240 



THE ANIMAL KINGDOM 



One cell 



Two cells 



Pour ceils 



Ei^ht ceils 



Sixteen cells 



Thirty-two cells 



General ectoderm 



■Stomodeum 

 (and proctodeum, 



if present) 



Endocierm,a.nd,in one 

 rfaadrant,mesoderm. 



Figure 13.2. Spiral cleavage. One quadrant (the progeny of one cell of the four-cell 

 stage) is shaded. Lines indicate the axes of the preceding mitoses. The lower diagram 

 shows the fates of the cells of one quadrant. Numbers indicate the three quartettes or their 

 progeny. 



of egg cleavage occurs that is essentially identical throughout. In any 

 given species it is a fixed pattern, so that all of the steps can be identi- 

 fied. The following description refers to a common, basic pattern that 

 is found in several of the phyla. 



As in most eggs, the first and second cleavages, forming the two-cell 

 and then the four-cell stages, are meridional and at right angles to each 

 other, dividing the egg from animal to vegetal pole into four quarters 

 (Fig. 13.2). . 



The third cleavage is not transverse as in many other eggs, but is 



