The same result has been obtained in all other cases in which the character of the se creted product is such, that it can be de tected, when in a finely divided state, by the assistance of the microscope. Thus Prof. Goodsir has shown* that the pigrnentary matter of the " ink" of the cuttle-fish is con tained within the cells that line the ink-bag ; that the purple fluid secreted from the edge and internal surface of the mantle of Ianthina fragilis (which is supposed to have furnished the Tyrian dye) is contained within a layer of nucleated cells situated on the secreting surface ; and that a fluid resembling milk may be found in the cells contained within the ultimate follicles of the mammary gland in a lactating animal. We seem perfectly justified in concluding, therefore, that in cases where the transparency and freedom from colour of the secreted product prevent our distinguish ing it in the cells of the organ by which it is eliminated (as in the case of the urine), it is nevertheless contained within them and eli minated by their agency.
It would probably be too much to affirm, that the elimination of the secretion always involves the continual exuviation of' the cells, which are the instruments of the process. On the con trary, it seems probable that where the solid matter of the secretion bears but a small pro portion to the liquid, and is in a state of per fect solution, the secreting cells may be con tinually drawing in their peculiar pabulum on the side neare'st to the capillary network, and may be as constantly allowing it to transude by the free surface, so as to permit its pas sage into the cavity of the tube or follicle,—the cells themselves remaining attached to its walls, and continuing to perform this function for a considerable time. Such is probably' the case with regard to the epithelial cells which line the tubuli of the kidney, and which eli minate the secretion of urine ; and those which line the tubes of the perspiratory glan dule are probably as permanent.
In the case of cells, however, whose secre tion contains a large quantity of solid matter, and especially where this is of an adipose character, it seems impossible to suppose that their contents can be given up, xvithout the rupture or deliquescence of the cell-walls. This may take place either whilst the cells are yet in the follicles within which they were generated, or after they have been cast entire into the ducts, or have been even conveyed through them to their outlet. 'We have seen that in the biliary follicles of the Invertebrata, the discharged contents of secreting cells are usually to be met with, indicating that this rupture or deliquescence has taken place with in the follicles ; and this is probably the fact in regard to the biliary cells in general. An
extreme case of another kind is furnished by Mr. Harry Goodsir, in regard to the cells of an organ which is essentially one of secretion as to its structure, though its function has a different direction; the peculiarity of this case being, that even after the complete exuviation of the cells, they retain so much of inde pendent vitality, as to proceed in their own development to a stage much beyond that at which they were set free. The case referred to is that of the seminal secretion of the deca podous crustaceans; the cells of which, when thrown out of the cwca of the testis, are very immature, and undergo important changes in their progress along the tubuli of that gland. The final changes, however, whereby they are fitted for the fertilisation of the ova, only take place after they have been discharged from the male organs, and have been lodged in the spermotheca of the female.* Now in every case in which the secreted product can only be given up by the rupture or solution of the cell-wall, it is obvious that there must be a continual succession or new production of the secreting cells ; and a ques tion naturally arises as to their origin and mode of development. Few facts are as yet known upon this subject. It may, however, be stated with some certainty, that, in many of the simpler glands at least, the follicle with its contained secreting cells was originally a single closed cell, of which the secreting cells are the progeny. This is the case with the Peyerian glands, which are best known to us in this condition, but which afterwards open and discharge their contents into the intestinal canal. Dr. Allen Thomson has ascer tained that the primitive condition of the gas tric gland also is that of a closed vesicle ; and Henle has extended this view to the terminal follicles of the more complex secreting glands, which he considers to have originated in the same condition. The observations of Prof. Goodsir upon the testis of Squalus cornubieus show that this is the true account of the changes occurring in that organ ; the follow ing stages being distinguishable in its structure, when it is in a condition of activity :-1st, Isolated nucleated cells attached to the side of the duct, and protruding as it were from its outer membrane (fig. 321. a). 2nd, A cell containing a few young cells grouped in a mass within it, the parent cell presenting itself more prominently on the side of the duct. 3d, A cell attached by a pedicle to the duct, the pedicle being tubular, and communicating with the duct ; the cell itself being pyriform, but closed and full of nucleated cells (b).