Bundles of sperm in smear from testes of American Robin (Turdus migratorius), upper, and Rufous-sided Towhee (Pipilo erythrophthalmus), lower, by phase contrast microscopy. Stain: Feulgen and Light Green. Approximately 320 X. Note the relatively long length of the towhee sperm bundles. OSCINE SPERMATOZOA 43
Abstract.-The living spermatozoa of the flatworm Mesostoma georgianum have a sperm body about 100 u long and 0.5 , wide, and two motile free flagella, ca. 200 A long. In sections examined with the electron microscope, these flagella have the usual nine pairs of peripheral doublet microtubules and have a single central core unit which is connected to the A members of the doublets by spokelike structures. There are also short connections between the doublets and the flagellar membrane. In material negatively stained with phosphotungstic acid, the doublet microtubules seem to have very different elastic properties than the core; they tend to fall on the copper grids in coils of rather uniform diameter (2-4 ,A), while the core is much more rigid and is often found extending alone, along a relatively straight course, for very long distances (up to 73 1A).After negative staining, the core has a striking appearance with a dense center around which are wound two hollow structures in a double helix of 450 pitch.The center-to-center distance of each gyre is approximately 650 A, and the hollow structures are ca. 180 A in diameter.Observations on patterns of microtubules in spermatozoa which do not have the usual 9 + 2 configuration are undoubtedly of considerable significance with respect to a general theory of flagellar motility. We have for some time been making a systematic survey of such patterns in spermatozoa of turbellarians, and have reported on the occurrence of a 9 + 0 pattern in motile spermatozoa of the acoels Polychoerus,l Childia,2 and Anaperus.3 The present paper is concerned with a "9 + 1" pattern in spermatozoa of a rhabdocoel, Mesostoma georgianum Darlington, 1959, and to our knowledge this is the first report of the occurrence of such a pattern in this order.Other reports of the "9 +
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