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Hematologic manifestations and the ultrastructure of a platelet-specific microorganism isolated from a dog in Florida were studied. The agent was readily transmitted experimentally to adult dogs by intravenous inoculation with infected blood. Parasitemias and concomitant thrombocytopenias were cyclic in that both recurred within relatively constant periods of one to two weeks following experimental infections. Hemorrhage was not a manifestation of the disease even though thrombocytopenias were severe. Microorganisms were visualized by light and electron microscopy. They were observed only in platelets and were composed of single or multiple subunits (morula forms). The microorganisms were ultrastructurally very similar to those reported in Ehrlichia canis infections of dogs and Anaplasma marginale infections of cattle. Microorganisms were surrounded by single membranes which more or less conformed to the external surfaces of subunits that were surrounded by double membranes. From electron microscopic studies, it is suggested that these organisms be classified in the order Rickettsiales.

Erythroblast denucleation in the peripheral blood was studied by electron microscopy. Blood was used from dogs anemic either by infection with Babesia canis or from injections of phenylhydrazine hydrochloride. One of the earliest stages of denucleation was the migration of nuclei to the plasmalemma. Mitochondria and coalesced vesicles, derived from the cell membrane of the erythroblast, congregated at the underside of the nuclear envelope unapposed by erythroblastic cell membrane. The coalesced vesicles apparently provided the limiting membrane which surrounded the deep circumference of the extruded nucleus and its associated hemoglobin rim, and also furnished a new plasma membrane for the cell in the area where the nucleus, in denucleation, had utilized the original erythroblastic plasmalemma.It is not known whether loss of nuclei from erythroblasts occurs by extrusion, by karyolysis, or both. Most studies to date have used phase-contrast microscopy to determine the mechanism involved, and the predominant opinion seems to favor nuclear expulsion as the method of denucleationThe present study of erythroblast denucleation was made with the electron microscope, with blood from dogs with either induced or spontaneous anemia. MATERIALS AND METHODSThe blood studied was from dogs anemic as a result of severe parasitism with a hematozoic parasite, Babesia canis, and from dogs made anemic by treating them every other day with three subcutaneous injections of phenylhydrazine hydrochloride at a dosage of 16 mg per kg of body weight. Five to six drops of whole blood were collected in fixative when approximately 10% of the erythrocytic cells were nucleated, as determined by examination of Wright-Giemsa-stained blood smears. 1% OsO4 (12) was employed as a fixative for blood obtained from dogs parasitized with B. canis, while blood from phenylhydrazine-treated dogs was fixed in 2% glutaraldehyde and postfixed in 1% Os04. The fixed blood was embedded in Araldite (13), and cut into thin sections with a dimond knife. Sections on grids were stained with uranyl acetate and lead citrate (14), and were examined with a Philips EM200 electron microscope. RESULTS Light MicroscopyNuclei were located centrally, peripherally, or partially protruded from erythroblasts (Fig. 1). Occasionally nuclei were observed extracellularly. Basophilic, polychromatic, and orthochromatic erythroblasts were present in blood smears. Electron MicroscopyThe mechanism of denucleation was the same in both types of anemia, and thus the results were combined for this study.Erythroblasts with centrally located nuclei contained numerous polyribosomes and a scattering of mitochondria as well as open-spaced vesicles throughout the cytoplasm. Such vesicles ap-

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