Short / Long Answer question Type :
Describe the events that take place from the time of entry of human sperm into the ovum till fertilization is complete.

• Fertilization of the ovum occurs in the isthemic-ampullary region of the fallopian tube.
• One spermatozoon pierces the zona pellucida and enters the ovum. As soon as the spermatozoon enters the ovum the second meiotic division is completed and the second polar body is extruded.
  
• When a spermatozoon comes in contact with the oocyte, plasma membranes of the two cells fuse. This probably occurs at receptor sites. The head of the spermatozoon enter the cytoplasm of the ovum.
  
• Alterations taking place in the plasma membrane of the oocyte and in the zona pellucida ensure that no other spermatozoon can enter the oocyte.
  
• The nucleus of the ovum becomes the female pro nucleus.
  
• The head of the spermatozoon separates from the middle piece and tail and transforms itself into the male pro nucleus.
  
• Soon the pro nuclei loose their nuclear membranes. The 23 chromosomes of the female pro nucleus and 23 of the male pro nucleus get mixed up and form 23 pair of chromosomes.
  
• These 46 chromosomes go through the typical mitotic division and lead to the formation of an embryo having two cells.