152 DON W. FAWCETT 



peripheral doublets. The mid-portion of each of these radial laminae 

 is slightly thickened. Our observations are based mainly upon the ex- 

 amination of unstained cross sections (Fig. 2 and 3). Although the 

 micrographs lack the high contrast and diagrammatic clarity of the 

 elegant micrographs of protozoan flagella published by Gibbons and 

 Grimstone, it is quite possible that they retain more detail. When 

 heavy metal staining is used and every photographic device is em- 

 ployed to gain contrast, the more delicate portions of the radial pat- 

 tern of connections in the matrix may be lost and only the thicker 

 central portions of the dense laminae are then recorded as a series 

 of dots midway between the central and peripheral fibrils. This may 

 be the basis of Gibbons and Grimstone's conclusion that there is a 

 set of secondary filaments interposed between the central and pe- 

 ripheral fibrils. 



THE OUTER COARSE FIBERS 



A characteristic feature of mammalian sperm tails is the presence 

 of an extra outer row of nine coarse longitudinal fibers that are 

 lacking in the sperm tails of fish and many of the invertebrates. The 

 internal structure of the tails of invertebrates is essentially identical 

 to that of flagella and cilia generally. Instead of the familiar pat- 

 tern of nine-plus-two fibrils, the mammalian sperm tail has nine, 

 plus nine, plus two, or a total of 20. The fibers of the outer row are 

 very dense, homogeneous, and evidently composed of a substance 

 quite different from that of the inner fibrils. Except for a few ob- 

 servations on their solubility properties and resistance to enzymatic 

 digestion (Bradfield, 1955), very little is known about the physical 

 properties or chemical nature of these fibers. It is generally assumed 

 that they are accessory contractile elements possibly serving to over- 

 come the resistance to bending offered by the relatively thick mito- 

 chondrial and fibrous sheaths, thereby strengthening the propulsive 

 force of the tail. The nine coarse fibers vary in their thickness and 

 cross-sectional shape. The differences are least conspicuous near the 

 junction of the head and tail (Fig. 4) but become more obvious in 

 the midpiece and principal piece. In several common rodent species 

 three of the fibers are distinctly thicker than the others. It will fa- 

 cilitate description to be able to designate these fibers by number. 



