During week two, the cell structure is now called the blastocyst. Hormones
of the woman have created a soft and absorbent atmosphere in the uterus - an
environment perfectly suited for implantation.
As the blastocyst floats through the uterus, outer cells begin to release
enzymes which will break down the soft uterine lining, enabling it to implant
itself. The best area for this is the back of the uterine wall. The
enzymes rupture the capillaries in the uterus as the egg is imbedded. The
cells form a tough membrane, now called the chorion. At
this stage, a woman does not yet know she is expecting.
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On the thirteenth day, the placenta is growing - serving as a mechanism to
exchange nourishment, removing waste and protecting the embryo from germs.
A yolk sack now emerges, although it differs from that of a typical egg.
This yolk produces blood cells for the embryo until the embryo is able to do
this on its own. The umbilical cord is now formed. A mesoderm is
formed between the ectoderm and the endoderm, now creating three layers that
will become a tiny baby.