Embryonic Development; Differentiation of Tissues - 275 



Fig. 15-6. Diagram of fertilization. 1, contact of sperm with surface of egg. 2, response of the egg: formation of 

 the fertilization cone, which engulfs the sperm, and of the fertilization membrane. 3, sperm head drawn into the 

 egg; appearance of aster. 4, 5, enlargement of sperm nucleus and approach of the two gamete nuclei; appear- 

 ance of the haploid number of chromosomes (here three) in each gamete nucleus. 6, union of the two gamete 

 nuclei. 7-9, first division of the zygote nucleus; each daughter nucleus receiving a replica of each chromosome 

 present in the zygote nucleus. 



bryo while further development proceeds 

 (Fig. 11-10). 



Usually, only the head and middle piece 

 of the sperm enter the egg; and even when 

 the whole sperm becomes engulfed, the flag- 

 ellum soon disintegrates. The sperm head 

 consists mainly of nucleus, and from this 

 nucleus half the chromosomes in all of the 

 cells of the new individual will be derived. 



The sperm nucleus swells gradually as it 

 moves toward the center of the egg where it 

 is to join the egg nucleus (Fig. 15-6). Simul- 



taneously a large single aster forms in the 

 surrounding cytoplasm; and the aster appears 

 to be concerned with bringing the gamete 

 nuclei together. Finally the gamete nuclei 

 fuse to form the zygote nucleus. This event, 

 strictly speaking, is fertilization. At this stage, 

 the chromosomes of both nuclei are individ- 

 ually visible (Figs. 15-4D and 15-6); and it 

 is entirely certain that the chromosomes of 

 the diploid zygote represent a combination 

 of the chromosomes of the haploid gametes. 

 Half the chromosomes of the zygote are de- 



