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GENERAL EMBRYOLOGY 



what like the bricks in a wall or the bones of the wrist. They 

 may be cut off in this way on account of the obliquity of the 

 spindle in the parent cell, or they may shift to this position 

 after having been formed according to the radial plan. This 

 spiral arrangement may be foreshadowed in the four-cell stage 

 by the meeting of the first two cleavage planes at the poles of 

 the egg in the form of a zig-zag line instead of at a common 

 point. 



D 



FIG. 110. Cleavage in the Annulate, Polygordius. From Wilson, "Cell." 

 A, B. Four- and eight-cell stages, from the animal pole. C. Side view of eight- 

 cell stage. D. Side view of sixteen-cell stage. 



One of the simplest illustrations of this type is the adequal 

 cleavage of Polygordius (Fig. 110) but it is also well represented 

 by the markedly unequal cleavage of many Platyhelminthes, 

 Nemertines, Annelids, and Molluscs (Figs. Ill, 112, 119). 

 These forms illustrate at the same time a graduated series in the 

 inequality of the blastomeres. This inequality may appear in 



