Changes

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Mitosis continues to occur within the '''blastocyst''' and fluid accumulates to fill the blastocoele and pressure within the embryo increases. Concurrent with growth and fluid accumulation is the production of '''proteolytic enzymes''' by '''trophoblastic''' cells.

Mitosis continues to occur within the '''blastocyst''' and fluid accumulates to fill the blastocoele and pressure within the embryo increases. Concurrent with growth and fluid accumulation is the production of '''proteolytic enzymes''' by '''trophoblastic''' cells which begin to weaken the '''zona pellucida'''. The '''blastocyst''' then begins to contract and relax causing intermittent pressure pulses on the '''zona pellucida''' which eventually ruptures facilitating the development of a small fissure just large enough for the blastocyst to squeeze out.

* Enzymes weaken the '''zona pellucida'''.

* Blastocyst begins to contract and relax causing intermittent pressure pulses on the zona pellucida.

* Zona pellucida ruptures with the development of a small fissure just large enough for the blastocyst to squeeze out.

The '''blastocyst''' ''''hatches'''' from the zona pellucida, although the period after fertilisation that this occurs is variable depending on species. '''Blastocyst''' hatching occurs in rodents, dogs, cats and primates 8 days after fertilization whilst in ungulates it is 12 days after fertilization. Once hatched, the '''blastocyst''' is free-floating within the uterus, totally dependent on the uterine environment for survival.

* Blastocyst ''''hatches'''' from the zona pellucida.

** '''Rodent, Dog, Cat, Primate''': 8 days after fertilization

** '''Ungulates''': 12 days after fertilization.

* Blastocyst free-floating within the uterus, totally dependent on the uterine environment for survival.