Why doesn't the outer layer of cells in the mammalian blastocyst become the ectoderm during mammalian gastrulation?
A - All the cells in the layer die
B - It become the endoderm
C - It becomes the placenta
D - The outer layer is shed when the blastocyst hatches out of the zona pellucida
E - It becomes the mesoderm
Answer: C - It becomes the placenta
The outer layer becomes the mammalian placenta so the ICM has to form the three germ layers.