Fertilization

If the ovum receives sperm it is in the fallopian tube, the two unite to form a
zygote. This is called fertilization.
Fertilization occurs in the fallopian tube. The zygote immediately begins to
divide and forms a mass of cells called morula, which passes down to the uterus
and fixes itself to the wall of the uterus (known as implantation). Menstruation does not occur and the female is said to be pregnant. The developing young one or the foetus is attached to the uterus by a tissue called placenta. Placenta supplies oxygen and nourishment from the maternal blood to the foetus. It also transports carbon dioxide and excretory waste from the foetal blood to the maternal blood. Placenta also produces two hormones— progesterone and oestrogen. Under the influence of these hormones neither ovulation nor menstruation take place till pregnancy continues. The Umbilical cord is a tough structure that serves as the blood vascular connection between the foetus and uterine wall. From the first few weeks of development, the embryo is enclosed in a sac called amnion, which is filled with amniotic fluid. This fluid acts as a shock-absorber and helps to protect the embryo from damage. Enzymes from many sperm are necessary to digest cumulous mass surrounding oocyte, but only one can penetrate. The cortical reaction: enzymes released from cortical granules cause rearrangement of fibers in zona pellucidaand prevent further sperm penetration. Sperm is engulfed in cytoplasm –nucleus becomes pronucleus for first division, midpiece breaks apart, flagellum returns to role as centrioles.