Date post: | 16-Aug-2015 |
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Fertilization is the process in which gametes—a male's sperm and a female's egg or ovum—fuse together, producing a single cell that develops into an adult organism.
THE FERTILIZATION PROCESS
Fertilization is a precise period in the reproductive process. It begins when the sperm contacts the outer surface of the egg and it ends when the sperm's nucleus fuses with the egg's nucleus.
THE FERTILIZATION PROCESS
Fertilization is complete when the sperm's nucleus fuses with the egg's nucleus.
THE FERTILIZATION PROCESS
1. The first step is the sperm approaching the egg. - In some organisms, sperm just swim randomly toward the egg (or eggs). - In others, the eggs secrete a chemical substance that attracts the sperm toward the eggs.
THE FERTILIZATION PROCESS
2. The second step of
fertilization is the attachment of several sperm to the egg's surface coat.
- All animal eggs have surface coats, which are variously named the vitelline envelope.
- This attachment step may last for just a few seconds or for several minutes.
THE FERTILIZATION PROCESS
3. The third step is a
complex process in which the sperm penetrate the egg’s surface coat. - The head, or front end, of the sperm of almost all animals except fish contains an acrosome, a membrane-enclosed compartment.
THE FERTILIZATION PROCESS
4. The next step in
fertilization—the fusion of sperm and egg cell membranes.
- When the membranes fuse, a single sperm and the egg become one cell.
THE FERTILIZATION PROCESS
After fusion of the cell membranes the sperm is motionless. The egg extends cytoplasmic fingers to surround the sperm and pull it into the egg's cytoplasm.
THE FERTILIZATION PROCESS
Filaments called microtubules begin to grow from the inner surface of the egg cell's membrane inward toward the cell's center, resembling spokes of a bicycle wheel growing from the rim inward toward the wheel's hub.
THE FERTILIZATION PROCESS
As the microtubules grow, the sperm and egg nuclei are pushed toward the egg's center.
THE FERTILIZATION PROCESS
5. Finally, the egg and sperm nuclear envelopes (outer membranes) fuse, permitting the chromosomes from the egg and sperm to mix within a common space. A zygote is formed, and development of an embryo begins.
THE FERTILIZATION PROCESS