466 WILSON. 



PLATE XIII. 



[Figs. I to 6, ']a, and lo from living specimens; the others from preparations.] 



Fig. I. Unsegmented egg, from the upper pole, before the extrusion of the polar 

 cells. Zona radiata still present. 



Fig. 2. Side-view after the formation of the polar cells and disappearance of 

 the zona. 



Fig. 2.a to 2d. Four stages in the first cleavage; 2.a and 2b are side-views of suc- 

 cessive stages -of the same egg; 2c is from the upper pole in the earlier part of the 

 division, and 2d from the lower pole at a later stage, showing the still undivided 

 bridge of protoplasm on the lower side. 



Fig. 3. Completed two-celled stage from the upper pole; AB the anterior, CD 

 the posterior blastomere (i h. 55 m.). 



Fig. 4. The same egg from the upper pole; second cleavage in progress; the 

 anterior blastomere divided into A and B, the posterior incompletely divided 

 (2 h. 10 m.). 



Fig. 4a. View from the lower pole of slightly later stage, showing the cross-furrow 

 f, although CD is still incompletely divided (2 h.). 



Fig. 5. Four-celled stage from the upper pole. The fainter lines represent the 

 cleavage-lines of the lower pole, seen through the transparent vitellus. The deuto- 

 plasm-spheres omitted (2 h. 19 m.). 



Fig. 6. Four-celled stage, from the right side. 



Fig. 7. View from the upper pole of an egg (preparation) about to divide into 

 eight cells, showing the position of the spindles. The stars represent the upper 

 asters; the small circles, the centres of the lower asters. 



Fig. 7a. Side-view of third cleavage (2 h. 20 m.). 



Fig. 8. View from the upper pole of an egg later in the third cleavage, to show 

 the position of the spindles. 



Fig. 9. Side-view of the last. 



Fig. 10. Side-view of completed eight-celled stage. 



