On the growth of the baleen plate of the Fin Whale and the Blue Whale

1. In Whales variations in the thickness of the baleen plates are supposed to give an insight into certain cyclical processes in the life of the animal. To a certain extent, by means of these variations, it is possible to reach conclusions about the age of the animal and/or about its recent period of life. In order to get a better insight in the significance of the variations in thickness of the baleen plate, researches are made in the histogenesis of the baleen plate of Fin Whales and Blue Whales, caught in the Antarctic waters. The variations in thickness are caused by variations in thickness of the cortex of the baleen plate. 2. The “root” of the baleen plate consists of the following parts: the corium wall, the epithelium that covers the corium wall, the cortex and the epithelium of the gum. 3. In the epithelium of the gum arched bands of varying width are visible. These bands run into the outer layer of the cortex. There is a correlation between the width of these bands and the height of the thickenings of the cortex, in which they end. These bands are not present in the epithelium that covers the corium wall. In both epithelia corium papillae are found. On the top of each papilla a row of spherical cells is formed. In the epithelium that covers the corium wall these rows of spherical cells all run into the cortex and there they cannot be distinguished from the surrounding cornified material. This is also correct for the very few rows of spherical cells originating from the corium papillae near the base of the corium wall. These rows run into the first band of flattened cells of the gum. They become flattened with the cells of the stratum spinosum of the gum in this band, forming the very first layer of the cortex. The other rows of spherical cells in the gum, even those immediately adjacent to the cortex, all run parallel to each other and to the outer surface of the cortex. They never run into the cortex. This shows clearly that the cell material of the gum forms the first and outer layer of the cortex of a baleen plate near the base of the corium wall. Cell material of the gum is not added to the cortex in any other place. 4. The cortex of a baleen plate is formed by two epithelia, the epithelium of the gum and the epithelium that covers the corium wall. Variations in the thickness of the cortex and consequently of the baleen plate, which are visible as peaks and hollows in the record, are only formed by variations in the addition of material by the epithelium of the gum. 5. One of the functions of the epithelium that covers the corium wall is that it serves as a layer by which the cortex slides from the corium wall. The long range and gradual increase in thickness of the baleen plate, and consequently also of the cortex, is mainly a result of the addition of material to the inner side of the cortex by this epithelium. 6. It is shown that the peaks and hollows are instantaneous formations, resulting from increased mitotic activity in the epithelium of the gum. These extra thickenings are only formed at a special point at the very first origin of the cortex. So the “growth periods” in which the records of baleen plates can be divided, by means of the “regular” occurrence of peaks and hollows, are conclusive in age determination. 7. The variations in thickness of the cortical layer of the baleen plate visible as peaks in the records, are probably caused by changes in the physiological balance of the Fin Whale. These changes are of comparatively short duration. They may e.g. be caused by the mitotic stimulating activity of hormones, produced by the ovaries during ovulation. Owing to such a stimulus more material is added to both sides of the cortex. This is visible as a thickening only on the outer surface because here all the extra material is added at one point, whereas to the inner side of the cortex material is added along the whole length. 8. The cells of the stratum spinosum in the “root” of the baleen plate maintain their mutual contact in the desmosomes. These structures are always visible in all microscopic sections of the various parts of this stratum. 9. The coarse tonofibrils run through the cells from the desmosomes in one part of the cell wall to the desmosomes in another, mostly opposite, part. 10. The tonofibrils are not present in all cells of the stratum spinosum of the gum and nowhere in the stratum spinosum of the epithelium between the corium wall and the cortex. 11. The presence of tonofibrils in the stratum spinosum of the gum of the Fin Whale and the Blue Whale is most probably caused by shrinkage of the cells, e.g. under the influence of the fixing fluid. The main direction of this shrinkage of the cells and the direction in which the tonofibrils run, is determined by forces that work on the cells in the epithelium, caused by mitotic activity.