CLEAVAGE 



235 



different stages; in the very first division of the egg (Nereis)] 

 at the second (ClaveUna) ', third (Cerebratulus, Fig. 112); fourth 

 (sea-urchin), or still later (Synapta). The direction which the 

 spiral takes is fixed in each species; it is described as dextral 



FIG. 111. The eight-cell stage of four animals showing gradations in the 

 inequality of the third cleavage, and in the extent of the spiral rotation of the 

 micromeres. From Wilson, "Cell." All viewed from the animal pole. A. The 

 leech Clepsine (Whitman). B. The chaetopod Rhynchelmis (Vejdovsky). 

 C. The lamellibranch Unio (Lillie). D. Amphioxus. 



(dexiotropic) or sinistral (keotropic) when the upper cells are 

 rotated clockwise or counter-clockwise respectively, as viewed 

 from the animal pole. 



(3) Bilateral. In this form we see a second modification 

 of the radial type, which is first indicated by the fact that the 

 third of the meridional cleavages fails to reach precisely the 

 poles of the egg but meets either the first or second plane at 

 some distance from the pole. In this way it comes about that 



