(a) Implantation: The embryo enters uterus at stage of blastocyst and becomes attached to wall of uterus, where it is to perform its subsequent development. This attachment of blastocyst to uterine wall is known as implantation.

Each seminiferous tubule is lined on its inside by two types of cells called male germ cells (spermatogonia) and a few large pyramidal Sertoli cells. The male germ cells undergo meiotic divisions finally leading to sperm formation, while Sertoli cells provide attachment and nutrition to germ cells.