[Debatable questions in the theory of parenchymella]

Certain variants of the theory are compared. As the precursors of parenchymella Mechnikov's blastea, Lang's blastea, amphiblastea, homogenous parenchymella were supposed. The parenchymella could be formed either via multipolar or unipolar immigration of blastodermal cells. It is not clear whether the differentiation of the parenchymal cells took place before or during their immigration. The initial-parenchymal structure of Metazoa evidences in favour of a divergent differentiation and not the immigration of the external and internal cells. In order to evaluate further evolutional ways of the parenchymella, it is important to notice that there are no specialized digestive cells in the parenchyma of calcareous sponges and that endomesodermal myocytes in lower invertebrates are similar to myoepithelial cells. The theory of genitogastrea is discussed. It is noted that the genitoparenchymella should be considered as the precursor of the genitogastrea. Preliminary calculations have demonstrated that small genitoparenchymellas could be stable, and when the genitoparenchymella is formed from the blastea, including more than 10,000 cells, it is possible that the parenchymella is converted into the genitoblastea much more quickly. Weak points and possible suggestions of Zakhvatkin's theory on the origin of Metazoa from metagenetic protists are discussed.