Comparative embryology of nematodes and the law of embryo similarity

Two types of embryonic development can be distinguished within nematodes, with a variable (Enoplia) or invariant (remaining species) cleavage. In the case of invariant cleavage two main variants of cell lineage are presented in nematodes, with the posterior (Rhabditea) or anterior (Dorylaimida) localization of endoderm material at the two-cell stage. This classification is in a good agreement with some modern nematode taxonomy and it is supported by molecular phylogeny studies. The variable cleavage is plesiomorphic. Traditional concept of "mosaic" cleavage is not applicable for nematodes as inductive interactions and a regulation of experimental interventions are usual attributes of any mode of nematode development. The representatives of order Rhabditida have almost identical cell lineage, but at the same time they have strong interspecific differences in mechanisms of ooplasmic segregation any early inductive interactions. The diversity of geometric patterns in the early cleavage, often at the level of individual random variations, is a usual characteristic of nematodes including species with the invariant cleavage. Thus, the early stages of nematode development are evolutionary very flexible, but at the course of embryonic development similarity of different species is progressively increased up to the uniform morphogenetic stages. The dynamics of variation in nematode development contradict to the von Baer's law but are in an agreement with the modern "hourglass model" (Doboul, 1994; Raff, 1986).