2IO CONK LIN. 



EXPLANATION OF PLATE I. 



Fig. I. Egg of Crepidula fornicata, from side, showing male and female pro- 

 nuclei in contact. 



Fig. 2. First cleavage spindle, seen from above. 



Fig. 3. Appearance of first cleavage furrow, seen from side. Daughter nuclei 

 not yet reconstituted. 



Fig. 4. Same egg as Fig. 3, seen from above. 



Fig. 5. Completion of first cleavage furrow. Nuclei and asters opposite each 

 other in the two blastomeres. Polar bodies between the two nuclei. 



Fig. 6. " Resting stage " after first cleavage, showing flattening of blastomeres 

 against each other and dexiotropic turning of nuclei, asters, and protoplasmic 

 areas. 



Fig. 7. Beginning of second cleavage. Laeotropic turning of spindles and 

 protoplasmic areas. 



Fig. 8. Three-cell stage ; an unusual condition, in which one of the first two 

 blastomeres divides before the other. 



Fig. 9. Second cleavage; usual condition, in which the two blastomeres divide 

 simultaneously. Polar furrow appearing at // 



Fig. 10. Completion of second cleavage. Asters nearly in position of the 

 poles of preceding spindles. Polar furrow well formed. 



Fig. ii. " Resting stage " after second cleavage. Four cells. 



Fig. 12. Third cleavage. Spindles nearly radial. 



