PLATE 1 



EXPLANATION OF FIGURES 



All the drawings were made with the camera lucida. Unless otherwise stated 

 the magnification is X 875. Figures 1 to 15 inclusive show the development of 

 Nereis under the conditions described; figures 16 to 23 are of Arbacia. 



1 Drawn from the living egg. X 475. This egg had been radiated with 

 rapid beta rays at 4 mm. for 1 hour; drawn just before cleavage. The very wide 

 perivitelline space is traversed by delicate strands representing the walls of the 

 alveoli. 



2 Section of egg treated as above; 35 minutes after insemination; the softened 

 protoplasm at the periphery has been pulled in under the influence of the aster. 



3 Egg radiated for 1§ hours with both rapid and slow beta rays, 40 minutes 

 after insemination. The protoplasm at the periphery lacks the usual granular 

 appearance. 



4 Same treatment, 40 minutes after insemination; the protoplasm is vacuo- 

 lated. 



5 Exposure as in fig. 1. 35 minutes after insemination; the sperm has begun 

 to enter the egg. 



6 Same, 45 minutes after insemination; the sperm is now in the center of the 

 egg. 



7 Egg exposed to the slow and rapid rays for 30 and 50 minutes after. 



8 Same, 70 minutes after insemination. 



9 Same, 60 minutes after insemination. 



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