Embryological method of determination of the relationships between the orders of Placentalia
Shmidt, G.A.
Arkhiv Anatomii Gistologii i Embriologii 73(9): 35-47
1977
ISSN/ISBN: 0004-1947 PMID: 563714 Document Number: 121209
Embryological data should be taken into consideration to determine systematic position of animals. For the peculiarities of the Mammalia Placentalia provisory organs are of special importance. Sharp specialization of morpho-functional peculiarities in mature species results in specialization of these organs. These relations are especially clear in small placentals, e.g., the Rodentia. An increased death rate produced in the Rodentia an intensified reproduction, specialization of the provisory organs and reduction of life span. The allantoic placenta underwent alterations which concentrated transport of the nutrients and O2 to different parts of the body. Inversion of the germ layers and the provisory organs serves the same end. In the Duplicidentata (representatives of an ancient order) provisory organs are very primitive (central implantation, folding amnion, large yolk sac and yolk placenta of prolonged functioning). The allantoic placenta of Duplicidentata is highly specialized, it is a perfect hemoendothelial placenta. Preservation of the provisory organs in close to the initial state is determined by antiquity, lack of precise specialization owing to large body dimensions and rather weak annihilation (Manidae, Tylopoda, lemurs and Centetes ecaudatus). Embryological data permit the problem of systematic position of Tupaidae to be solved. These ancient insectivores give some idea as to the original forms in lemurioid evolution. Embryological data supported Havenson's view (1961, 1963) of a distant relationship between Tylopoda and Nonruminantia on the basis of comparative-morphological peculiarities. Provisory organs in these groups differ sharply. In Nonruminantia, the amnion develops at the stage of primitive streak, the yolk sac is medium-sized, the yolk placenta functions for a very short time and the allantoic placenta is invasive. In Tylopoda, the amnion develops when the 1st paired somites form, the yolk sac is large, the yolk placenta functions for a long time and the allantoic placenta is non-invasive.