SEXUAL REPRODUCTION AND DEVELOPMENT OF INDIVIDUAL 213 



Cleavage is a series of mitotic cell divisions beginning in the 

 zygote immediately following its formation. These divisions occur 

 in rapid order with but very little intervening growth, and the 

 resulting cells adhere to each other in a body. In eggs where the 

 yolk material is scant and evenly distributed, the ensuing cleavage 

 divisions extend completely through tiie zygote, forming nearly equal 

 cells. If the yolk is concentrated in one end of the egg, the divisions 

 of the developing embryo are unequal. During the early divisions all 

 of the cells of the body divide at so nearly the same rate that it 

 appears as if the zygote were being cut with a knife or cleaver into 



s 



3- itS^'. ...... • ..•>Z\- 





Fig. 62. — Cleavage in the embryo of Asterias (starfish). 1, fertilized ovum 

 (zygote); 2, two-celled embryo following first cleavage division; 3, the four-cell 

 stage; J,, the eight-cell stage; 5, the sixteen-cell stage; 6/morula stage (solid); 

 7, blastula stage (hollow) ; 8, early gastfula stage (infolding of cell layer at one 

 side) ; 9, later stage of gastrulation. The infolded layer is the endoderm. (Drawn 

 by Titus C. Evans.) 



smaller parts. This process provides for the rapid increase in the 

 number of cells and growth of the embryo which is necessary before 

 any special parts can be formed. Cleavage has been described briefly 

 in an earlier chapter under the discussion of the development of 

 the frog. 



As divisions proceed, a Uastula is formed by the development of 

 a cavity (blastocoele) within the spherical mass of cells, the wall of 



