CLEAVAGE 



239 



(4) Irregular. In many phyla, scattered forms are known 

 in which cleavage adheres to no single or simple type and may 

 truly be said to be irregular. This does not mean that no 

 definite plan is followed, for each species follows a fixed rule; 

 these cleavage forms are in this sense regular, but cannot 



FIG. 115. Irregular cleavage in the Turbellarian, Mesostomum ehrenbergi. 

 After Bresslau. X 700. A. Three-cell stage, in section. B. Four-cells 

 becoming five. Side view. C. Seven-cell stage, from animal pole. D. Twelve- 

 cells. A. Macromere, giving rise to A\ and At in the seven-cell stage. B. First 

 micromere, forming Bi and Bi in the five-cell stage. C. Second micromere 

 formed in three-cell stage, and giving rise to Ci and Cz in the seven-cell stage. 



be described in general terms. We cannot stop to describe 

 any of these instances in detail. Irregular cleavage may be 

 found among the Porifera, Coelenterates, Platyhelminthes, 

 Molluscoids, Enteropneusta, and Teleosts; a typical example 

 is illustrated in Fig. 115. 



The remaining forms of cleavage are grouped as incomplete^ 

 and are found among those species with markedly telolecithal 



