The changes which most likely occurred in the egg to cause accidental and obligatory parthenogenesis have been discussed. As regards obligatory parthenogenesis, in the case of restoration of diploidy by alteration of meiosis (that the Author calls intrameiotic regulation), it has been assumed that the stages of meiosis following the sperm entry into sexual eggs could have been more easily modified than those preceding it. Finally it has been pointed out that in heterogonic species the preservation...
The different modes of restoration of somatic chromosome number in both accidental and obligatory parthenogenesis have been summarized. In particular, the automictic parthenogenesis of previous Authors has been divided in parthenogenesis with postmeiotic and intrameiotic regulation, according to whether restoration of diploidy takes place after or during meiosis.
24 specimens of Triops cancriformis from a maleless population living in a rice field of Sibari plain (Cosenza) have been examined. All the animals possess ovario-testes; the degree of hermaphroditism is variable. The testis lobes do not show an uniform distribution along the reproductive system; they are almost exclusively placed in the portion behind the efferent oviduct. On the ground of this observation the Authors think that in Triops there is not a cephalo-caudal maleness gradient, but a progressive...
Parthenogenetic oocytes of Daphnia pulex autonomously syntetize RNA and do not receive RNA from the nurse cells during the growth period.
Origin and fine structure of the germ-cell determinant in summer and winter eggs of Daphnia pulex and D. magna were studied. The germ-cell determinant appears in the perinuclear cytoplasmic region of the oocyte still undifferentiated from its nurse cells, and then it will migrate to the cortical ooplasm. The completely developed germ-cell determinant consists of a coarse network of fine granular and fibrillar material, which is extruded from the nucleus through the pores of the nuclear envelope....
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