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The evolution of reproduction in marsupials, and speculation about the ancestral state of mammalian reproduction, have engaged discussion since the end of the 19th century. Both sexes possess a cloaca , [ 17 ] although modified by connecting to a urogenital sac and having a separate anal region in most species. [ 18 ]
Marsupials' reproductive systems differ markedly from those of placentals, [10] [11] though it is probably the plesiomorphic condition found in viviparous mammals, including non-placental eutherians. [12] During embryonic development, a choriovitelline placenta forms in all marsupials.
The mammals included are only viviparous (marsupials and placentals) as some mammals, which are monotremes (including platypuses and echidnas) lay their eggs. A marsupial has a short gestation period, typically shorter than placental. For more information on how these estimates were ascertained, see Wikipedia's articles on gestational age.
Marsupials have a separate genital tract, whereas most placental females have separate openings for reproduction (the vagina), urination (the urethra), and defecation (the anus). In monotremes, only semen passes through the penis while urine is excreted through the male's cloaca. [ 21 ]
As marsupials, female opossums have a reproductive system that includes a bifurcated vagina and a divided uterus; many have a pouch. [29] The average estrous cycle of the Virginia opossum is about 28 days. [30] Opossums do possess a placenta, [31] but it is short-lived, simple in structure, and, unlike that of placental mammals, not fully ...
In marsupials, the genital tract is separate from the anus, but a trace of the original cloaca does remain externally. [102] Monotremes, which translates from Greek into "single hole", have a true cloaca. [105] Urine flows from the ureters into the cloaca in monotremes and into the bladder in placentals. [102]
Because the amnion and the fluid it secretes shields the embryo from environmental fluctuations, amniotes can reproduce on dry land by either laying shelled eggs (reptiles, birds and monotremes) or nurturing fertilized eggs within the mother (marsupial and placental mammals).
Due to the fact that placental mammals and marsupials nourish their developing embryos via the placenta, the ovum in these species does not contain significant amounts of yolk, and the yolk sac in the embryo is relatively small in size, in comparison with both the size of the embryo itself and the size of yolk sac in embryos of comparable developmental age from lower chordates.