An ultrastructural study of the spermatozoid of the fern, Marsilea vestita.

The ultrastructure of the mature spermatozoid of Marsilea vestita was studied after its release from the microspore and prior to its penetration of the egg. The psermatozoid is a pear-shaped cell with a complex spiral structure coiled around the edge in the narrow anterior end. This coil is composed of a large mitochondrion, elongated nucleus with highly condensed chromatin, a ribbon of microtubules, and a dense band of material (flagellated band) into which the flagella are inserted. There are over a hundred flagella protruding from each spermatozoid along the length of the coil. At the anterior tip of the coil is a short multilayered structure. It is not known what maintains the helical shape of the coil. The microtubular ribbon could be involved, but it is also possible that either the flagellated band, the condensed chromatin, or both, are sufficiently rigid to retain their shpaes unaided. When the spermatozoid is first released from the microspore it includes a cytoplasmic vesicle in the posterior region containing plastids, mitochondria, and other organelles. This vesicle is shed, taking the nuclear envelope with it, before the spermatozoid reaches the egg.