Given below is the diagram of a human ovum surrounded by a few sperms. Study the diagram and answer the following questions :

(a) Which one of the sperms would reach the ovum earlier ?
(b) Identify 'D' and 'E'. Mention the role of 'E'.
(c ) Mention what helps the entry of sperm into the ovum and write the changes occuring in the ovum during the process.
(d) Name the specific region in the female reproductive system where the event represented in the diagram takes place.

Explanation

(a) Sperm 'A' would reach the ovum earlier. As sperm comes in contact with the zona pellucida layer of the ovum and induces changes in the membrane that block the entry of additional sperms.
(b) D - Cells of Corona radiata.
E - Perivitelline space.
(c ) The secretions of the acrosome help the sperm enter into the cytoplasm of the ovum through the zona pellucida and the plasma membrane. This induces the completion of the meiotic division of the secondary occyte. The second meiotic division is also inequal and results in formation of a second polar body and a haploid ovum. The haploid nucleus of the sperm and that of the ovum fuse together to form a diploid zygote.
(d) Fertilisation takes place at ampullary-isthmic junction in fallopian tube of the female reproductive system.