P. acericola presents a clear case of physiological haploidy in the [male] line. In both sexes 2n=12. The cytology of the [female] is orthodox in all eggs. In the [male], 1 whole haploid group of chromosomes maintains a permanent pycnosis from the early blastula stage on. In meiosis the 1st division is equational for both the pycnotic and the normal chromosomes. In the 2nd division a single half-spindle forms from the pycnotic group and these chromosomes alone pass to its single pole, thus effecting a reduction without synapsis. No cytoplasmic division accompanies this, and the binucleate spermatids thus formed immediately fuse in pairs. In each quadrinucleate spermatid the 2 nuclei of pycnotic origin degenerate in situ; only that half of the spermatid nuclei derived from the normal haploid chromosome group produces sperm.