As a result, they have found a much higher volume density of mitochondria in germline cells than in somatic cells and it has been suggested that the energy requirements of germline cells might be higher than those of somatic cells. Such an interpretation of stereological results corresponds with the ��disposable soma theory��, which claims that all of the syntheses in NVP-BEZ235 germ-line cells must be very accurate in order to avoid the accumulation of defective molecules and to keep the cells fit for further generations. Therefore, germ-line cells should possess more mitochondria than somatic cells because such a LY2109761 strategy requires high energy consumption. Recently, studies using substances that directly show the level of mitochondrial activity have provided us with contradictory results. Some studies on germ-line cells have reported a high level of activity of mitochondria in Xenopus and zebrafish oocytes or in the germinal plasm in Drosophila embryos while other studies that were recently carried out on human, mouse, bovine and, Xenopus oocytes and eggs using confocal microscopy and substances that indicate the level of mitochondrial activity have shown that the mitochondria in germ-line cells behave in a different way than the mitochondria in somatic cells and that during oogenesis or at least at certain stages of this process, a large number of these organelles remains inactive. Moreover, the studies that have been carried out on the early embryos of different mammals have also confirmed that the majority of mitochondria remain inactive not only in germ-line cells but also in some blastomeres and embryoblast cells. The data from invertebrates are few and scarce. There are several chemicals that can be used for the assessment of mitochondrial activity in living cells. Rhodamine-123, Mitotracker, DiOC6 and JC-1 have been used most frequently. The substances must be used very carefully in order to avoid the misinterpretation of results. JC-1 was the most suitable for our study because it can show both the active and inactive mitochondria in the same cell. After entering into the mitochondria, it changes color from green to red as the mitochondrial membrane becomes polarized and the aggregates of the JC-1 stain form. Therefore, we decided to use this substance to study the activity of the mitochondria in the gonads of the annelid Dendrobaena veneta. In addition, we used stereological methods to assess the volume density of these organelles in the gonads of the annelid in order to confirm the results for Collembola embryos that were presented in our previous papers.