ИСТИНА

Cnidarians and particularly hydroids display a great variety of modes of embryonic development. Details of this process strongly depend on the general mode of reproduction and details of sexual stage development which not always correlate with group phylogeny. Oocytes of Sarsia lovenii develop in slightly reduced but attached medusoid stage. Mature eggs are released in surrounding water. After fertilisation eggs become sticky, adhere to the substrate and further development takes place within the mucous sheath. The cleavage is complete, equal and polyaxial. Blastula with minute blastocoel develops. Gastrulation proceeds at the 128-cells stage in a form of primary delamination and ends with parenchymula development. Further on ectodermal cell differentiation starts and in 24 hours post fertilisation competent planula escapes from the sheath. First cells of the nervous system were detected soon after. Few tubulin-positive cells form a belt of sensory cells at the level of endodermal core. A bit later some cells arranged in rosette-like structure around the anterior larva pole start displaying anti-RFamide immunoreactivity. Other cells at the apex of anterior pole also display weak and diffuse anti-RFamide immunoreactivity. RFamide-positive cells also look like the sensory cells. No real ganglionic cells were found. Embryonic development of S. loveni has certain peculiarities: (1) the larva consists of relatively few cells; (2) cell proliferation after cleavage quickly slows down, particularly in endoderm; (3) nervous system consists of few cells and precisely organised; (4) competent planula include only about 10 nematocytes; (5) embryonic development ends within 24 hours under 15°C.