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After one month of pregnancy, the embryo's $...A....$ is formed. By the end of the $.....B...$ month of pregnancy, the foetus develops limbs and digits. By the end of $\ldots$... most of the major organ systems are formed for example, the limbs and external genital organs are well-developed. By the end of $...D....$ the body is covered with fine hair, eyelids separate, and eyelashes are formed. Here$ A$ and $D$ refers to
$A-$heart, $B$-second, $C-$first trimester, $D-$second trimester
$A-$heart, $B-$second, $C-$first month, $D-$second month
$A-$heart, $B-$second, $C-$first week, $D-$second week
$A-$heart,$B-$fourth, $C-$first trimester, $D-$second trimester
Solution
Summary of important development changes in the human embryo
| Time from Fertilisation |
Organ Formed |
| $Week\; 1$ |
Fertilisation cleavage starts about $24\; hours$ after fertilisation leavage to form a blastocyst $4.5$ days after fertilisation. More than $100\; cells$ implantataion $6-9$ days fter fertilisation |
| $Week\; 2$ |
The three primary germ layers (ectoderm, endoderm and mesoderm) develop |
| $Week\; 3$ | Woman will not have a period. This may be the first sign that she is pregnant. Beginning of the backbone. Neural tube develops, the beginning of the brain and spinal cord (first organs) |
| $Week\; 4$ | Heart, blood vessels, blood and gut start forming. Umbilical cord developing |
| $Week\; 5$ | Brain developing, 'Limb buds', small swelling which are the beginning of the arms and legs. Heart is a large tube and starts to beat, pumping blood. This can be seen an ultrasound scan |
| $Week\; 6$ | Eyes and ears start to form |
| $Week\; 7$ | All major intermal organs developing. Face forming. Eyes have some coloun Mouth and tongue develop. Beginning of hand and feet |
| $Week\; 12$ |
Foetus fully formed, with all organs, muscles, bones toes and fingers. Sex organs welldeveloped. Foetus is moving |
| $Week\; 20$ |
Hair beginning to grow including eyebrows and eyelashes. Fingerprints developed.Fingernails and toenails growing. Firm hand grip. Between $16$ and $20$ weeks baby usually felt moving for first time |
| $Week\; 24$ | Eyelids open. Legal limit of abortion in most circumstances |
| $Week\; 26$ | Has a good chance of survival if born prematurely |
| $By\; Week\; 28$ | Baby moving vigorously. Responds to touch and loud noises. Swallowing amniotic fluid and urinating |
| $By\; Week\; 30$ | Usually lying head down ready for birth |
| $By\; Week\; 40$ | Birth |
