842 PKOF. G. C. BOURNE ON THE [Nov. 17, 



the community of structure is due to community of origin ; and if 

 it can be shown that there is a transition between the secretory 

 epithelia of the glands and the epitheha of other regions, there is 

 a presumption in favour of the view that the glands are derived 

 from those other epithelia. A transition can be demonstrated 

 between the epithelium of the mantle-cavity and that of the 

 shell-gland. The epithelium, lining the mantle-cavity differs con- 

 siderably in different parts, but in the neighbourhood of the 

 ovipository aperture it consists of fairly high columnar ciliated 

 cells, among which are numerous gland-cells with granular contents, 

 which in SejJtaria stain bright green in picro-indigo-carmine. 

 This epithelium is continued over the lip of the ovipository 

 aperture into the terminal part of the lumen of the ootype, and, 

 extending further do^vn on the right side than elsewhere, forms 

 the lining of the crystal-sac. Elsewhere it quickly undei'goes a 

 change : the gland-cells disappear, and the ciliated cells increase 

 in length, become attenuated, and are separated by considerable 

 intercellular spaces (fig. 24). This simple ciliated epithelium is 

 continued downward for some little distance, and only gradually 

 becomes complicated by the appearance at first of a few club- 

 shaped gland-cells lying between the ciliated cells. The gland- 

 cells contain large granules, highly eosinophilous, or staining 

 bright green in picro-indigo-carmine. The gland-cells soon 

 become more abundant, and some of them, while retaining their 

 connection with the surface, tend to take up a deeper position. 

 Their swollen inner ends, containing the nucleus and most of the 

 cytoplasm, pass through the thin layer of muscle-fibres under- 

 lying the ciliated epithelium and embed themselves in the 

 surrounding connective tissue, their distal ends being drawn out 

 into fine tubes which pass between the ciliated cells and oj)en into 

 the cavity of the ootype. In good preparations the walls of these 

 tubes are quite distinct, and each tube contains a single row of 

 eosinophilous granules, but swells out in the intercellular space 

 between the ciliated cells, so that it appears to terminate in a 

 claviform vesicle distended with granules (fig. 25). A little 

 further down the gland-cells increase in number and form groups, 

 and these groups passing into the surrounding connective tissue- 

 form at first shorter, but in the greater part of the ootype 

 relatively long, club-shaped masses surrounding its cavity. The 

 club-shaped masses have the appearance of and have been 

 described as crypts, but are not to be regarded as such, for what, 

 appears to be the cavity of the crypt is occupied by the tubes, and 

 there is no lumen into which the secretion is discharged, but each 

 cell has its own duct opening on the surface. This may readily 

 be seen in transverse sections of the so-called crypts in good 

 preparations. It would be more correct to describe each group of 

 cells as a bunch, the deeper cells having very long hollow stalks 

 and the more superficial cells shorter stalks ; all the stalks pass 

 between the ciliated epithelial cells, become slightly swollen, and 

 open on the surface. The manner in which this somewhat 



