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The class Mammalia is divided into two subclasses based on reproductive techniques: egg-laying mammals (yinotherians or monotremes - see also Australosphenida), and mammals which give live birth . The latter subclass is divided into two infraclasses: pouched mammals ( metatherians or marsupials ), and placental mammals ( eutherians , for which ...
The traditional "Theria hypothesis" states that the divergence of the monotreme lineage from the Metatheria and Eutheria lineages happened prior to the divergence between marsupials and placentals, and this explains why monotremes retain a number of primitive traits presumed to have been present in the synapsid ancestors of later mammals, such ...
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.
An early birth removes a developing marsupial from its mother's body much sooner than in placentals; thus marsupials have not developed a complex placenta to protect the embryo from its mother's immune system. Though early birth puts the tiny newborn marsupial at greater environmental risk, it significantly reduces the dangers associated with ...
The class Mammalia is divided into two subclasses based on reproductive techniques: monotremes, which lay eggs, and therians, mammals which give live birth, which has two infraclasses: marsupials/metatherians and placentals/eutherians. See List of monotremes and marsupials, and for the clades and families, see Mammal classification ...
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.
Their function in non-placental mammals is to stiffen the body during locomotion, [5] but in placentals they would inhibit the expansion of the abdomen during pregnancy.) [6] the rearmost bones of the foot fit into a socket formed by the ends of the tibia and fibula, forming a complete mortise and tenon upper ankle joint. [7]
Modern mammals (class Mammalia) are divided into Monotremes, Marsupials, and Placentals. The subclass Prototheria (Monotremes) comprises the five species of egg-laying mammals: platypus and four echidna species. The infraclasses Metatheria (Marsupials) and Eutheria (Placentals) together form the subclass Theria. [4]