<prompt> At what stage of embryonic development does an embryo normally begin to implant in the endometrium?BlastocystFour-cell stageMorulaTrilaminar embryo  <response>: Ans. A BlastocystRef: I.B. Singh, 9th ed. pg. 43-44 After fertilization, the fertilized ovum begins to divide as it migrates through the uterine tube. It undergoes cleavage till a 16 cell stage and is called morula. Cells of morula differentiate into an inner cell mass which is completely surrounded by an outer layer of cells, which further gives rise to a structure called trophoblast. It reaches the blastocyst stage approximately 110 cells on about day 5 and it enters the uterus on about day 6. Implantation normally begins on day 6 with the syncytiotrophoblast of the embryonic pole of the blastocyst eroding into the endometrium. 