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ae 
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Fic. 10.—Diagrams illustrating the fertilization of the egg. 
‘ 
A, egg surrounded by spermatozoa ; on the right, one has just penetrated the egg 
membranes and is entering the egg cytoplasm; egg nucleus in the centre. B, egg 
nucleus with chromatin reticulum on left ; on right, the sperm nucleus preceded by 
its centrosome and attraction sphere. C, egg nucleus on the left, sperm nucleus on 
the right of the centre of the egg; stage immediately preceding the division of the 
centrosome. D, the centrosome has divided, the two attraction spheres separate to 
form the first cleavage spindle ; the chromosomes of the egg and sperm nuclei clearly 
visible and indistinguishable (in the figure the egg chromosomes are black, the sperm 
chromosomes shaded). E, the first cleavage spindle, with splitting of chromosomes. 
F, completion of first cleavage ; two-celled stage, each nucleus contains four chromo- 
somes—two from the egg and two from the sperm. (After Boveri.) 
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