90 LILLIE. 



EXPLANATION OF PLATE I. 



Fig. I. Egg-plates of Unio complanata magnified three times. 



(a) The flat surface. 



(b) From the side. 



Fig. 2. Part of Fig. i (a) more highly magnified. 



Fig. 3. Ovum within vitelline membrane ; polar globules have been formed 

 opposite to the micropyle {m k). 



Fig. 4. Early two-cell stage. 



Fig. 5. Later two-cell stage in outline. 



Fig. 6. Preparatory to three-cell stage from the upper pole. 



Fig. 7. Three-cell stage from the upper pole. 



Fig. 8. Three-cell stage from the lower pole. Actual appearance of egg 

 shown. 



Fig. 9. Early four-cell stage from the upper pole ; cross-furrow small. I-I, 

 first cleavage-furrow; II-II, second cleavage-furrow. 



Fig. 10. Later four-cell stage from the upper pole ; cross-furrow large. I-I, 

 first cleavage-furrow ; II-H, second cleavage-furrow. 



Fig. II. The four-cell stage from in front. 



Fig. 12. The five-cell stage from the upper pole. This egg is remarkable 

 inasmuch as A, not D, is the largest of the macromeres. The lower ends of the 

 spindles in B and C are outlined more faintly than the upper ends. 



Fig. 1 3. Six-cell stage from the upper pole. 



Fig. 14. Six-cell stage from in front. 



Fig. 1 5. Eight-cell stage from above. 



