the stomach and in the crypts of Lieberkühn in the intestine. Without losing the shape of a tubule, the glands of this type may be more or less coiled (pyloric glands, sweat glands, and the ceru

minous glands of the external ear). Again, the secretory portions of the glands may divide, forming branched tubular glands (pyloric glands, uterine glands).

A compound tubular gland is one in which two or more secretory tubules empty into each branch of a system of excretory

Fig. 51. Schematic diagram of glandular classification: a, Simple tubular; b, branched tubular; c, simple alveolar; d, compound alveolar (without alveoli); e and f, alveolar (with alveoli).

ducts, as the result of repeated division of the primary duct (kidney). The secretory tubules may anastomose with each other, forming a reticulated tubular gland (liver).

In alveolar or saccular glands the secretory portion usually takes the form of a winding tube, the caliber of which is somewhat enlarged at its extremity (alveus).

Glands of this class are divided into simple and compound types, as in the case of the tubular glands. To the former belong Ebner's glands.of the tongue and Brunner's glands of the duodenum; to the latter, the salivary and the larger mucous glands.

Certain glands have the shape of a flask, the neck representing the excretory duct of the gland (integumentary glands of salamandra). To these the term saccular is often restricted. Still more complicated forms of alveolar or saccular glands are produced by the bulging here and there of the walls of the tube or alveus. The protrusions thus formed are known as alveoli.

According to the above description, multicellular glands may be classified as follows:

belong also the reticu- without alveoli). or without alveoli). lated glands).

The secretory and excretory epithelia rest upon a thin membrane (membrana propria), which has, according to some authors, a connective-tissue origin, while, according to others, it is the product of the glandular cells themselves. In some cases it appears structureless, in others a cellular structure can be distinguished; in the latter case the cells are flattened, with very much flattened nuclei, and show irregular outlines.

Macroscopically, compound glands present a more or less lobular structure, the separate lobules being held together by connective tissue. In the immediate neighborhood of the gland and its larger lobes, the connective tissue is thickened to form the so-called tunica albuginea or capsule. In this fibrous-tissue sheath are found numerous blood-vessels which penetrate between the lobes and lobules of the gland and form a dense capillary network about the tubules and alveoli immediately beneath the membrana propria. Nerve-fibers are also plentiful.

(c) Remarks on the Process of Secretion.-The gland-cell varies in its microscopic appearance according to its functional condition. In its phases of activity it shows vacuoles filled with secretion (as in the liver-cell), or a granulation (pancreas), or even a distinct striation of its protoplasm (kidney).

The secretory process varies. In one case the cell remains intact throughout the process (salivary glands); in another a portion of its own substance is used up in the production of the secretion, only the basal portion containing the nucleus being preserved. When this occurs, the upper part of the cell is reconstructed from the remaining basal portion, and the cell is ready to renew the

process (mammary glands).

In a third type the whole cell is destroyed, and is replaced by an entirely new cell (sebaceous glands).

4. NEURO-EPITHELIUM.

In certain of the organs of special sense (inner ear and taste-buds) the epithelial cells about which the nerves terminate undergo a high degree of specialization. This differentiation is more apparent in the outer portions of these cells, resulting in the formation of one or several stiff, hair-like processes, which appear especially receptive to stimuli. Such cells are known as neuro-epithelial cells. In the epithelia in which they occur they are surrounded by supporting or sustentacular cells.

5. MESOTHELIUM AND ENDOTHELIUM.

The pleural, pericardial, and peritoneal cavities are lined by a single layer of flattened epithelioid cells which develop from the mesothelium lining the primitive body cavity (celom). For this reason, as has been suggested by Minot (90), the term mesothelium may with propriety be applied to this layer in its developed condition. A mesothelial cell is a very much flattened cell, resembling those of squamous epithelium, with faintly granular protoplasm, possessing a flattened, oval, or nearly round nucleus. These cells are of polyhedral shape, and are united into a single layer by a small amount of intercellular cement substance. The borders of these cells may be quite regular or slightly wavy (Fig. 52); more often they are serrated (Figs. 53, 54). The quantity of intercellular cement substance is so small in amount, and the cell boundary so indistinct, that it is necessary to resort to special staining methods to bring out clearly their outline (silver nitrate or intra vitam methylene-blue method).

Fig. 52.-Mesothelium from pericardium of rabbit. Silver nitrate preparation, stained in hematoxylin.

The cavities lined by mesothelium communicate directly with lymph-vessels or -spaces beneath the lining membrane by means of small openings known as stomata. The stomata are surrounded by a layer of cubical cells with granular protoplasm, spoken of as germinal cells. They are numerous in the diaphragm, and may be readily demonstrated in the frog in the membrane separating the abdominal lymph-space from the peritoneal cavity (in the region of the kidneys). Small accumulations of the intercellular cement substance, found at the place of union of several mesothelial cells, are described as pseudostomata or stigmata.

Endothelial cells are differentiated mesenchymal cells. They line the blood- and lymph-vessels and lymph-spaces (arachnoidal and

Nu

cleus.

Cell boundary.

Fig. 53.-Mesothelium from mesentery of Fig. 54.-Mesothelium from peritoneum of

rabbit.

frog; X 400. Technic No. 123.

Fig. 55.-Mesothelium covering posterior abdominal wall of frog. Stained with silver nitrate and hematoxylin.

Fig. 56.-Endothelial cells from small artery of the mesentery of a rabbit. Stained with silver nitrate and hematoxylin.

synovial spaces, anterior chamber of the eye, bursæ, and tendon sheaths). Endothelial cells are in structure like those of the meso

thelium. In blood- and lymph-vessels they are of irregular, oblong shape, with serrated borders. The boundaries of these cells are clearly brought out by silver nitrate.

TECHNIC.

127. Epithelium may be examined in a fresh condition. The simplest method consists in placing some saliva under a cover-glass and examining it with a moderate power. In it will be found a number of isolated squamous epithelial cells, suspended in the saliva singly and in groups. The cells that are cornified still show the nucleus and a small granular area of protoplasm.

128. In order to examine isolated epithelial cells of organs, it is necessary to treat the epithelial shreds or whole epithelial layers with the so-called isolating or maceration fluids. These are: (1) Iodized serum; (2) very dilute osmic acid (0.1% to 0.5%); (3) very weak chromic acid solution (about 1:5000 of water); (4) 0.5% or 1% solution of ammonium or potassium bichromate; and, above all, the one-third alcohol recommended by Ranvier (28 vols. absolute alcohol, 72 vols. distilled water). The mixture recommended by Soulier (91), consisting of sulphocyanid of potassium or ammonium, and the mixture of Ripart and Petit (vid. T. 13) serve the same purpose. All these solutions are used by allowing a quantity of the isolation fluid to act upon a small fresh piece of epithelium for from twelve to twenty-four hours, according to the temperature of the medium and quality of the tissue. As soon as the isolation fluid has done its work, it is easy to complete the isolation of the cells by shaking the specimen or teasing it with needles. Separation of the elements may be accomplished either in the isolation solution itself or in a so-called indifferent fluid (vid. T. 13), or in gum-glycerin (vid. T. 98). The macerated preparation may be stained in a hematoxylin or carmin solution before teasing and mounting in gum-glycerin.

129. The movement of the cilia can be observed in mammalian tissues by scraping the epithelium from the trachea with a scalpel and examining it in an indifferent fluid. As the ciliated epithelium of mammals is very delicate and sensitive, specimens with a longer duration of ciliary movement are more desirable. They can be obtained by using the mucous membrane from the palate of a frog (examine in normal salt solution, vid. T. 13). Particularly large epithelial cells, as well as very long cilia, are found on the gill-plates of mussels or oysters.

130. In order to study the relations of mesothelial and endothelial cells, the silver method is the most satisfactory. The outlines of the mesothelial cells may be clearly brought out by placing pieces of the pericardium, central tendon of the diaphragm, or the mesentery in a 0.75% to 1% solution of silver nitrate. Before placing in this solution, they should be rinsed in distilled water in order to remove any adherent foreign bodies, such as blood-corpuscles, etc. In this solution they remain until opaque, which occurs in from ten to fifteen minutes. They are then again rinsed with distilled water, in which they are exposed to sunlight until they begin to assume a brownish-red color. Once again they are washed with distilled water, and either placed in glycerin, in which they may be mounted, or dehydrated and mounted in Canada balsam, according to the