The 3 groups of mammal are classified by the differences in their reproductive characteristics.
For example; (platypus and echidna) have a single urogenital opening and give birth to their young inside an egg. At the moment of birth, the young are already partly developed but still inside the shell.
Metatheria (the marsupials) also have a single urogenital opening, but give birth to live young. The third group, Eutheria, are all other mammals and are distinguished by a separate opening for birthing the young. The name 'Eutheria' , meaning 'true' mammals was not a good choice, as marsupials are also 'true mammals'. They have all the requisites for producing larger young (like eutherian mammals) but Natural Selection throughout the 120 million years of separate evolution between eutherians and metatherians has resulted in marsupials surviving quite satisfactorily with the birth of a tiny young. Maybe, it's because the mothers are so good!!
For example; (platypus and echidna) have a single urogenital opening and give birth to their young inside an egg. At the moment of birth, the young are already partly developed but still inside the shell.
Metatheria (the marsupials) also have a single urogenital opening, but give birth to live young. The third group, Eutheria, are all other mammals and are distinguished by a separate opening for birthing the young. The name 'Eutheria' , meaning 'true' mammals was not a good choice, as marsupials are also 'true mammals'. They have all the requisites for producing larger young (like eutherian mammals) but Natural Selection throughout the 120 million years of separate evolution between eutherians and metatherians has resulted in marsupials surviving quite satisfactorily with the birth of a tiny young. Maybe, it's because the mothers are so good!!