CLEAVAGE 



6i 



is quite irregular; indeed the blastomeres tend to fall apart and are only- 

 held loosely together by the jelly in which they are embedded. Perhaps 

 one can also include, under the heading of irregular cleavage, the pheno- 

 mena found in insects, in which the cleavage consists only in the separa- 

 tion of daughter nuclei within the undivided mass of the yolky egg (see 

 P- 119). 



Unequal, partial and superficial cleavage 



The most important examples of unequal cleavages are found in 

 vertebrates, which present a complete series of types, from those in which 



Figure 4.2 



Spiral cleavage (dextral) seen from the animal pole. At the 4-cell stage, 

 the right-or left-handedness of the cleavage can already be recognised by the 

 direction of the cross-furrow at the animal pole. At the third cleavage, the 

 macromeres A, B, C, D give off micromeres la, ib, ic, id, the cleavage 

 spindles being tilted in the direction indicated by the arrows. At the next 

 cleavage, the second quartet of micromeres, la, 2b, etc., are formed, with the 

 spindles tilted in the other direction; and la divides into lai and ia2,ib into 

 ibi and ibi, etc. This system of cleavage continues until four quartets of 

 micromeres have been formed ; but the divisions of the micromeres and 

 macromeres are not always synchronous, cleavage of la into lai, ia2 being 

 sometimes delayed till after the formation of the second quartet, and so on. 



