FIG. IO. Diagrams illustrating the fertilization of the egg. 



A, egg surrounded by spermatozoa ; on the right, one has just penetrated the eg 

 membranes and is entering the egg cytoplasm egg nucleus in the centre. B, eg 

 nucleus with chromatin reticulum on left ; on right, the sperm nucleus preceded b 



its centrosome and attrac 



spher 



right 

 C, egg nucle 



sperm nuceus pre 

 he left, sperm nucleus o 



the right of the centre of the egg ; sta^e immediately preceding the division of th 

 centrosome. D, the centrosome has divided, the two attraction spheres separate t 

 form the first cleavage spindle ; the chromosomes of the egg and sperm nuclei clearly 

 visible and indistinguishable (in the figure the ecg 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.) 



174 





