Ryuichiro MACHIDA and Hiroshi ANDO
Sugadaira Montane Research Center, University of Tsukuba, Sanada, Nagano 386-22, Japan The cleavage of Microcoryphia was reported by Heymons and Heymons (1905) for Trigoniophthalmus alternatus (Machilinae) and by Larink (1969) for Petrobius brevistylis (Petrobiinae). Those observations are all coincident in that the cleavage is of the typical superficial type as those of the other Thysanura s. lat. (Microcoryphia + Thysanura s. str.) and the Pterygota. On the basis of the cleavage type insect comparative embryologists have conceived a closer affinity between the Microcoryphia and Thysanura s. str., and they have taken it for granted the unity of the Thysanura s. lat. and further of the Euentomata (Thysanura s. lat. + Pterygota) (Sharov, 1966; Jura, 1972). We have been studying the early embryogenesis of a microcoryphian, Pedetontus unimaculatus Machida (Petro- biinae) in detail, with employing Karnovsky's fixative and metacrylate or epoxy resins as an embedding medium, and we came to a conclusion that the cleavage of this insect is, at least in earlier stages, holoblastic. Here we briefly report our observations. First six or seven nuclear divisions are performed radially, with the centrifugal migration of cleavage nuclei. First at the third division, furrows appear between adjacent nuclei on the egg surface. In the following divisions the furrows gradually deepen to divide the yolk mass completely into yolk blocks or blastomeres. The third to eighth nuclear divi- sions are accompanied by cytoplasmic division. Each resulting blastomere contains 'a single nucleus. Figure 1 a and b are photographs of a 32-nucleus embryo, in which the cytoplasmic division is in half the way of the process, and constrained blastocoels are found between adjacent blastomeres. From the seventh to eighth divisions, tangential divisions take place, although radial ones are predominant, resulting in the formation of blastomeres localizing away from egg surface. The nuclei of these off-surfaced blastomeres are future primary yolk nuclei. After the eighth or ninth nuclear division no cytoplasmic division was observed except in the periplasm. In ca. 250-cell embryos, the nuclei other than the yolk nuclei have settled in the periplasm. The peripheral nuclei proliferate to form blastoderm. A part of their daughter nuclei are liberated into yolk to become secondary yolk nuclei. Both the prim- ary and secondary yolk nuclei undergo mitoses to increase in number up to ca. 1000 are counted, when 2000-3000 nuc- lei are in the just completed blastoderm. With the proceeding of blastoderm formation, boundaries of blastomeres gra- dually fade out to vanish. As mentioned above, in P. unimaculatus the embryonic development starts with the total cleavage, and then comes to the superficial one, with the restriction of cytoplasmic division to the egg periphery. Similar cleavage pattern is observed in Haslundichilis sp. (Machilinae) (unpublished). Here, we conclude that the cleavage is, at least in earler stages, holoblastic in two subfamilies of the Machilidae, Microcoryphia. Our observataion makes a clear contrast with Heymons and Heymons' (1905) and Larink's (1969) observation for the species of the same two subfamilies that the cleavage is of typical superficial type. Cleavage of T. alternatus and P. brevistylis should be reexamined by a method we used. The cleavage of P. unimaculatus, and also of Haslundichilis sp., resembles those of the Collembola (Jura, 1965). and some myriapods (Tiegs, 1940, 1947; Dohle, 1964), and it is fairly different from the superficial cleavage of the Thy- sanura s. str. (Heymons, 1897; Sharov, 1953; Wellhouse, 1954; Woodland, 1957) and Pterygota. The phylogenetic rear- rangement of the lower hexapods may be desired through the consideration on the type of cleavage. Further phylogene- tic discussion and details of early embryogenesis including the cleavage of P . unimaculatus will be published near future. This work was supported by Grants-in-Aid from the Ministry of Education, Science and Culture, Japan (63740433) and from the University of Tsukuba Project Research.