12 AUX. 10. N. YATSU : 



2. li" a })orti()n of cytoplayii), whutlier IVuni ono or two blasto- 

 ineres, is removed iluring the first cleavage, tlie mode of cleavage 

 is disturbed, flight injury or horizontal cut alVects the cleavage 

 but little. 



3. ]f the blastomeres are separated by a vertical cut near the 

 end of the first cleavage, partial cleavage takes place as in the 

 blastomeres isolated at the 2-cell stage. 



4. If a portion of cytoplasm is cut off from the vegital region 

 at the 2-cell stage, the size-relation of the upper and lower cells 

 of the 8-cell stage is normal, irrespective of the angle of the section. 



5. In the egg compressed between two planes parallel to the main 

 axis of the egg, the second cleavage is equatorial ; and upon relieving 

 the pressure, the third cleavage is vertical and perpendicular to 

 the first. 



The following results were obtained from the study of the egg 

 of both Cerebî'atulus lacleus and C. 7uarginatus. 



6. In the eggs kept in Ca-free sea-water, the third cleavage is 

 vertical resulting in eight blastomeres arranged either in a ring 

 or in two parallel rows. 



7. In the trefoil egg the second cleavage is vertical, giving rise 

 to six blastomeres arranged in a ling. 



8. Irrespective of the number of the initial blastomeres (basal 

 cells) formed by vertical cleavages, the division goes on normally 

 in each of them. 



X. General Discussion. 



The results of the foregoing eight series of experiments, if 

 considered with those of other investigators obtained from the 

 egg of nemertine and other forms, seem to throw some light on 



