PLATE 2 



EXPLANATION OF FIGURES 



10 Same, 60 minutes after insemination; the sperm is still external. 



11 Egg exposed to the rapid beta rays at 4 mm. for IJ hours; the chromosomes 

 have split in a normal fashion; 45 minutes after. 



12 Same, 55 minutes after insemination. 



13 Same, ^5 minutes after; the whole egg is filled with these vesicles. 



14 Same, 70 minutes after insemination; cleavage is superficially regular 

 but the distribution of chromosomies is abnormal. 



15 Same, 70 minutes after insemination; the sperm has not entered and no 

 polar bodies have been given off. 



16 to 23 Sea urchin eggs ; in each case the eggs were exposed to the rapid and 

 slow rays acting at 4 mm. distance. 



16 Entrance of the sperm head; 20 miirutes after insemination. 



350 



