1. The layer zone pellucida is retained around the embryo and thus, there is no change in the overall size from zygote to morula.
2. This layer is important because it prevents the implantation of the blastocyst at an abnormal site such as fallopian tube.
3. Though the number of blastomeres increase, the size of morula remains almost same as that of ovum till it reaches the uterus by the end of the day 4.
