ИСТИНА

Oogenesis of phoronids was studied in only a species <Phoronopsis harmeri>, whose development completely occurs in sea water. According to previous results, in phoronids, there is a combination of simply developed follicle and autosynthesis. We describe here for the first time the new type of oogenesis, which is discovered in viviparous phoronid <Phoronis embryolabi>. This species broods embryos in the trunk coelom until stage of completely developed larva. Oogonia develop around the blood vessel in the posterior part of the trunk. Cytoplasm of previtellogenic oocytes contains centriole, a few rough endoplasmic reticulum, and germ plasm: several clusters of numerous osmiophilic particles of different size (from 0.1 to 0.8 µm) and shape (from round to irregular), which are surrounded by mitochondria. Mature oocytes are about 40 µm in diameter; their cytoplasm is filled with numerous lipid droplets and proteinaceous granules. The surface of mature oocyte bears short branching microvilli. Each oocyte is attached to the basal lamina of the blood vessel and surrounded by cells of vasoperitoneal tissue (VPT). These are flat cells with long thin projections, which contact projections of other VPT cells via desmosomes. Cytoplasm of VPT cells is electron dense and is fully occupied by glycogen particles. There are cytoplasmic bridges between developing oocytes and VPT cells. These roundish spaces between cells are about 20 nm in diameter and lack cell membrane. The presence of these bridges together with poor developed synthetic apparatuses in oocyte allows to suggest the transport of nutriments or (and) RNAs from VPT cells to oocyte. Thus, oogenesis of viviparity phoronid can be characterized as nutrimental with developed follicle.