Fig. 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 e^g ; 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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