[The fine structure of the spermatozoid of Sphaeroc arpos donnellii Aust. (Hepaticae)].

The structure of the spermatozoid of the liverwort, Sphaerocarpos donnellii, was investigated under the electron microscope after fixation in potassium permanganate, osmium tetroxide or glutaraldehyde with postfixation in osmium tetroxide. Mature, newly emerged spermatozoids consist of three parts: Head end, nuclear piece and the attached cytoplasmic part. The two flagella originate in the head end. They show the typical structure of nine outer double fibers around two fibers in the middle. Connections exist between the central and the outer fibers. At least to some extent they are composed of thin, tubular fibers, about 70 Å in diameter. The head end contains a body which may be regarded as a modified mitochondrion or a plastid.Nearly the whole space of the nuclear piece is occupied by the nucleus, with a length of a about 13 μ and a thickness up to 0,4 μ. The dense nuclear content shows above all, approximately 25-40 Å thick fibers, which are often packed closely together. A typical double membrane as nuclear envelope is not recognizable. Rarely one observes a dark line, not thicker than 40-60 Å which may be interpreted as limiting membrane of the nucleus. A thin band of cytoplasmic material may be interposed between the nucleus and the double membrane, which has a thickness of about 80-100 Å and surrounds the whole spermatozoid.The cytoplasmic piece includes the big leucoplast, mitochondria, membranes of the endoplasmic reticulum, multivesicular bodies, small vesicles and occasionally vacuoles. The interior of the leucoplast is filled with numerous starch granules, up to 0,2 μ in diameter. Only rarely some remains of the thylacoid system appear. The mitochondria show a modified fine structure compared with the corresponding organelles in the spermatids. The cytoplasmic end comprises bodies which are limited by a double membrane and which contain double membranes. They are absolutely alike the membraneous body in the head piece and have to be regarded as modified mitochondria or plastids.Immediately below the membrane of the spermatozoid one recognizes a structure, named "Fibrillenscheide"=fibrous sheath, which in most cases expands from the nuclear piece into the cytoplasmic part. The 400-800 Å thick fibrous sheath consists of up to 30 fibers lying side by side, each with a diameter of about 180-220 Å. The fibers at both ends of the fibrous sheath possess a diameter of about 300 Å. A double membrane encloses all the fibers together and separates them from the limiting membrane and other components of the spermatozoid.The cytoplasmic end is lost, at the latest when the spermatozoid enters the open neck canal of the archegonium. The spermatozoid which has reached the egg cell is composed only of the head end and the nuclear piece.