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Preimplantation Period: Week 1
During the first week, the woman's ovum is penetrated by and united with a man's sperm during fertilization.
-> A zygote is formed from this union.
The final stages of meiosis occur in the ovum during fertilization.
The zygote now undergoes mitosis that splits it into more and more cells due to cleavage. -
Preimplantation Period: Week 1
With ongoing mitosis, the zygote now becomes a blastocyst.
As it grows by cleavage, the blastocyst travels from place of fertilization to the uterus.
At the end of the first week, the blastocyst stops traveling and undergoes implantation and embeds into the prepared endometrium. -
Embryonic Period: Week 2
The first physiological process is induction which is the action of one group of cells on another that leads to the establishment of the developmental pathway in the responding tissue.
The implanted blastocyst grows by increased proliferation of the embryonic cells, with differentiation also occurring resulting in changes in cellular morphogenesis.
- A bilaminar embryonic disc is eventually developed from the increased number of embryonic cells within the blastocyst. -
Embryonic Period: Week 3
During the beginning of the third week, the primitive streak forms within the bilaminar disc.
- The primitive streak causes the disc to have bilateral symmetry.
During the latter part of the third week, the central nervous system begins to develop in the embryo.
Near the end of the third week, the neutral groove deepens further and is surrounded by the neural folds. -
Embryonic Period: Week 4
During the fourth week of the embryonic period, the trilaminar embryonic disc undergoes anterior and lateral embryonic folding.
- Places forming tissue types into their proper positions for further embryonic development.
The face and neck begin to develop with primitive eyes, ears, nose, oral cavity, and jaw areas during the fourth week.
- The primitive mouth has become the stomodeum. -
Embryonic Period: Week 5-8
During the fifth to sixth week, primitive muscle cells from the mesoderm in the mandibular arch begin to differentiate.
During the sixth week, the upper lip begins formation when each maxillary process fuses with each medial nasal process on both sides of the stomodeum.
During the seventh week, the muscle cells from the mesoderm of the hyoid arches have begun to differentiate. -
Fetal Period: Week 12
During the twelfth week, the posterior part of the primary palate meets the secondary palate due to increased growth and these structures gradually fuse in an anterior to posterior direction.
- When these three processes completely fuse, they form the final palate including the hard and soft palate. -
Fetal Period: Week 17
By the end of the fourth month, the fetal heartbeat and fetal movements are present. -
Fetal Period: Week 37-40
The fetus's major organs and body systems have fully developed and matured. The fetus is ready to be born.