1. The behaviour of the cell organs has been studied during keratinization in normal epidermal cells, in cases of hypertrophy of the epidermis, and in malignant growths of both experimental and spontaneous origin. All observations have been confined to small laboratory animals (mammals), chiefly rats and mice.
2. In epidermal cells, apart from keratinization, extrusion of particles of the nucleolus has been observed. This has been seen to take place in the lowest layer of cells of the rete mucosum (fig. 5, Pl. 2).
3. The first definite indication of impending keratinization is the dispersal of the cytoplasm organs, Golgi apparatus, and mitochondria. In malignant cells the mitochondria are normally scattered throughout the cytoplasm, so that only the Golgi apparatus disintegrates and becomes scattered.
4. In the simplest cases, e. g. the squamous-cell carcinoma (630), the mitochondria enlarge at the onset of keratinization, then disappear (fig. 6, Pl. 3).
In some tar tumours and in the hypertrophied epithelium surrounding them, the mitochondria apparently give rise to epidermal fibrillae, which may, or may not, break up at the onset of keratinization into granules which ultimately disappear (figs. 7-9, Pl. 3).
5. The Golgi apparatus, after dispersal throughout the cytoplasm, fails to show in osmic-acid preparations (fig. 10, Pl. 4) ; however, by the silver technique, granules are seen until an advanced stage of keratinization is reached (fig. 12, Pl. 4). It is not certain to what extent these are related to the Golgi apparatus.
6. Usually, after these changes in the cytoplasmic organs, the nucleolus breaks up, and extrudes fragments into the cytoplasm, which swell up, apparently owing to imbibition. Finally, they disintegrate and diffuse into the cell cytoplasm (figs. 1-4, Pl. 2).
7. Keratohyalin granules are principally of nucleolar origin. A part of the granulation in some cases seems to be due to the breaking up of the mitochondrial complex of the cell (fig. 9, Pl. 3).
8. At the onset of keratinization, the cytoplasm usually increases in volume, relatively to the nucleus. Lobed and fragmented nuclei, as well as amitotic nuclear division, are probably indications of attempts at readjustment of the normal nuclear cytoplasmic relationship (Text-fig. 8).
9. Keratinization is mainly a function of the ground cytoplasm, although the cytoplasmic organs and the nucleus undoubtedly contribute their part.
10. The cytological processes occurring during keratinization are essentially of the same kind in both normal and malignant cells. The only morphological difference seen is the difference in cell polarity, as denoted by the mode of distribution of the cytoplasmic organs in the cytoplasm. While in the normal epidermal cell it is usual for the Golgi apparatus to occupy a position at the more distal pole of the nucleus, and for the mitochondria to be heaped up at the other end of the cell, no such regularity of distribution of these cell organs is found in malignant cells growing en masse. A polarized distribution, however, occurs in the acinus parts of adenocarcinomata. It is suggested that the polarity of cells is an indication of their reaction to the stimuli of the external environment, or is the result of their functional activity being directed towards certain interfaces of the cell.