage rate is accelerated with each of the 

 early divisions, and, since the blastomeres are of unequal size and 

 have varying amounts of cytoplasm and yolk, synchronous cleavage is 

 lost and there is an obvious overlapping of the divisions. The upper- 

 most cells (micromeres) divide the most rapidly so that for a period 

 there may be a 12-cell stage, made up of 8 upper and 4 lower blas- 

 tomeres. Perfect symmetry in cleavage and in the blastomeres from 

 this point on is a rarity, and yet the embryo will develop perfectly 

 normally. 



The fifth cleavage (also double), which should provide a 32-cell 

 embryo, is more apt to be out of line, and the symmetry and regularity 

 of the earlier cleavages is lost. The gradient of cleavage rate from the 

 rapidly dividing animal hemisphere blastomeres to the slowly dividing 

 vegetal hemisphere cells now becomes more apparent. These new 

 furrows tend to be horizontal (latitudinal), as was the third cleavage. 

 One cuts the animal hemisphere blastomeres and the other the vegetal 

 hemisphere blastomeres horizontally, so that tiers of blastomeres are 

 formed. The smallest cells, which contain the most pigment, are now 

 found at the animal pole, and the largest blastomeres with no pigment 

 and the most yolk are at the vegetal pole. There is evident, however, 

 some movement of the surface (cortical) pigment in the direction of 

 the vegetal pole accompanying each division of the blastomeres. This 

 process is known as epiboly, to be described later in this book. 



