During the second week, cells in the wall of the blastocyst will begin to fold inward.
Consequence of gastrulation :
A hole is now present on the side of the blastocyst – it is called the blastopore .
Cell layers are formed in the gastrula .
The innermost layer of cells is called the endoderm .
The middle layer of cells is called the mesoderm .
The outer layer of cells is called the ectoderm .
Cells are becoming specialized. Cell differentiation is occuring – they are becoming differentiated. They are changing from undifferentiated differentiated
In vertebrates, Gastrulation is followed by Neurulation … the development of the nervous system. (change in body symmetry)
By day 21, embryonic membranes have also formed:
The Amnion develops into a fluid filled sac. (cushions and protects baby)
The Chorion along with the lining of uterus forms the Placenta , the embryo’s organ for respiration, nourishment, and excretion. (The placenta allows the embryo to make use of the mother’s organ systems while its own are developing.)
Materials diffuse between baby and mother – there is no direct blood link between the two…