The platypus and the echidnas are mammals that lay eggs. The young hatch as very immature individuals for some time after birth. The pouched mammals, for example Kangaroos, do not lay eggs, but they give birth to very undeveloped young after they have spent an extremely short period of development within the mother. The young continue their development clinging to the nipples of the mother inside the pouch. The young of placental mammals, however, have a long period of development within the mother. This is permitted by means of an elaborate organ, the placenta. It allows extremely close contact between the blood vessels of the mother and those of the growing embryos so that adequate supplies of food (e.g. sugar molecules) are continually carried to them. Within the mother they are also protected. Placentals have an almost constant body temperature, maintained above that of the surroundings. An embryo enclosed in this way within remarkably constant surroundings for a long period of time, can reach a high degree of development (though not all placentals are well developed at birth). A placenta is one of several important factors that allow the great development of the nervous system associated with placental animals, and so their whole behavior patterns are more complicated. A young giraffe can run almost as fast as its parents soon after birth; a young zebra can leap in the air. A newly born opossum, on the other hand, is able merely to make a journey to the pouch.
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The placenta is formed from the lining of the mother's uterus and certain of the embryonic tissues - the allantois. For this reason it is known as allantoic placenta. The placenta is rich in blood vessels, partly supplied by the mother and partly by the embryo or fetus. Food materials and oxygen pass from the blood vessels of the mother into those of the fetus, and waste substances (e.g. urea and carbon dioxide) in the reverse direction. The placenta persists throughout the development of the young.
The length of time that the young are carried by the mother varies from 3 weeks in the house mice to 22 months in elephants.
The majority of female placentals will pair only at certain times of the year. each of these is referred to as "estrus" or "heat." Estrus is preceded by a preparatory phase called "proestrus." The beginning of this period marks the onset of the sexual season. Eggs begin to develop in the ovaries and there are other marked changes in the reproductive organs. Activities of the endocrine system "prepare" the nervous system of an individual so that it reacts in the correct way to stimulation by individuals of the opposite sex during estrus.PlatypusOrnithorhynchus anatinus
The whole variety of internal and external changes is designed to bring male and female together at the right time - the chances of fertilization are high, therefore. To ensure that the fertilied egg is protected and nourished from the start, the uterus is fully prepared to receive it. If an egg is fertilized successfully, and is retained witin the body of the female, gestation follows. This is the period during which the young are nourished and protected within the mother. Gestation is followed by nursing or lactation - the suckling phase during which the young feed on the mother's milk.
A resting, non-breeding phase (anestrus) may follow lactation. In the rat the birth of the young is followed by another proestrus and estrus during lactation. Alternatively, lactation may be followed almost immediately by another period of "heat."
If fertilization does not occur, estrus is followed by a brief recovery period - metoestrus. Then the changes that occurred during proestrus and estrus slowly subside. Alternatively changes occur similar to those that take place during pregnancy. The pseudopregnant period is longer than metoestrus. Pseudopregnancy is then succeeded by another estrus or in some instances anestrus. The changes described above constitute the estrous cycle.
There is considerable variation in the length of the estrous cycle in different species. Some species have only one estrous cycle a year (bears). They are said to be monoestrous. Polyestrous species (most primates) have several cycles in the course of a year. The act of pairing usually takes place on land, but whales and their relatives, the Muskrat, the European otter and sertain seals, pair in water.