The physiological relations of the gland vesicles, and especially their duration and origin, are as yet little known. The facts may probably be stated as follows. The gland-vesicles arise from cells which enlarge, so that the cell-membrane becomes the wall of the vesicle, and its contents form those of the vesicle also. At least I have ob served exactly this occurrence to take place in formation of vesicles in a fish of very low development which inhabits our brook (Ammoccetes branchialis). In this fish a num ber of glands of this kind may be found at the dilated part of the intestinal canal, on both sides of it near the spiral fold : these remain tolerably small, although they experience con siderable variations of diameter. They are densely filled with large, pale, smooth nuclei, of oval or roundish form, the number of which is greater or smaller according to the size of the vesicle. Beside these are seen in the tissue of the wall of the intestine separate cells, which enclose two nuclei in no way distin guishable from those of the gland-vesicles. From these cells they pass, by all intermediate gradations, to the large vesicles. If one may generalise on this observation, it may be stated therefrom, in accordance with the proposition mentioned above, that the shut gland-vesicles proceed from cells. The gland-vesicles in the mucous membranes of the Mammalia are very unfavourable objects for examination ; so that, unfortunately, in spite of many attempts, I have not been so lucky as to observe their de velopment. So also I am unable to state in what way the multiplication of nuclei takes place, whether by a fission of the nuclei already present, or whether new nuclei arise inde pendently of the old ones in the fluid of the gland. But I conjecture that the latter is the case.
By their further increase of size, the gland vesicles come to lie more on the surface of the substance in which they are imbedded. But the membrana propria does not possess an unlimited extensibility, so that a period arrives when it can no longer withstand the contents, andis burst by them. This rending occurs at the place of least resistance, namely, at the upper and most external part, and en gages not only the membrana propria of the gland-vesicle, but also the thin layer of fibrous tissue present superficially to this. By the occurrence of this so-called dehiscence of the gland-vesicles, its fluid contents become effused amongst the other constituents. This process has long been known to occur in the Graafian follicle of the ovary ; but it may also be seen with the greatest distinctness iu the vesicles of the mucous membranes, as well in the separate as in the Peyerian glands. The burst gland-vesicle is thus brought into connection with the outer world, end now resembles a wide-bellied and short-necked flask : it has thus suddenly become converted into a small sac, such as we have already found as a constituent of the glands treated of as belonging to the second variety.
After their dehiscence, the gland-vesicle4 I appear to behave differently. Part of them by bursting arrive at the end of their career. They do not become distended with secretion a second time, but the gland-vesicles cease to exist as such in consequence of further meta morphosis. The ovary, the vesicles of which
disappear by the formation of a corpus luteuni, affords a proof of the occurrence of this pro cess. While, on the contrary, other gland vesicles seem to close again, and to become distended with new secretion, in order to their going through the process for a second or perhaps a third time. It is exceedingly diffi cult to come at the truth respecting this matter, but an examination of the l'eyerian glands may afford us some help in the investigation.
Each Peyerian patch consists of but a single layer of gland-vesicles, which in the adult ap pear all to be at pretty much the same stage of development. No second row of unde veloped vesicles is ever found beneath this layer, such as might be supposed to serve as a compensation for those which have burst ; so that a Peyerian patch of glands, after all , the vesicles had burst, would necessarily dis appear, if no closure of the vesicles followed, unless indeed a power existed to develop a new group of vesicles at some other point. Now although the Peyerian glands of Man and Mammalia certainly show individual varieties in number and extent, yet these appear too small to prove a destruction of the Peyerian vesicles after the manner of the Graafian fol licles of the ovary. Besides this, I have never been able to remark the least appearance which could lead me to imagine a new development of the glandular patches. Moreover, it is a well ascertained fact, that in many Mammalia certain Peyerian gland-patches show a con stant position and size at all periods. To this category belongs a very large Peyerian patch which exists in the rabbit at the point where the ileum opens into the large intestine. Such structures will scarce admit of any other conclusion than that the gland-vesicles re close themselves after their dehiscence.
From these observations, which in part de pend on researches not yet published, we shall be able to come to a decision respecting the glandular nature of the supra-renal cap sules.
In them the same gland-cells appear ; the same membrana propria, capable of with standing alkalies. Exteriorly this issurrounded by capillaries, and interiorly it contains the same mass, consisting of molecules of a protein substance, fatty granules, nuclei, and cells de veloped by circumposition of a membrane. We also find a development of gland-vesicles from cells identical with that which was met with in the Ammoccetes. The multiplication of the nuclei here occurs by the method of fission. And since, in the supra-renal capsules, these undeveloped gland-vesicles occur in predomi nant quantity, we may, therefore, well imagine that the old gland-vesicles are destroyed in these organs, in which they undergo dehis cence. But their contents are not extruded outwardly, as are those of the gland-vesicles previously described,but into the fibrous frame work of the organ, in which they exist in the fluid form, and from which they are subse quently received into the vascular system either by immediate or mediate resorption.