Maxillary and mandibular molars of the American opossum, Didelphis virginiana L., were viewed in the scanning electron microscope (SEM) after acid‐etching or after cutting and acid‐etching. Observations were made on enamel prism patterns as they relate to functional properties of the tooth at a particular site. Molars at different stages of wear were also observed under a dissecting microscope; worn surfaces were correlated with function and enamel ultrastructure. Pounding surfaces of molar cusps wear more rapidly than near‐vertical shearing surfaces or crushing basins (i.e. the trigon and talonid basin). Pounding surfaces are subjected to abrasion by food and arc not normally involved in tooth‐tooth contact. Near‐vertical shearing surfaces and basins used for crushing do experience tooth‐tooth contact, but are surprisingly more resistant to wear. Prisms at pounding sites approach the occlusal surface at a near 90° angle and are surrounded with very thick interprismatic (IP) enamel parallel to the occlusal surface of the tooth. The pounding pattern is present at tips of cusps and at occlusal surfaces of ridges of the tooth. At near‐vertical shearing surfaces, the prisms approach the outer surface obliquely and are surrounded with IP crystals which are perpendicular to the vertical surface. The angle between prismatic and IP enamel in these patterns is 60–90° in a cervical to occlusal direction. In basins of the tooth used principally for crushing and some shearing, IP enamel is perpendicular to the changing slope of the basin and the prisms are usually at a 55–65° angle to the IP enamel. When the pounding and shearing‐crushing patterns meet at a ridge, a distinct seam is observed. Pounding forces occur parallel to the long axis of the prisms and perpendicular to the thick IP enamel (i.e. perpendicular to the long axis of the IP crystals) lying on either side of the prisms. Shearing and crushing forces occur at an oblique angle to the prism, and interprismatic enamel is more evenly distributed about the prism. A spiral pattern is found at the bottoms of the trigon and talonid basins, but not at the bottom of the trigonid which is a non‐occluding basin. It is concluded that the differential rates of wear of the enamel surfaces are necessary in maintaining the sharp cutting edges and effective crushing basins of the tribosphenic molar, and the ultrastructural arrangements of the enamel prisms are of functional significance.

No abstract

SUMMARYThe rates of formation of transformed cells in mixtures of transforming DNA and transformable HaemophiZus inJuenxae bacteria have been measured. The data were interpreted as follows. About 4-5 sec. are required for the penetration of a DNA particle of 8-9 x 106 mean molecular weight. The average number of penetration sites per bacterium is probably two. There is no reversible attachment of DNA to these sites. Crude calculations show that about one out of every two encounters between a bacterium and a DNA molecule results in the penetration of the latter into the cell.

No abstract