122 MR. W. E. HOYLE ON THE [ 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 tothe 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 veatral 
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 layer is the one which is active in 
secreting the pen. 
A little further back than the region first described the pen under- 
goes 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. Tis 
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. Uniy. xxiv. figs. 34, 58, 
62, 85, 87 (1877). 
