[Kinematics and ultrastructure of plasmic factor regions in the egg of Wachtliella persicariae L. (Diptera) : II. The behaviour of ooplasmic partial systems after centrifugation of eggs in the stage of four cleavage nuclei].

Kinematics and ultrastructure of centrifuged and untreated eggs fromWachtliella persicariae were investigated for the micromorphological properties of ooplasmic factor regions and their role in early developmental processes by means of time-lapse motion pictures and electron microscopic analysis (see part I).After centrifugation the eggs show up to five different layers, among them a pole of fatty yolk with lipid droplets, a region of clear plasm (rich in ground plasm) which itself may become subdivided into a centripetal region with nuclei and endoplasmic reticulum, followed by a centrifugal part with mitochondria and ribosomes, another region containing orange clods of proteid yolk and finally a cup of glycogen. Displacement of pole plasm from the posterior pole always is accompanied by dislocation of the basophilic oosome material contained therein. At sufficient r.p.m. both of them enter the centripetal area of clear plasm. Structures of "di-polar density" type are orientated by centrifugation. The initial phase till the centrifuge reaches its final r.p.m. may act decicively upon the site of certain egg components after centrifugation as upon the nuclei, and thus may essentially influence the experimental results. In case centrifugation coincides with certain dividing phases of energides, the nuclear envelope becomes fragmented. The fragments then may appear piled up to form annulated membranes which have been recognized as pathological structures in centrifuged eggs. Besides lamellar cytosomes are often found. In centrifuged as well as in untreated eggs the nuclear envelope either consists of two layers as usual or may be of the complex multi-layered type (see part I). As for the movement of nuclei, the possible role of the complex nuclear envelope is not yet clear. The pigment halo of cleavage nuclei does not play an active part in nuclear migration. In centrifuged eggs yolk nuclei are of the usual type i.e. either roundish, horse-shoe-shaped or multi-lobed. They mostly appear in parts of the entoplasm which are poor in yolk. A surrounding rich in yolk does not seem to be essential for transforming normal cleavage nuclei into vitellophagues. For their changing into the multi-lobed type yolk nuclei must be surrounded by a sufficient amount of ground plasm.Pole cells have been found in the posterior pole region only. Their formation requires an abundant amount of ground plasm, the presence of cleavage energides, as well as pole plasm and oosome material, if not either of the two latter systems. Since in centrifuged eggs pole plasm and basophilic oosome material are always shifted together into the region of clear plasm, contrary to the opinion of other authors (p. 42; part I, p. 124) the technique of centrifugation does not permit any decision as to which of both ooplasmic systems controls the karyotic differentiation of the germ line or the formation of pole cells, respectively, or whether both systems are essential to promote these processes.The oolemma may also become invaginated to form cell membranes when no nuclei are present ("pseudoblastoderm"), the formation occurring in regions with sufficient amounts of ground plasm only. For that reason the formation of pole cells is restricted to the posterior pole rich in ground plasm, whereas blastoderm cells exclusively occur in the area of preblastoderm plasm. The ground plasm plays a decisive part in the dynamics of cell membrane formation. As for blastoderm cells, the nuclei seem to be necessary only to control their regular shape. In contradiction to the opinion of other authors (p. 42, part I, p. 124), the periplasm of young eggs cannot range among the essential prerequisites of blastoderm formation. During centrifugation it does not stay at the surface of the egg poles where nevertheless a blastoderm may be formed. Yet blastoderm formation is only possible if, in spite of the compact condition of polar yolk material, the egg poles become covered with preblastoderm plasm from the region of clear plasm, rich in ground plasm, and thus replacing sufficient amounts of periplasm and ground plasm shifted by centrifugation.