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Show 122 MR. W. E. HOYLE ON T H E [Mar. 5, in histological structure to that just mentioned. The matrix of this cartilage is perfectly hyaline and does not take up the staining-fluid (borax carmine), and there is comparatively speaking a considerable thickness of it between the adjacent cells. In the centre of the cartilage the cavities are subspherical, but towards the surfaces, particularly towards that one which is directed to the pen, they show a tendency to become flattened. The cell-contents appear pale and structureless, and are slightly retracted from the margin of the cavities in which they lie. The nucleus is variable in form and is always pushed quite to one side of the cell, usually towards that side which is directed to the pen. In the two lateral pads of cartilage it is larger, rounder, and more frequently shows traces of cell-division. The inferior surface of the median cartilage is, of course, covered by the epithelial lining of the mantle (e.m.), which here becomes ventral and has very distinct round nuclei. The structure of the pen-sac undergoes various modifications in its different parts. At the anterior extremity (fig. 4), for example, its structure is much simplified. Merely the two layers of epithelium are to be found, but even here the lower one is much thicker than the upper owing to the different form of the cells. At this point no trace of the cartilaginous pads is visible. This preponderating thickness of the lower epithelium may also be observed in embryos, as is shown in several of Bobretzky's ** beautiful figures. There can be little doubt that it indicates that this lower laver is the one which is active in * secreting the pen. A little further back than the region first described the pen undergoes a slight change in the form of its transverse section. It not only becomes thicker, but each limb of the arch gives off a prominence near its end, towards the middle line, the limb itself being prolonged outwards to a thin sharp edge. Opposite the prominence the lower epithelium is thinner than elsewhere, but it thickens out into a triangular pad between the prominence and the extremity of the limb of the arch, thus forming a kind of mould upon which the pen is shaped. Still further back, on a level with the stellate ganglia, both layers of epithelium have the same appearance, the inferior one having become reduced to a layer of simple pavement epithelium. This point is posterior to the region of the nuchal cartilages, hence no cartilage is to be seen below the pen. The two upper cartilages have also disappeared, and the concavity of the arch is filled with connective tissue. The posterior extremity of the pen-sac showed some points worthy of being recorded. This part of the animal was entirely digested away in the larger examples, and the observations here recorded were based upon sections of two of the smaller specimens. At the posterior extremity of the body both the superior and inferior tracts of epithelium are extended laterally and their edges 1 Bobretzky, " Izsliedovaniya o Razvitie Golovonogikh " [Investigations on the Development of the Cephalopoda], Izvest. Mosk. Univ xxiv fitrs 34 58 62, 85, 87 (1877). s ' * |
