The histology of freshly fixed adrenal glands of Ornithorhynchus and of Tachyglossus has been investigated.
In the adrenals of both species the bulk of the chromaffin tissue was found to occur at one pole, the lower or caudal part of the gland. The adrenal cortex of Ornithorhynchus has a complicated histological appearance and comprises three main types of tissues, designated groups I-III. The major portion of the cortex was made up of groups II and III tissues which were in contiguous patches associated with blood vessels. Group I tissue was confined, for the most part, to a layer of cells lying against the chromaffin tissue. Group III tissue consisted of big cells with large nuclei which frequently contained prominent globules or vacuoles. It is possible that group III represents the secreting part of the cortex and group II the formative layer.
In Tachyglossus, the cortex had a more homogeneous appearance and was more akin to that of reptiles. There was, however, some gradation of cell types. The peripheral cells had plentiful cytoplasm with faintly basophilic nuclei, while centripetally the cells were closely packed and the nuclei densely stained.