PLATE 6 



EXPLANATION OF FIGURES 



17 A tube egg of a brown rat, showing the second polar body at the top, below 

 the deeply staining female pronucleus, the entrance cone to the right, the male 

 pronucleus with the sperm aster in the lower left-hand margin and the sperm 

 tail extending diagonally through the egg. 



18 A somewhat later stage of the- egg of the brown rat, showing in addition 

 to the above-mentioned points the nucleolus vacuolated in the female pronucleus. 



Figures 17 and 18 were drawn by Dr. W. R. Coe, and are here published with 

 his kind permission. 



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