CLEAVAGE. 



41 



Holoblastic Cleavage. 



(A) EQUAL. In this form of cleavage the entire egg divides and the cells 

 resulting from the early cell divisions are of approximately the same size. One 

 of the Echinoderms Synapta presents a beautiful example of this, the sim- 

 plest type of cleavage (Fig. 25). The egg of synapta is meiolecithal, contain- 

 ing very little yolk. The first cleavage is in a vertical plane at right angles to the 

 long axis of the central spindle and divides the egg into halves. The second 

 plane of cleavage is also vertical but is at right angles to the first cleavage plane 

 and results in four equal cells. The third cleavage plane is horizontal, cutting 

 the four cells resulting from the second cleavage into eight equal cells. The 

 fourth cleavage is vertical, the fifth horizontal and so on, regular alternation of 



FIG. 25. Cleavage of the ovum of Synapta (slightly schematized). Selenka, Wilson. 

 A-E, Successive cleavages to the 32-cell stage. F, Blastula of 128 cells. 



vertical and horizontal cleavage planes being continued through the ninth set 

 of divisions, resulting in 512 cells. At this point gastrulation begins and the 

 regularity of the cleavage planes is lost. Amphioxus is another classical ex- 

 ample of equal holoblastic cleavage, being classed as such, although after the 

 third cleavage the cells are not of exactly the same size. In Amphioxus the 

 first two cleavage planes are vertical and at right angles, as in Synapta. The 

 third cleavage plane is horizontal, as in Synapta, but the cells lying above the 

 third cleavage plane are smaller than those lying below it. The eight-cell stage 



