Comparative spermatology of 11 species of Polyplacophora (Mollusca) from the suborders Lepidopleurina, Chitonina and Acanthochitonina

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Abstract
A theory of the three-dimensional structure of the collagen fibril outlined by Woodhead-Galloway (1977) is discussed in greater detail and an account of the low angle X-ray diffraction pattern (Miller & Wray 1971) is obtained. Square-packed dimers form the overlap region of the fibril. In the gap region, clusters of four dimers at each of the points of a square lattice of side 3.8 nm provide the structure. Molecules are parallel to the axis of the fibril in the overlap region and tilted from the axis by a few degrees in the gap region. A brief discussion of some aspects of the self assembly of such a structure is included. In particular, it is noted that the axially projected D (= 66.8 nm) period, which is a property of the whole fibril, is not necessarily also a local property; there may be no well defined and finite small grouping of molecules that is itself D-periodic, such as is proposed in the microfibril model of the fibril (Smith 1968). On the other hand, there is strong circumstantial evidence of a D-periodic crystallographic unit cell.