CLEAVAGE. 



43 



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 simplest 

 type of cleavage (Fig. 25). The egg of Synapta is alecithal, containing 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 result- 

 ing from the second cleavage into eight equal cells. The fourth cleavage is 



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, the fifth horizontal and so on, regular alternation of vertical and hori- 

 zontal cleavage planes being continued through the ninth set of divisions, re- 

 sulting in 512 cells. At this point gastrulation begins and the regularity of the 

 cleavage planes is lost. Amphioxus is another classical example 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 of Amphioxus thus 

 presents four upper smaller cells and four lower larger cells (Fig. 26). 



