1. Fertilization
While the ovarian follicle is growing the oogonium within it undergoes maturation. Oogonium enlarges to form primary oocyte.

2. Primary oocyte undergoes first meiotic division to shed off the first polar body & becomes a secondary oocyte.at the time of ovulation
The second meiotic division is in progress & a spindle has formed for the separation of the second polar body
The pre embryonic period begins when an oocyte is fertilized.

3. Female gametogenesis

4. Fertilization
Fertilization is a sequence of events that begins with contact between a sperm & a secondary oocyte & ends with the fusion of the nuclei of the sperm &ovum & the intermingling of maternal & paternal chromosomes.
Fertilization process lasts about 24 h
Fertilization of the ovum occurs in the ampulla of the uterine tube.

6. Phase of fertilization
1.passage of the sperm through corona radiata.
The dispersal of the follicular cells of the corona radiata results mainly from the action of the enzyme hyaluronidase release from the acrosome. Movements of the tail of the sperm also help it to penetrate to corona radiata

8. 2. penetration of the zona pellucida by the sperm.
Pathway through the zona pellucida also results from the action of enzymes released from the acrosome. The enzymes acrosin & neuraminidase cause lysis of the zona pellucida, thereby forming a path for the sperm to follow to the oocyte.

9. Once the sperm passes through the zona pellucid, a zona reaction occurs in this amorphous layer that makes it impermeable to other sperms.
This reaction believed to results from the action of lysosomal enzymes released by cortical granules near the plasma membrane of the secondary oocyte.
The contents of the granules which are released in to the perivitelline space also cause changes in the plasma membrane of the oocyte that make it impermeable to sperms.
Although several sperms begin to penetrate the zona pellucida, usually only one sperm enters the ovum & fertilize it.

10. 3. fusion of the oocyte & sperm cell membrane.
The plasma membranes of the oocyte & sperm fuse and soon break down at the area of fusion. The head and tail of the sperm enter the cytoplasm of the oocyte, but the sperm’s plasma membrane remains behind.

13. 4. completion of the second meiotic division of the secondary oocyte.
After entry of the sperm, the secondary oocyte completes its second meiotic division, forming a mature oocyte (ovum) & second polar body. The nucleus of the mature oocyte is known as the female pronucleus.

14. 5.Formation of the male pronucleus
Within the cytoplasm of the oocyte the nucleus of the head of the sperm enlarges to form the male proucleus. During this process the tail of the sperm degenerate. Morphologically the male & female pronuclei are indistinguishable. During growth of the pronuclei, they replicate their DNA.

15. 6.The male & female proneuclei contact each other,
Lose their nuclear membrane & fuse to form a new cell called zygote. Fertilization is completed within 24 h of ovulation.
Male & female pronuclei meet but they do not fuse to form one nucleus. There nuclear membranes disappear and their chromosomes become distinct. Each pronucleous has 23 chromosomes so that the fertilized ovum now has 46 chromosomes in all.

16. Each of these 46 chromosomes split in to two
Each of these 46 chromosomes split in to two. Spindle forms & mitosis occurs leading to the formation of 2 daughter cells. This is called the 2 cell stage of the embryo.
Note that strictly speaking there is no one cell stage of the embryo.

18. Important points
Two daughter cells are still surrounded by zona pellucida.
Each daughter cell is much smaller than the ovum. As subsequent divisions occur the cells become smaller &smaller until they acquire the size of most cell of the body. This process is called cleavage.