In this article is presented a novel subunit model for the tropocollagen (TC) macromolecule, the monomeric unit of soluble collagen.1 The TC macromolecule is a rigid rod of definite length composed of three helical polypeptide strands that are equal (or very nearly equal) in length.2 4 The strands are of two types chemically, designated a1 and a2.4 Each macromolecule contains two al strands and one a2 strand.4 Our thesis is that each a1 strand is a repeating sequence of a subunit

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SummaryThe morphology of the fowl spermatozoon, as revealed by use of the electron microscope and such techniques as partittl enzymic digestion and disruption with distilled water, is described in detail, and compared with that observable by light microscopy.The sperm head carries at its anterior extremity a spindle-shaped body, the apical spine, which normally is closely covered by a conical membranous cap. The apical cap, which has been overlooked by previous workers, may be detached by dilution of the semen with distilled water. It seems likely that these structures are intimately concerned in the penetration of the vitelline membrane during fertilization.The axial filament, which contains nine L fibrils arising from the al}terior distal centriole and two M fibrils, passes through the mid-piece and continues the full length of the tail, approaching 100 Il. in length. It is surrounded in the mid-piece by a number of granules, presumably of mitochondrial origin, which are arranged to give an appearance of bilaterally symmetrical segmentation. The mid-piece is externally surrounded by a delicate membrane easily disrupted in distilled water. There is no evidence for the presence of a spireme or other helically-wound structure in the mid-piece.In the tail, the axial filament is encased in an amorphous sheath, which decreases in thickness towards the tip of the tail and is easily disrupted by distilled water, allowing the axial filament to fray into eleven fibrils. Two of these fibrils are differentiated from the remaining nine by their dimensions and greater susceptibility to distilled water. It is suggested that the nine L fibrils constitute the locomotor organ of the sperm. It is possible that the two M fibrils function as a rudimentary nervous system.In direct contrast with the state prevailing in mammalian sperm, there is no helically-wound cord surrounding the axial filament in the tail. This seems to explain why the tails of fowl and of certain other sperm fray easily in distilled water, while those of mammalian sperm do not.Certain dilution phenomena are explained by the presence of an adsorbed layer of colloidal material, which is removable by great dilution or repeated washing of the sperm. The layer greatly modifies the rate of osmosis in hypotonic solutions. There is no trace of a lipoid or other capsule external to the cell-wall, as has been postulated to explain similar protective phenomena occurring with other sperm .

Electron microscope studies of chloroplasts have confirmed the lamellar structure postulated from polarization optical and other studies (9). In the chloroplasts of lower plants, e.g., Euglena (28), Chlamydomonas (22), Chlorella (1), S~rogyra (25), and Nitella (18), and in the chromoplasts of brown algae (Fucus (17)), the lamellae are fairly well ordered, but grana have not been demonstrated. In the higher plants, grana are present in the chloroplasts, and both the grana and the intervening regions (intergrana regions) are laminated. The grana consist of well ordered stacks of lameUae, e.g., Beta (15), Aspidistra (7,15,16,26), Nicotiana (4), and Zea (12).The present paper is concerned with the finer structural details of the lamellae and grana, and with the interrelationships existing between the lamellae of the grana and those of the intergrana regions. It will be shown that the individual lamellae of both the grana and intergrana regions of Zea exhibit a characteristic compound layer structure when ultrathin sections are examined in the electron microscope. A possible interpretation of this compound structure in terms of the lipide, protein, and other components of the chloroplast will be given. Materials and MethodsThe chloroplasts were examined in situ. Small portions of the leaves of 3 to 4 week old maize plants were excised and fixed for 3 to 6 hours in 2 per cent osmium tetroxide solutions buffered to pH 7.2 with acetate-veronal (19) and adjusted to 0.2 ~ with sodium chloride. After washing and dehydration in an ethanol series, the leaf portions were embedded in n-butyl methacrylate, with or without the addition of up to 10 per cent of the methyl monomer. Portions of the leaves of efiolated seedlings were excised at various time intervals after exposure of the plants to daylight, and were prepared for examination in a similar manner. Thin sections were obtained using a microtome and the technique already described (11), and were examined in the electron microscope (RCA model EMU-I, fitted with an externally centerable objective aperture) without removal of the embedding medium. 6O5J.