Penetration of the fertilising sperm allows the oocyte to resume meiosis and the polar body is extruded.
The pronuclei move towards each other and eventually compress their envelopes where they lie adjacent near the centre of the wall.
[2] Carnegie stage 3 begins when a cavity first appears in the morula and ends after the hatching from the zona pellucida when the embryo makes contact with the endometrial lining of the uterus.
[3] There are four characteristic processes that CS3 embryos go through cavitation, collapse and expansion, hatching, and discarding of cells.
In vitro, the blastocyst rapidly collapses and slowly re-expands before hatching from the zona pellucida.
TEM inspection of in vitro blastocysts has allowed us to identify two types of cells that the developing embryo apparently discards.
Development of the Olfactory nerve and the early stage foot and hand plates Ectoderm: sensory placodes, lens pit, otocyst, nasal pits moved ventrally, fourth ventricle of brain Mesoderm: heart prominence, ossification continues Head: forebrain, eye, external acoustic meatus Body: straightening of trunk, heart, liver, umbilical cord Ectoderm: sensory placodes, lens pit, otocyst, nasal pits moved ventrally, fourth ventricle of brain Mesoderm: heart prominence, ossification continues Head: forebrain, eye, external acoustic meatus hearing - otic capsule connected with the basal plate and with the future exoccipitals.
Ectoderm: sensory placodes, nasal pits moved ventrally, fourth ventricle of brain Mesoderm: heart prominence, ossification continues Head: nose, eye, external acoustic meatus Body: straightening of trunk, heart, liver, umbilical cord Limb: upper limbs longer and bent at elbow, foot plate with digital rays begin to separate, wrist, hand plate with webbed digits Mesoderm: heart prominence, ossification continues Head: nose, eye, external acoustic meatus Body: straightening of trunk, heart, liver, umbilical cord Limb: upper limbs longer and bent at elbow, foot plate with webbed digits, wrist, hand plate with separated digits Final embryonic stage, after this development is described as "fetal" through the entire second and third trimester.