622 WILSON. 



PLATE XXX. 



[Figs. 13 to 18, from a living specimen ; the others from preparations ; 270 diameters.] 



Figs. 13 to 18. Successive stages of a living embryo, viewed from the lower 

 pole, followed from the undivided egg up to the 1 28-celled stage. Strict bilateral 

 form //. The straight lines represent as nearly as could be determined the axis 

 of the spindles in the stage following each figure. 



Fig. 13. 8-celled stage; 14, fourth cleavage; 15, i6-celled stage (cf. Figs. 21, 

 27); 16, 32-celled stage. 



Fig. 17. 64-celled stage. Note the extreme lateral position of C^ D^ and the 

 equality of D and d.^ First appearance of cross-furrow by displacement. 



Fig. 18. 1 28-celled stage. Restoration of bilateral symmetry. Cross-furrow. 



Fig. 19. Fourth cleavage, radial form, upper pole. 



Fig. 20. i6-celled radial form, upper pole. 



Fig. 21. The same embryo from the lower pole. 



Figs. 22 to 24. Three examples of modified bilateral forms, fourth cleavage. 



