text: The<unk>  is an early stage of the mammalian development that occurs at the beginning of pregnancy. It is the structure formed in early embryogenesis, after the formation of the blastocoel, but before implantation.  It possesses an inner cell mass, or embryoblast which subsequently forms the embryo, and an outer cell mass, or trophoblast whoch forms the placenta.  The human<unk>  comprises 70-100 cells. It is preceded by a zygote, the fertilized egg cell, and succeeded by an embryo. Other definitions consider the<unk>  an intermediate stage of the embryo, and begin using the term<unk>  at day 5 (16-20 cells.)
  Recently the inner cell mass has become a source for embryonic stem cells.
option_0: Blastocyst
option_1: Ovum
option_2: Gametocyte
option_3: Decidua
option_4: Dermatophyte
option_5: Clavicle
option_6: Spermatid
option_7: Nucleosol