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Human Reproduction — BIOLOGY STD 12 Science — Question

Bihar BoardEnglish MediumSTD 12 ScienceBIOLOGYHuman Reproduction5 Marks
Question
Describe briefly about embryonic stages in human being.
✓

Answer

Summary of embryonic development stages in human being :
1 day Fertilization, diameter of fertilized egg is about 0.15 mm.
2 days Two cell stage.
3 days 16 celled stage, Morula.
4 days Entry of blastocyst into uterine lumen.
7-8 days Partial Embedding of Blastocyst into endometrium of uterus implantation.
12 days Implantation completed. Formation of embryonic disc, extraembryonic membrane like Amnion, Chorion and Yolk sac etc.
14 days Formation of primitive streak.
18 days Formation of 3-5 pairs of somites.
19 days Formation of neural groove, neural plate, notochordal plate and 6-8 pairs of somites.
24 days Determination of cephalic and caudal parts, formation of 21-23 pairs of somites and heart.
28 days Imitation of heart beat, complete formation of nasal tube, three pairs of visceral arch, 30-31 pairs of somites as blood islands.
32 days Formation of 30-39 pairs of somites.
7 weeks Jaw, fingers, external ear begin to visible CR Length (crown rump-length) of Embryo becomes 19-20 mm.
8 weeks Embryo or foetus fully surrounded by Amnion. Fingers and toes distinctly visible.
All the organs have been formed and growing. By the end of 8th week Embryo takes shape of human and is now called foetus with CR length 28-30 mm.
5 months Blood formation in bone marrow begins, decidua capsularis fused with parietalis hair begin to grow.
9 months Placenta attains maximum size. Nails grow on fingers and foetus becomes ready to take birth in next 10 days.
Image
A – 14 somite stage (about 21 days)
B – 25 somite stage (about 25 days)
C – 30 somite stage (about 28 days)
D – 44 somite stage (about 34 days)
E – 5-6 week embryo
F – 6-5 week embryo stage
G – 7 week embryo stage
H – 8 week embryo stage
I – 9 week embryo stage
J – 10 week embryo stage
The above mentioned time period is approximate time period. Sometimes some babies take birth before their gestation time due to some unknown reasons. 

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