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Question

The second maturation division of the mammalian ovum completes
  1. in the Graafian follicle following the first maturation division
  2. after the nucleus of the sperm has fused with that of the ovum
  3. shortly after ovulation before the ovum makes an entry into the
    fallopian tube
  4. after the ovum is penetrated by a sperm


Solution

The correct option is D after the ovum is penetrated by a sperm
i) Oogenesis is initiated during the embryonic stage when a couple of million gamete mother cells (oogonia) are formed within each fetal ovary

ii) These cells are diploid. They start division and enter into prophase-I of the meiotic division and get temporarily arrested at that stage. These are now called primary oocytes 

iii) Each primary oocyte then gets surrounded by a layer of granulosa cells and is called the primary follicle

iv) The primary follicles get surrounded by more layers of granulosa cells and a new theca and are now called as the secondary follicles

v) The secondary follicle soon transforms into a tertiary follicle which has a fluid filled cavity called antrum

At this stage the primary oocyte within the tertiary follicle grows in size and completes its first meiotic division resulting in the formation of a large haploid secondary oocyte and a tiny first polar body

vi) The secondary oocyte proceeds with meiosis II (second maturation division) but the division gets arrested at metaphase stage

It is in this stage of secondary oocyte that the ovum is shed during ovulation. It passes into the fallopian tube. The cell cycle resumes and the second maturation division is completed only after the penetration of sperm into the ovum.

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