Differentiation of primordial germ cells during embryogenesis of Allacma fusca (L.) (Collembola : Symphypleona)

The youngest primordial germ cells (PGCs) of Allacma fusca (L.) (Collembola : Sminthuridae) can be identified in embryos at the blastoderm stage as sca ttered in the yolk mass. They are arranged in pairs connected via intercell ular bridges and dispersed among the yolk granules over a relatively small area but they never form multicellular clusters. With progressing developme nt, the mesoderm of the germ band differentiates, the PGCs migrate to the a bdominal part of the germ band and enter among mesoderm cells making two cl usters of cells in the left and right parts of the abdomen. The mesoderm ce lls neighbouring the PCC cluster differentiate into a one-layered gonad env elope and produce a thin basal lamina separating the gonad from the rest of the mesoderm. The PGCs are still connected in pairs. At the end of the emb ryonic development, the gonads have regular spherical shapes and are enclos ed within the envelope built up by a layer of flat somatic cells. Now, the PGCs do not occur only in pairs, but chains of cells connected with a seque nce of intercellular bridges can also be seen. (C) 1999 Elsevier Science Lt d. All rights reserved.