Concept preview

What you will be able to do

1. 01Describe fertilization in the fallopian tube and the formation of a diploid zygote.
2. 02Describe cleavage, the blastocyst, and implantation in the uterus.
3. 03Explain the structure and exchange role of the placenta.
4. 04Outline the stages of pregnancy: embryo versus fetus, and the three trimesters.

When an egg is released at ovulation, it has only about a day to be fertilized, and the sperm that reach it have completed a long and punishing journey. Of the hundreds of millions of sperm deposited in the vagina, only a few hundred survive the swim through the cervix and uterus and up into the fallopian tube. Fertilization is the moment a single successful sperm fuses with the egg, and it normally takes place high in the fallopian tube, not in the uterus. Many sperm may reach the egg and release enzymes from the caps on their heads to digest a path through the egg's protective coat, but the instant one sperm's membrane fuses with the egg's, the egg rapidly changes its surface to lock all the others out — ensuring that exactly one sperm, and therefore one set of paternal chromosomes, enters. Fertilization is thus both a fusion and a gatekeeping event, and both halves of that description matter: the fusion supplies the second set of chromosomes, while the gatekeeping guarantees that the set is single, so the new cell has exactly 46 chromosomes and not more.