The Development of the Vertebrate Embryo 55 



Cleavage performs at least three functions in embryonic develop- 

 ment: 



1. It provides an adequate number of cells, i.e., building blocks for 

 the future organization of tissues and organs. 



2. It lays the groundwork for the gross design of the embryo ( dorsal- 

 ventral axis, anterior-posterior axis, etc. ) by shifting around the material 

 (both cytoplasm and yolk) of the egg and compartmentalizing this ma- 

 terial within separate cells. 



3. It brings nuclear and cytoplasmic materials into balance. The 

 egg starts as a cell with a single nucleus and an enormous amount of cyto- 

 plasm. During cleavage, total cytoplasmic content does not change mark- 

 edly but thousands of nuclei appear as cell division proceeds. 



The pattern of cleavage is determined in large part by the amount 

 of yolk in the egg. In eggs that contain a little yolk that is homogeneously 

 distributed, cleavages are equal; that is, the daughter cells are always the 

 same size. Figure 27a shows this type of cleavage in Amphioxus. Eggs 

 with a large amount of inhomogeneously distributed yolk cleave un- 

 equally. Figure 27b illustrates this type of cleavage in the frog. The yolk 

 in the frog egg is concentrated in one hemisphere ( the vegetal ) , and de- 

 creases in a sharp gradient toward the opposite ( animal ) hemisphere. The 

 first two cleavages split the frog egg longitudinally to produce four cells 

 shaped like the segments of an orange. The third cleavage is transverse 

 and separates the yolky ( vegetal ) half from the non-yolky ( animal ) half. 

 The split is unequal, since the four animal blastomeres are much smaller 

 than the four vegetal ones. Subsequently, the yolky cells cleave far more 

 slowly than the non-yolky ones. The result is that in the completed 

 blastula, the dorsal part contains many small cells while the ventral part 

 contains fewer but larger cells. In fish and birds, the extreme of yolkiness 

 is reached. The cytoplasm and nucleus of the egg sit atop a huge ball of 

 inert yolk. The yolky area does not cleave at all and only the little cap on 

 top divides into cells ( Fig. 27c ) . 



The specific appearance of cleaving cells is much the same in all 

 embryos. A groove (called the cleavage furrow) appears at one point of 

 the egg. For example, in the frog the first furrow appears at the animal 

 pole. The furrow then deepens and extends downward on both sides. The 

 two ends meet at the vegetal pole. The furrow then extends inward ra- 

 dially, finally constricting the egg into two sister blastomeres. 



In most cases, as the cells cleave, a cavity appears in the middle of 

 the ball of cells. This is the blastular cavity. Thus, by the end of the cleav- 

 age stage, the embryo is a spherical or flattened hollow ball generally one 

 cell thick. The blastomeres can vary in size, yolk content, and cytoplasmic 

 organization, but no tissues and certainly no organs yet exist. 



