[Evolution of the allantoid placenta in placental mammals]

The article surveys the literature concerning comparative morphology of allantoic placenta and presents some evolutionary-morphological conclusions. The initial form of allantoic placenta is non-deciduate that occurs in 7 orders of placental mammals. The non-deciduate placenta is especially characteristic for lower primates and Cetacea. The non-deciduate placenta in lower primates and the fact that in some modern species of insectivores the non-deciduate placenta is also preserved speaks in favour of the opinion that insectivores of the Cretaceous period had non-deciduate placenta. The non-deciduate placenta in Cetacea that take their origin from ancient carnivores (from procreodonts) demonstrates, in its turn, that the non-deciduate placenta was the initial form. The deciduate placenta was the means for changing many of vital activities of the organism--it is connected with shortening or lengthening of individual life duration and duration of pregnancy. In the course of evolution the allantoic placenta improved towards intensified metabolism between the embryo (fetus) and the maternal organism. Relations between the evolution of mature specimens and evolutional changes in placenta are at their initial stage of investigation.