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 stainiug-fluiJ 

 (t)orax 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 tlie 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 ejjithelium 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 {)en under- 

 goes a slight change in the form of its transverse section. It not only 

 becomes thicker, but each liinb 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 tiieir edges 



^ Bobretzky, " Izsliedovaniya o Razvitie Golovonogikh " [Investigations on 

 the Development of the Cephalopoda], Izvest. Mosk. Univ. xxiv. figs. 34, 58, 

 62, 85, 87 (1877). 



