[Progressive character of embryogenesis in placental animals].

The article presents evidence of the placental animals being isolated from the rest of higher vertebrates according to many peculiarities of their structure and the embryonic development. The most important of these signs are the following: female genital tracts are disposed to the inside from the urinary tracts (in embryos the Müller's canals are on the inner side of the wolffian ducts); in placental animals there are no tertiary egg sheaths which are physiologically substituted for the trophoblast; the provisory organs reveal great variability; the allantoid placenta performs not only metabolic but also endocrinous functions. The evolution of peculiarities of the structure and development characteristic of placental animals was performed by a series of successive aromorphous phenomena. Theri mighty adaptive radiation is the result of greater perfectness of placental animals.