450 HOLMES. 



EXPLANATION OF PLATE XVII. 



Fig. I. Egg mass of Pla7iorbis trivolvis. 



Fig. 2. Undivided egg. 



Fig. 3. 2-cell stage, showing the second cleavage spindles and the torsion of 

 the dividing cells. 



Fig. 4. 4-cell stage viewed from the side, showing the laeotropic division 

 of B. 



Fig. 5. The same egg seen from the upper pole, showing the laeotropic incli- 

 nation of the spindles. 



Fig. 6. 8-cell stage seen from the side. 



Fig. 7. The same egg seen from the animal pole. 



Fig. 8. Apical view of the 24-cell stage. 



Fig. 9. Same egg seen from the side. 



Fig. 10. Same egg seen from the vegetal pole, showing the division of D 

 which produces the primary mesoblast cell D* or M. 



Fig. II. View of the lower pole of the 33-cell stage, showing the large meso- 

 blast cell lying partly in the cleavage cavity. 



Fig. 12. Lateral view of the same egg, showing the cleavage of the lower 

 tier of the second quartette. 



