Hypothesis for presence of two sequential forms of embryo in the early developmental stage in mammals
From 80<sup>th</sup><sub> </sub>years of past century the question is debated as may the fertilized oocyte in mammals be considered as true embryo, or it is pre-embryo, the function of which is the tissue formation for defence and nutrition of future proper embryo. The last suggestion is supported by the fact that in early embryo there is not any definite determination in the development directions of individual blastomeres. The data of comparative embryological investigations suggest that a blastocyst may be considered as parasitic larva whereas a true embryo (<i>embryo proper</i>) is formed within this larva. Most likely, the origin of future foetus is a small number of cells from the inner cellular mass. Whithin blastocyst there may be several groups of such embryonic cells which results in formation of one-egg twins. As this takes place, there is no need for blastocyst morhogenesis neither animal, or vegetative poles. In author's opinion, for final supporting of this hypothesis the study of differential activity of genes and intercellular relationships must be carried out during formation of primary strip and in the course of subsequent processes of gastrulation.