Improved electron micrographs of the shadow-cast bovine flbrinogen molecule have been obtained establishing its general morphology and dimensions in the dry state. It consists of a linear array of 3 nodules held together by a very thin thread which is estimated to have a diameter of from 8 to 15 A, though it is not clearly resolved. The two end nodules are alike but the center one is slightly smaller. Measurements of shadow lengths indicate that nodule diameters are in the range 50 to 70 A. The length of the dried molecule is 475 =t= 25 A.Adopting the molecular volume from previous physical chemical data and the general morphological features and length from electron microscopy, we calculate the diameters of the end nodules to be 65 A and the center one as 50 A. The model of the molecule so obtained is consistent with the electron microscopical observations and the data from physical chemistry.

Nagy et al. As noted, their morphology 11. H. C. Urey, Nature 193, 1119 (1962); Science 137, 623 (1962). 12. The sample was obtained through the courtesy of Dr. E. P. Henderson.publications. The particles of the first class are in a morphological no-man'sland, and to establish their possible biological origin, new techniques and new criteria will have to be developed. As for particles of the second class, proof must be given that they are not terrestrial contaminants. This, like all negative proof, may be very difficult to obtain, unless present techniques are improved very greatly (26).

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