Development cycle Of Blastocyst

Development  Cycle  Of  Blastocyst :

The blastocyst develops from the morula, a solid ball of about 16 undifferentiated, spherical cells within the zona pellucida. As cell division continues, the blastomeres change their shape and tightly align themselves against each other, forming the fluid-filled cavity called the blastocoele.
    The cells now forming differentiate into two types, an inner-cell mass growing on the interior of the blastocele and cells growing on the exterior of it. In 24 to 48 hours, the zona pellucida breaches, referred to as hatching. This removes the constraint on the physical size of the cell mass.
     The cells on the exterior of the blastocyst begin excreting an enzyme which erodes epithelial uterine lining and creates a site for implantation. The cells surrounding the blastocyst now destroy cells in the uterine lining, forming small pools of blood, which in turn stimulate the production of capillaries. This is the first stage in the growth of the placenta. The inner cell mass of the blastocyst divides rapidly, forming two layers. The top layer becomes the embryo, and cells from there occupy the amniotic cavity. At the same time, the bottom layer forms a small sac.
      The blastocyst stage gives way to the gastrula with the formation of the primitive streak, after which when the embryonic cells change form and differentiate into three layers.