(b) Progesterone: It is also a steriod hormone secreted by yellow-coloured endocrine gland, called corpus luteum, formed from empty Graafian folicle during more... | Time from fertilization | Stage/organs | Event | |||||||||||||||||||||||||||||||||||||||||||||||||
| 24 hours | Cleavage | Embryo is at two-cell stage | |||||||||||||||||||||||||||||||||||||||||||||||||
| 3 days | Modula | The module reaches uterus | |||||||||||||||||||||||||||||||||||||||||||||||||
| 7 days | Blastocyst | Implantation of blastocyst begins | |||||||||||||||||||||||||||||||||||||||||||||||||
| 2.5 weeks | Notochord | Notochord formed, differentiation of tissues that will give rise to heart, blood cells formed in yolk sac and chorionic. | |||||||||||||||||||||||||||||||||||||||||||||||||
| 3.5 weeks | Organ system | Neural tube formed, primordial eye and ear vesicle, pharyngeal pouches formed, liver bud differentiates, respiratory system and thyroid gland begin to develop, heart tube bends and begins to beat, blood vessels are formed. | |||||||||||||||||||||||||||||||||||||||||||||||||
| 4 weeks | Limb buds | Development and appearance of limb buds, brain forms three primary vesicles. | |||||||||||||||||||||||||||||||||||||||||||||||||
| 2 months | Muscles and gonads | Muscles differentiate, embryo capable of movement, gonads distinguishable as testes or ovaries, ossification of bones begins, cerebral cortex is differentiated, blood vessels assume final position. | |||||||||||||||||||||||||||||||||||||||||||||||||
| 3 months | Sex differentiation | By external examination sex can be determined, notochord degenerates, lymph glands develop. | |||||||||||||||||||||||||||||||||||||||||||||||||
| 4 months | Face |
Face begins to look human, eye, ear and more...
Extra embryonic membrane
An aquatic embryo is surrounded by water, which protects the embryo, keep it moist, removes wastes and permits gas exchange. In land vertebrate (reptiles, birds and mammals), these functions are taken over by the extra embryonic membranes. These membranes are formed outside the embryo from the trophoblastic only in amniotes and perform specific function. Some of these membranes take part in the formation of placenta in mammals.
(i) Yolk sac: It is formed below the embryo. It contains fluid, not yolk. The yolk sac is a vestigial organ inherited from the oviparous reptilian ancestors. Yolk sac encloses by outer mesoderm and inner endodermal layer.
Function: It is mainly digestive in function. It also absorbs the dissolved yolk and passes it to developing embryo in reptiles, birds and prototherian. In human beings, it is vestigial. In human embryo it act as the site of blood cell formation until about the 6th week, when the liver takes over this role.
(ii) Amnion: It is formed above the embryo. It consist of outer mesoderm and inner ectoderm. The amnion and the fluid filled amniotic cavity it encloses, enlarge and nearly surround the embryo. The embryo is suspended in the amniotic cavity by an umbilical cord. The latter is formed of the stalks of the yolk sac and allantois. The main blood vessel from the placenta reach the foetus through the umbilical cord. Amniotic fluid secreted by both embryo and amnion. The cells of amniotic fluid are the basis of parental test called amniocentesis, for the sex of the foetus and for checking chromosomal defects in it.
Functions
(1) The amniotic fluid cushions the embryo.
(2) It protecting embryo against bumps and bacterial infections.
(3) It maintains a constant temperature and pressure.
(4) It protects the embryo from jerk, injury and shocks.
(5) It prevents desiccation of the embryo.
(iii) Allantois: It is a fold of splanchnopleur developed from the hind gut of the embryo. It consist of outer mesoderm and inner endoderm. It is well developed in amniotes with polylecithal egg (e.g. reptiles, birds and prototherians) and stores the nitrogenous waste of the embryo so act as extra embryonic kidney. In most of eutherians, it combines with chorion to form allantochorion placenta. But in man it remains small or reduced and does not reach the chorion. However, it forms umbilical arteries and veins which grow up to the chorion to vascularise it.
Functions
(1) The cavity of the allantois serves as a urinary bladder. It stores the protein breakdown product in the form of water-insoluble crystals of uric acid and inside the egg up to the time of hatching. But with the acquisition of viviparity in the marsupials and the placental mammals, the original function of the allantois as a urinary bladder becomes altogether lost.
(2) The vascular “chorioallantoic membrane” lies in a close proximity to the inner more...
Placenta
(i) Definition: Placenta is defined as a temporary intimate mechanical and physiological connection between foetal and maternal tissues for the nutrition, respiration and excretion of the foetus.
(ii) Structure: Human placenta consist of chorion only. Hence, it is called a chorionic placenta. Allantois remains small. The allantoic blood vessels, however, extend to vascularize it. A large number of branching villi from the vascular chorion penetrate the corresponding pits, the crypts, formed in the uterine wall. The latter becomes very thick and highly vascular to receive the villi. The intimate connection established between the foetal membrane and the uterine wall is known as the placenta. The placenta has two parts: the part contributed by the foetus, i.e., chorionic villi, is called the foetal placenta; and the part shared by the mother, i.e., part of uterine wall, is termed the maternal placenta. The chorionic villi receive blood from the embryo by umbilical artery and return it by umbilical vein. These blood vessels are derived from the allantois and run between the foetus and the uterine wall in the tough umbilical cord covered with cells derived from the amnion and chorion. The choroinic villi come to lie in uterine lacunae that receive blood from the uterine arteriole and return it by uterine venule. The cells forming the wall of chorionic villi bear microvilli which increase their surface area for quick and adequate exchange of materials by diffusion, active transport and pinocytosis.
The placenta is fully formed by the end of the third month and it lasts throughout pregnancy. When complete, it is a reddish – brown disc. In the placenta, the foetal blood comes very close to the maternal blood, and this permits the exchange of materials between the two. Food (glucose, amino acids, simple proteins, lipids), water, mineral salts, vitamins, hormones, antibodies and oxygen pass from the maternal blood into the foetal blood, and foetal metabolic wastes, such as carbon dioxide and urea, also water and hormones, pass into the maternal blood. The placenta, thus, serves as the nutritive, respiratory and excretory organ of the foetus. The continuous uptake of oxygen by foetal blood is ensured by the difference in affinity for oxygen between foetal and maternal haemoglobin.
The maternal and foetal blood are not in direct contact in the placenta, because (i) the two may be incompatible; (ii) the pressure of maternal blood is far too high for the foetal blood vessels; and (iii) there must be a check on the passage of harmful materials (blood proteins, germs) into the foetal blood.
(iii) Functions
(1) Placenta helps in the nutrition of the embryo as the nutrients like amino acids, monosugars, vitamins, etc. pass from the maternal blood into foetal blood through placenta.
(2) It also helps in respiration of the embryo as \[{{O}_{2}}\] of the maternal blood more...
Gestation Period and Parturition
(i) Gestation period: Gestation period is the duration between fertilization and parturition.
Gestation period
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Flower : Bracteate or ebracteate, pedicellate, hermaphrodite, complete, zygomorphic, hypogynous.
Calyx : Sepals 5, polysepalous, imbricate aestivation.
Corolla : Petals 5, polypetalous, ascending imbricate aestivation.
Androecium : 10 stamens, or staminodes are found as in Cassia, free filaments of unequal size, anther lobes bilocular, introrse, versatile.
Gynoecium : Monocarpellary, unilocular, ovary superior, marginal placentation, stigma capitate.
Fruit : Legume.
Floral formula : \[%\,\,\,\,\,\,{{K}_{5}}\,{{C}_{5}}\,{{A}_{1+2+2+2+3\,\left( \text{staminodes} \right)}}\,\text{or}{{\,}_{7+3\,\left( \text{staminodes} \right)}}\,{{G}_{1}}\]
Subfamily – Mimosoideae
Inflorescence : Head or capitulum or spike, flowers arranged in acropetal succession.
Flower : Bracteate or ebracteate, sessile, hermaphrodite, complete actinomorphic, hypogynous, pentamerous.
Calyx : 5 sepals (4 in Mimosa) gamosepalous, connate at the base, valvate aestivation, rarely imbricate (e.g., Parkia).
Corolla : 5 petals (4 in Mimosa) gamopetalous or polypetalous, membranous, valvate aestivation.
Androecium : In most of the members, stamens are indefinite and polyandrous. However, there are only 4 stamens in Mimosa pudica and 10 each in Prosopis and Dichrostachys. Filaments are long, usually connate at the base, sometimes they are coloured and gland dotted. Anthers are dithecous and introrse.
Gynoecium : Monocarpellary, unilocular, ovary superior, style long, cylindrical, stigma single and capitate, marginal placentation.
Fruit : Lomentum.
Floral formula : \[\text{Br}\,\text{or}\,\text{Ebr}\,\,\oplus \,\,\,\,{{K}_{\left( 5 \right)}}\,{{C}_{\left( 5 \right)}}\,\,\text{or}\,{{\,}_{5}}\,{{A}_{\infty }}\,{{G}_{1}}\]
Systematic position
Division : Angiospermae
Class : Monocotyledonae
Series : Coronarieae
Order : Liliales
Family : Liliaceae
Habit : Usually perennial herbs growing by means of rhizomes (e.g., Aloe, Polygonatum), bulbs (e.g., Lilium, Allium) and corms (e.g., Colchicum). Some herbs are annual (e.g., Asphodelus). Shrubs occur in Aloe, Agave, Yucca (Dagger plants, Adam’s Needle), Dracaena (Dragon plant), and Ruscus (Butcher’s Broom). They mostly grow in arid areas and are hence xerophytic (e.g., Aloe, Yucca). Xanthorrhoea of Australia is tree-like. Climbers are seen in Smilax, Gloriosa and species of Asparagus.
Root : Adventitious, fibrous or tuberous (e.g., Asparagus).
Stem : Erect or climbing as Smilex, branched or unbranched, herbaceous, phylloclade as Ruscus. Cladode as Asparagus, Bulb as Allium cepa.
Leaves : Radical or cauline and ramal, cauline and ramal show various types of phyllotaxy (alternate, opposite or whorled), exstipulate, stipulate in Smilax where the stipules are prolonged into tendrils, sessile or petiolate with sheathing leaf bases, venation parallel but reticulate in Smilax, leaves may be scaly, leathery, fleshy or modified into spines (e.g., Asparagus), leaf apex is tendrillar in Gloriosa. The leaves of Phormium tenax (New Zealand Hemp) are 3 metres long and 10 cm broad.
Inflorescence : Recemose, sometimes solitary (e.g., Tulipa, Gloriosa) or umbellate condensed cymes (umbel cyme), e.g., Onion. In several cases the inflorescence possesses a leafless peduncle called scape.
Flower : Bracteate or ebracteate, pedicellate, regular, actinomorphic, zygomorphic in a few cases (e.g., Gilliesia), complete or incomplete, perfect, unisexual in Smilax and Ruscus, hypogynous, generally pentacyclic, trimerous (rarely bimerous or tetramerous).
