Holoblastic early cleavage of Tetrodontophora bielanensis (Collembola) eggs, with special reference to its irregularity.

The fertilized eggs of Tetrodontophora bielanensis start to cleave 6 to 8 days after oviposition and initially only karyokineses occur. The cytokinesis begins after two karyokineses, when four nuclei are observed in the ooplasm. Two cleavage furrows, perpendicular to each other, appear simultaneously at the egg poles where polar bodies are located and gradually the furrows encompass the whole egg diameter. The furrow formation is initiated by the bundle of microfilaments that contract and pull superficial fragments of the oolemma into the yolk and subsequently new membranes, separating the daughter cells, start to form. However, they do not grow towards the egg centre but bifurcate, leaving the central part of the ooplasm outside of the newly formed blastomeres. Starting from the fourth or fifth cleavage division, the bifurcations permanently occur and multiple cleavage furrows are formed on the embryo surface. Moreover, fragments of the ooplasm, enclosed within the cell membrane but devoid of cell nucleus are observed. During further development such cell fragments become reincorporated into the embryo. This mode of cleavage leads eventually to the formation of cellular blastoderm on the embryo surface. The results presented in the paper suggest that the control of cleavage in T. bielanensis acts not at the level of cytoplasmic determinants but rather at the level of positional information of blastomeres.