The echidna, Tachyglossus aculeatus, a monotreme mammal, is thought to possess an all-rod retina (O' Day, 1952). This study provides anatomical evidence for the presence of cone-like photoreceptors in the retina of the echidna. The cones, which constitute 10–15% of the photoreceptors, have all of the ultrastructural characteristics previously shown in the cones of placental mammals, and, like the cones of other animals (Blanks & Johnson, 1984), they bind peanut agglutinin. Unlike the cones of another monotreme, the platypus, the cones of the echidna retina do not possess oil droplets. Twin cones, pairs of cones in which there is no obvious difference in the size, shape, or ultrastructural features of the members of a pair, are common. The density of cones varies from 9000 cells/mm2 in the superior periphery to 22,000 cells/mm2 in the central retina. Nearest-neighbor analysis suggests that the cone mosaic in the echidna retina results from the presence of single and twin cones in a relatively regular array.

The anatomy and histology of the thymus in monotremes and marsupials were revisited and several new observations made. Tissues of the mediastinum and/or ventral neck of 134 animals altogether (12 Australian marsupial and monotreme species) were examined and the locations of thymic tissues documented. Descriptions not previously recorded include the gross anatomy of the thymus in monotremes, the bandicoot (Isoodon macrourus, Isoodon obesulus), the wombat (Lasiorhinus latifrons), and koala (Phascolarctos cinereus), as well as light and electron microscopic features in all species examined. In monotremes, thymic tissue was diffusely spread in the mediastinum and extended over the ventral surface of the pericardium. In the bandicoot, a small, deep cervical thymus was persistently present as well as the expected thoracic thymus and contrary to previous observations, a thoracic thymus was found in the wombat but not the koala. Both species also have a cervical thymus. In the marsupial species examined, additional thymic tissue was often present in the vicinity of the carotid bifurcations, in the deep ventral regions of the neck. In many animals, thymic tissue of deep cervical origin as well as thoracic thymus was mingled with parathyroid tissue. In several specimens, the two tissue types lacked intervening connective tissue barriers and at the electron microscopic level, attenuated processes of epithelial reticular cells were the only tissue components separating parathyroid and thymic tissues. The role of the cervical thymus in diprotodont marsupials remains to be elucidated. The presence of aberrant thymic tissue in marsupials is important in the interpretation of experiments involving thymectomy and immunological development because complete removal of thymic tissue may not occur. One aspect that has received little attention is the function of the thymic epithelial reticular cells in marsupial development and a suggestion is made that perhaps the extra thymic tissue found in marsupials provides extra epithelial reticular cells necessary for the production of cytokines and other secretions to sustain the development of immunological competence.

Spermatogenesis in the platypus (Ornithorhynchus anatinus) is of considerable biological interest as the structure of its gametes more closely resemble that of reptiles and birds than marsupial or eutherian mammals. The ultrastructure of 16 steps of spermatid development is described and provides a basis for determining the kinetics of spermatogenesis. Steps 1–3 correspond to the Golgi phase of spermatid development, steps 4–8 correspond to the cap phase, steps 9–12 are the acrosomal phase, and steps 13–16 are the maturation phase. Acrosomal development follows the reptilian model and no acrosomal granule is formed. Most other features of spermiogenesis are similar to processes in reptiles and birds. However, some are unique to mammals. For example, a thin, lateral margin of the acrosome of platypus sperm expands over the nucleus as in other mammals, and more than in reptiles and birds. Also, a tubulobulbar complex develops around the spermatid head, a feature which appears to be unique to mammals. Further, during spermiation the residual body is released from the caudal end of the nucleus of platypus sperm leaving a cytoplasmic droplet located at the proximal end of the middle piece as in marsupial and eutherian mammals. Other features of spermiogenesis in platypus appear to be unique to monotremes. For example, nuclear condensation involves the formation of a layer of chromatin granules under the nucleolemma, and development of the fibrous sheath of the principal piece starts much later in the platypus than in birds or eutherian mammals.