Transcript, Watch Development 12.3
One day after fertilization, the embryos are observed under the microscope. If fertilization has occurred, a single cell is seen containing two nucleii, male and female.
Two days after puncture, the embryo divides itself and contains two cells, with one nucleus per cell
… which in turn starts to divide into an embryo of four cells if its development is optimal.
Three days following puncture, the embryo starts to divide and consists of eight cells if its development is optimal.
On day four the embryo divides a number of times, becoming a morula.
This is when it begins compact.
On day five, the embryo contains more than 100 cells. In the center there is a fluid-filled cavity.
Finally, the blastocyst will shed the membrane that surrounds it. This is called hatching.