The placenta of human mainly secretes two steroid hormones like estradiol and progesterone, and two protein hormones like human chorional gonadotropin HCG and human placental somatomammotropin HCS. Large amount of HCG hormone is secreted, during early pregnancy, from the placenta. Because of this reason its quantity increases in the urine of pregnant lady. On the basis of this fact, pregnancy test is performed.
The above hormones are also held responsible for keeping the corpus luteum active, protection of embryo, prevention of abortion and growth of mammary glands.
Functions of placenta
1. Exchange of important materials between foetal and maternal blood.
2. The essential materials are exchanged by diffusion, pinocytosis or active transport.
3. The small molecules like O2, CO2, H2O etc. and other inorganic substances like chlorides, phosphates, sodium, potassium, magnesium etc. are also diffused through placenta.
4. Large molecules like lipids, polysaccharides, carbohydrates, proteins etc. are obtained by pinocytosis process.
5. The nutritional substances are supplied to embryo from the mother through placenta.
6. Placenta also serves as a respiratory medium for exchange of O2 and CO2 between embryo and mother.
7. The nitrogenous and metabolic wastes from foetus are released into the blood of mother by diffusion through placenta.
8. The antibodies for measles, chikenpox, polio etc. present in the blood of mother reach the embryo through placenta.
9. Pathogenic viruses may also enter in embryo through placenta.
10. If a female takes some harmful chemicals, liquor, drugs etc.during pregnancy, these may cross the placenta and on reaching into foetus may cause deformity during organogenesis.(eg. Thallidomide)
11. Placenta itself secretes some hormones like progesterone, estrogen, lactogen, HCG, HCS etc.
12. Progesterone, maintains and supports the foetus during the whole pregnancy period . At the time of parturition, relaxin is secreted by placenta which lubricates, and widens the birth canal to facilitate child birth.
- He observed and described human sperm for the first time.
- According to Hartsoeker and Leeuvenhock there is a small model of developing animal present in the head of the sperm of that animal. This small model is called homunculus. Both these scientists are called spermists, and this theory is called "Theory of spermist".
- Bio-genetic Law :- According to this each organism during its embryonal development, passes through all stages, through which its species has evolved or embryo repeats its ancestry. i.e. Ontogeny recapitulates its Phylogeny.
According to this law, during embryonal development, the development of general structures takes place earlier and specific structures develop at last or later on.
Somatoplasm dies but the germplasm is never destroyed, rather it is transferred to the progenies.
According to them, a metabolic gradient is present inside the eggs.
Different parts of the egg have different metabolic rates.
The rate of metabolism is faster at the animal- pole of the egg and is slower at the vegital pole of the egg.
Due to different metabolic rates, different structures are formed from different parts of the egg.
These are termed as the organizers. - These organizers secrete some special chemicals called evocators which induce the formation of specific structures. - Spemann got the Nobel prize for his t