268 



EVOLUTION AND ANIMAL LIFE 



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Fio. 153. Diagrams illustrating the fertilization of the egg: A, Egg surrounded by 

 spermatozoa; on the right, one has just penetrated the egg membranes and is en- 

 tering the egg cytoplasm; egg nucleus in the center. B, Egg nucleus with chromatin 

 reticulum on the left; on the right, the sperm nucleus preceded by its centrosome 

 and attraction sphere. C, Egg nucleus on the left, sperm nucleus on the right of 

 the center of egg; aster fibrils preceding the fission of the centrosome. D, The 

 centrosome has divided, the two attraction spheres separate to form the first 

 cleavage of the spindle; the chromosomes of the egg and sperm nuclei clearly visible 

 and distinguishable (in the figure the egg chromosomes are black, the egg sperm 

 chromosomes shaded). E, First cleavage, showing spindle with splitting chromo- 

 somes. F, Completion of first cleavage, two-celled stage, each nucleus showing four 

 chromosomes, two from the egg and two from the sperm. (After Boveri.) 



