[Whether variable cleavage of Enoplida (Nematoda) is primitive? Notes to D.A. Voronov article "Comparative embryology of Nematoda and the law of embryologic similarity].
The early embryonic development of Nematoda proceeds by three ways, which strictly correspond to three phylogenetic lineages. Under the first way the endodermal precursor is localized in the posterior blastomere at the two-cells stage (such a determination is the peculiarity of all the Chromadoria, including Secernentea and Caenorhabditis elegans). Under the second way the endodermal precursor is localized in the anterior blastomere of the egg. This feature is very unusual for Metazoa, but it is the only way of entoderm determination in all the Dorylaimia orders (Mononchida, Mermithida, Trichinellida, Dioctophymida, Dorylaimida). The third way described for the sea Enoplida is characterized with variable location of blastomers and changeable localization of endodermal precursor before eight-cells stage. It is still unknown of these three variants was typical the most recent common ancestor of present Nematoda. D.A. Voronov (2001) produced argument in favour of variable cleavage as primitive one for Nematoda. This opinion is rejected because of the similarity in development between sea Enoplida and C. elegans. Both of them share such features as low-cell gastrula and neurula, identical phylotypic lima bean stage of embryogenesis, identity of some geometrical figures 4 or 8 blastomers, isolating of the endodermal precursor at the eight-cells stage, the lack in development of any plesiomorphous features, which are widely distributed outside Nematoda (under the variable cleavage of Enoplida there are no such locations of blastomers, which are typical for spiral or radial cleavage, there are no embryonic leaves as well). One can see the homology of separate cells at adult Enoplida and Rhabditia. Cell lineage of Triplonchida as far as it is described at Tobrilus gracilis doesn't exclude the hypothesis on their origin from the cleavage similar to one of present Dorylaimia with localization of the endodermal precursor in the anterior blastomere. In view of all the considerations mentioned above one should interpret variable cleavage of Enoplida as derivation from invariant cleavage.