Life Cycle - Human Biology (6A)



5-9 days - Remains free in uterine cavity.

10-11 days -
Attaches to and begins to become imbedded in the prepared lining of the uterus. The different tissue layers are developing.

14 days -
Irregular, blob-like oval body with a longitudinal depression from which cells are pushed into an enlarging body.

18-21 days -
Thickening of the neural plate, which is the first sign of the central nervous system. The primitive heart is a simple tube. There are primitive lung buds. Two faint depressions are the sites of eyes. The embryo begins to curve head-to-tail to fit its environment.

4th week -
Beginning of gallbladder and liver tubules. Parts of brain begin differentiation. Local dilations indicates beginning of stomach. Heart tube becomes slightly bend. Nose parts suggested. Tiny liver and belly stalk. Primitive head parts, mount, brain, eyes, ears, are forming. Opening from mouth to gut breaks through; a little later, the anus. Primitive thyroid cells. Windpipe and larynx beginning. The heart is under the chin. First heartbeat occurs. Blood corpuscles form, circulation begins.

5th week -
Nasal pit, buds that will be arms and legs, cells that will develop into pancreas gland, tiny thickening that will be tongue appear. The gut elongates. Primitive blood vessels function. Beginnings of eye lens, cranial nerves, retinal layer.

6th week -
Arm and leg buds lengthen, faint grooves suggest toes and fingers. Brain recognizable. Lung buds bifurcate. Primitive kidney established. Eyes far to either side of head. Epithelium and primitive ear parts begin to form. Nasal pits recognizable as nostrils. Stomach suggests adult form. Salivary glands identifiable. Skeleton system begins.

7th week -
Distinct beginnings of fingers, toes, eyelids, delicate fibrils that will be muscles, autonomic nervous system. Nasal opening break through, optic nerve fibers extend, gallbladder elongates.

8th week -
Centers of bone growth established. Thumb and big toe begin to diverge. Local buds destined to be teeth. Rapid growth of nose and upper jaw. Ears are very low on head. Recognizable human form.

3rd month -
Eyelids meet and fuse, eyes remain closed until seventh month. Bony parts of skull develop from base of skull upward. Inner ear structure almost complete. Hair follicles appear. External genitals evidenced by swellings; sex not obvious.

4th month -
Brain a recognizable miniature of adult brain; large bulge of forebrain distinguishable from cerebellum and brain stem. Sweat glands appear. Outer skin thickens into distinctive layer.

5th month -
Structures of testes and windpipe are well established. Branching structures will become kidney tubules.

6th month -
Eyebrows, eyelashes are visible. Fetus is coated with downy hair. Skin ridges form on palms and soles, the lifelong basis of fingerprints and sole prints. The bronchial tree branches out actively, continues to do so after birth.

7th month -
Eyelids unfuse, Testes begin to descent. Fat begins to be deposited under translucent skin layers and fetus becomes plumper.

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