580 



L 



R 



M/5 6 a P 



3 



4 



Figs. 1 — 8. The maturation and cleavage in dextral and sinistral snails; R dex- 

 tral forms, L sinistral ones. In Fig. 1, fp is the free pole and ap the pole of attach- 

 ment, the head of the arrow indicating the place at which the polar bodies will probably 

 form in each case. In Figs. 3 — 8 the arrows indicate the direction of division ; the 

 quadrants of the egg are labelled with the letters A, B, C, D; the Mesomere with the 

 letter M. 



In Figs. 1, 2 and 5 the chief axis of the egg lies in the plane of the paper; in 

 Figs. 3, 4, 6, 7, 8 it is perpendicular to the plane of the paper. Fig. 8 represents a 



