Synthetic hydroxyapatite was fabricated into a white translucent polycrystalline ceramic that displays the hardness of natural apatite. The mineral composition of this product after sintering was chiefly dehydrated hydroxyapatite. In addition, a portion of the apatite was converted into α-whitlockite. Because of the similarities among the mineral compositions of bone and teeth and this ceramic, it offers prospects for implant prosthetic devices.

Biological hydroxyapatites and closely related compounds form the inorganic or mineral phase of tooth enamel, dentine and bone.' The chemical composition of hydroxyapatite is Ca ,O(POa),(OH)2. The chemistry, structure and physical properties of biological, geological and synthetic hydroxyapatitites and related apatite compounds have undergone intensive investigation for a number of However, the potential uses of polycrystalline apatites for orthopedic prosthetic devices have not been reported. This is, in part, a result of the lack of an effective method of forming apatite powder into polycrystalline shapes. This difficulty has now been overcome by the application of a high temperaturehigh pressure forming method called hot pressing.The starting material in this investigation was a large single crystal of fluorapatite, Ca 10(P04)6F2. Grinding to -200 mesh powder was accomplished using an agate mortar and pestle. The powder was then placed in an inductively heated graphite mold and subjected to a pressure of 5000 p.s.i.Reaction and densification occurred over the temperature range 920 to 1230°C during a 30 minute period. The resulting polycrystalline body was in the form of a ring exhibiting desirable mechanical properties. Both X-ray and infrared spectrographic patterns were characteristic of highly crystalline apatite.The microstructure displayed (Fig. 1) is that of a typical sintered compact with grains forming equilibrium boundaries of 120" with one another. The temperature-pressure-time relationships as well as initial particle size distribution, atmosphere and doping agents can all be modified. This shouId allow adequate control over the porosity and other microstructural aspects of the final form, and thus of the resulting mechanical porperties.Calcified tissue ingrowth into a hot pressed apatite prosthetic device would serve to anchor it to adjacent hard tissues. Based on recent studies,4 it is expected that pore sizes of I50 to 200p would be necessary for this to occur. Whether apatite shapes would best serve as permanent orthopedic prostheses or as templates for directed or catalyzed bone growth with subsequet dissolution of the apatite, remains to be determined. Long term compatability studies must also be undertaken.Recent experiments indicate that apatite forms may be produced by standard sintering techniques. The extension of forming methods to synthetic hydroxyapatites is of immediate interest. In addition, the extremely large capacity for isomorphic substitution into the apatite lattice (ex. CO,, Na+, K+, F-) provides interesting possibilities for altering the physical and chemical properties of apatite prostheses.

The ultrastructure of 24 form-species of Ordovician conodonts has been examined with the scanning electron microscope. The conodonts represent two provincial and three subprovincial faunas and include hyaline forms, neurodonts (a subgroup of the hyalines), and cancellate forms (conodonts with white matter).The robust neurodont elements are constructed of cone-in-cone lamellae separated by distinct interlamellar spaces. There is limited fusion between adjacent lamellae and between the long, needle-like crystallites within each lamella. Crystallites become granular in form near the base in some neurodonts. A sheet-like septum bisects the elements longitudinally, but does not pass through the central growth canal. The latter is surrounded by fused lamellae producing a strengthened wall. Minute spheres occur along crystallites in some neurodonts. In the non-neurodont hyaline conodonts, greater fusion occurs between lamellae and between crystallites, and white matter may develop along the central growth canal; no septum or spheres have been noted within this group. Increased fusion is considered to have produced stronger elements capable of acquiring lateral compression, costae, and keels. Cancellate conodonts develop white matter primarily in their cusps and denticles. White matter is finely crystalline with no lamellar structure, but with abundant circular and linear voids. There is no pattern to the occurrence of the holes, but most linear voids are oriented transversely. White matter is formed by secondary transformation from lamellar material; this change is reflected in a transitional zone of incipient white matter, where 1 2 BARNES, SASS, AND MONROE a reorientation of the hard tissue is evident. Minute spheres occur in white matter; the central growth canal is destroyed and does not penetrate beyond the transition zone. White matter is believed to provide extra strength to the element allowing marked lateral compression and sharp margins.Although separated on structural criteria, these three conodont groups appear to occupy distinct and evolving ecosystems during the Ordovician. Three theories are proposed which, individually or in combination, may indicate the functional advantages conveyed by the development of white matter:(1) that the factors of element weight and phosphate availability were important, (2) that greater strength to cope with variable stresses was achieved, and (3) that white matter was induced by a disruption in vascular supply, possibly resulting from a partial eruption through the secreting tissues.

In the indigo snake, a pattern of undulating lines on the surface of the skin, formed by the junction of rows of cells, acts as a two-dimensional optical diffraction grating to produce the play of colors. The distance between repetitive units of the pattern measured from observations made with an electron microscope are in agreement with those found by spectrometric analysis.