Describe Fertilisation
Sexual intercourse of a male and a female is normally the way in which a sperm and egg cells meet although artificial insemination can also be possible . Through this sexual intercourse , there is a penetration of the penis inside the vagina . Therefore , sperm is deposited in to vagina through ejaculation , which is a reflex via muscle contractions .
Normally , the vagina kills most sperm but a few hundred make it to the fallopian tubes via the cervix and uterus . The sperm that makes it there in time penetrates the egg and the egg then disallows any other sperm to enter . The other sperm die .
The 23 egg chromosomes and 23 sperm chromosomes form a full amount of chromosomes to create a human . This is initially referred to as a zygote . On day 1 and 2 of this process , the zygote begins to divide through mitosis , and a 4 cell embryo is formed . On day 5 the cells form something called a blastocyst which is what will eventually attach to the uterus wall . By day 10 it is implanted . This is when the process of precidualisation and decidualisation occurs .
After ovulation , the endometrium becomes more vascularised , and produces cells that coat the uterine cavity called decidual cells . If pregnancy takes place , the decidualisation process begins . Decidual cells fill with lipids and glycogen . The outer wall is termed the trophoblast and creates a placenta .
The inner area of the cell mass creates 3 or 4 blastocyst cells which form the yolk sac and amniotic cavity . The chorion develops from the trophoblast to create a placenta . The surface of the chorion is then coated with chorionic villi which attaches the placenta to chorion . The inner cells form 3 germ layers , the mesoderm , endoderm and ectoderm . These create individual parts of the foetus , such as the various organ systems . ( Biology-online . org 2000 )
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