while if an excess of chromic acid be employed, the nuclei will appear coarsely reticulated. This method leads to the formation of granula in the cells as well as in the nucleus.

126. By fixing and staining cells of widely different vegetable and animal types Bütschli believes he has demonstrated the existence of protoplasmic structures having the form of bubbles and termed by him microscopic foam-structures. Fixing is done either in picric acid solution or in weakly iodized alcohol. The specimens are then stained with iron-hematoxylin-i. e., first treated with acetate of iron, rinsed in water, and transferred to a 0.5% aqueous solution of hematoxylin (similar to the method of R. Heidenhain (vid. T. 85). Very thin sections are required (1⁄2 to I). Mounting is done, when the lighting is good, in media having low refractive indices, which emphasize the alveolar or foam-like structure of the protoplasm. Of various animal objects, Bütschli especially recommends young ovarian eggs of teleosts, and blood-cells and intestinal epithelium of the frog, etc. It is still a matter of uncertainty whether or not the structures are actually present in living protoplasm.

II. THE TISSUES.

The first few generations of cells which result from the segmentation of the fertilized ovum have no pronounced characteristics. They are embryonic cells of rounded form, and are known as blastomeres. As they increase in number they become smaller and of polygonal shape, owing to the pressure to which they are subjected. From the mass of blastomeres, known as the morula mass, there are formed, under various processes described under the name of gastrulation, two layers of cells, the so-called primary germ layers, of which the outer is the ectoderm, the inner the entoderm. To the primary germ layers is added still a third layer, called the mesoderm; it is derived from both the ectoderm and entoderm, but principally from the latter. From these three layers of cells, known as the primary blastodermic layers, are developed all the tissues, each layer developing into certain tissues that are distinct for this layer. In their further development and differentiation the cells of the blastodermic layers undergo a change in shape and structure characteristic for each tissue, and there is developed an intercellular substance varying greatly in amount and character in the several tissues. In the tissues developed from the ectoderm and entoderm the cellular elements give character to the tissue, while the intercellular substance is present in small quantity; in the majority of the tissues developed from the mesoderm, the intercellular substance is abundant, while the cellular elements form a less conspicuous portion. The tissues derived from the ectoderm are:

The epidermis of the skin, with the epidermal appendages and glands; the epithelium lining the mouth, with the salivary glands. and the enamel of the teeth; the epithelium and glands of the nasal tract and the cavities opening into it; the lens of the eye and retina,

and the epithelium of the membranous labyrinth of the ear; and finally, the entire nervous system, central and peripheral.

From the entoderm :

The epithelium lining the digestive tract, and all glands in connection with it, including the liver and pancreas; the epithelium of the respiratory tract and its glands; the epithelium of the bladder and urethra (in the male, only the prostatic portion, the remainder being of ectodermal origin).

The cells of the mesoderm are early differentiated into three groups (Minot, 99):

(a) Mesothelium.-The mesothelial cells retain the character of epithelial cells. They form the lining of the pleural, pericardial, and peritoneal cavities, and give origin to the epithelium of the urogenital organs (with the exception of the bladder and urethra), and striated and heart muscle tissue.

(b) Mesenchyme, from which are derived all the fibrous connective tissues, cartilage, and bone, involuntary muscle tissue, the spleen, lymph-glands, and bone-marrow; and cells of an epithelioid character, lining the blood and lymph-vessels and lymph-spaces, known as endothelial cells.

(c) Mesameboid cells, comprising all red and white blood-cells. It would be extremely difficult to attempt a classification of tissues according to their histogenesis, as identical tissue elements owe their origin to different germinal layers. The classification adopted by us is based rather on the structure of the tissues in their adult stage.

We distinguish :

A. Epithelial tissues with their derivatives.

B. Connective tissues; adipose tissue; supporting tissues (cartilage, bone).

C. Muscular tissue.

D. Nervous tissue.

E. Blood and lymph.

A. EPITHELIAL TISSUES.

Epithelial tissues are nonvascular, and composed almost wholly of epithelial cells, united into continuous membranes by a substance known as intercellular cement. They serve to protect exposed surfaces, and perform the functions of absorption, secretion, and excretion.

The epithelia are developed from all of the three layers of the blastoderm.

They secrete the cement-substance found between their contiguous surfaces. This takes the form of thin lamellæ between the cells, gluing them firmly together. In certain regions the epithelial cells develop short lateral processes (prickles), which meet like structures

from neighboring cells, thus forming intercellular bridges. Between these bridges are intercellular spaces filled with lymph-plasma for the nourishment of the cells. Epithelia do not, as a rule, possess processes of any length. However, it would appear that the basement membranes, situated beneath the epithelia, consist chiefly of processes from the basal portion of the cells. Some authors ascribe to them a connective-tissue origin, a theory which conflicts with the fact that such membranes are present in the embryo before connective tissue, as such, has been developed (membrana prima, Hensen, 76).

The free surfaces of epithelia often support cuticular structures which are to be regarded as the products of the cells. The cuticulæ of neighboring cells fuse to form a cuticular membrane or marginal zone which can be detached in pieces of considerable size (cuticula). In longitudinal sections the cuticula show, in many cases, a striation which would seem to indicate that they are composed of a large number of rod-like processes cemented together by a substance possessing a different refractive index. The cell-body is also striated for more than half its length, corresponding to the rods of the marginal zone. In the region of the nucleus at the basal portion the striation disappears, the cell here consisting of granular protoplasm of a more indifferent character.

Since one surface of each epithelial layer lies free, and is consequently exposed to other conditions than the inner surface, certain differences are noticed between the two ends of each cell. The cells may develop cuticular structures as above stated. In other cases motile processes (cilia) are developed on their exposed surface, which move in a definite direction in the medium surrounding them, and by means of this motion sweep away foreign bodies. It is not strange that the free surface of the epithelia, exposed as it is to stimulation from without, should develop special structures for the reception of sensations (sense cells).

On the other hand, the inner or basal surfaces of the cells usually retain a more indifferent character, and serve for the attachment of the cells and the conveyance of their nourishment. For this reason the nuclei of such cells are usually situated near the basal surface.

From the above it is seen that the two ends of the epithelial cell undergo varying processes of differentiation, the outer being adapted more to the animal, the inner more to the vegetative functions. This differentiation has recently been known as the polarity of the cell. This polarity appears to be retained even when the cell loses its epithelial character and assumes other functions (Rabl, 90).

With few exceptions, blood- and lymph-vessels do not penetrate into the epithelia, but the latter are richly supplied with nerves. The finer morphology of the epithelia will be described in the chapters on the different organs in Part II.

Epithelia are classified according to the shape and relation of the epithelial cells.

We give the following classification :

1. Simple epithelia (with or without cilia).
(a) Squamous epithelium.
(b) Cubic epithelium.

(c) Columnar epithelium.

(d) Pseudostratified columnar epithelium.

2. Stratified epithelia (with or without cilia).

(a) Stratified squamous epithelium, with superficial flattened cells (without cilia).

(b) Transitional epithelium.

Stratified columnar epithelium, with superficial columnar cells (with or without cilia).

3. Glandular epithelium.

4. Neuro-epithelium.

1. SIMPLE EPITHELIUM.

In simple epithelia the cells lie in a single continuous layer. Simple epithelia are very widely distributed. They line almost. the entire alimentary tract, the smaller respiratory passages and air

sacs, the majority of the gland ducts, the oviducts and uterus, and the central canal of the spinal cord and ventricles of the brain.

(a) Simple Squamous Epithelium.-In simple squamous epithelium the cells are flattened. Their contiguous surfaces appear regular, forming, when seen from above, a mosaic. The nuclei lie, as a rule, in the middle of the cell, and if the latter be very much flattened, the position of the nucleus is made prominent by a bulging of the cell at this point. It occurs in the alveoli of the lung.

(b) Simple Cubic Epithelium.-Epithelial cells of this type differ from the above only in that they are somewhat higher. They appear as short polygonal prisms. Their outlines are, as a rule, not irregular, but form straight lines. Cubic epithelium occurs in the

smaller and smallest bronchioles of the lungs, in certain portions of the uriniferous tubules and their collecting ducts, in the smaller ducts of salivary and mucous glands, liver, pancreas, etc.

(c) Simple Columnar Epithelium.—In this type the cells take the form of prisms or pyramids of varying length. Cuticular structures are especially well developed. Columnar epithelium occurs in the entire intestinal tract from the cardiac end of the stomach to the anus, in certain portions of the kidney, etc.

Goblet cell.

Cuticular border.

Fig. 38. Simple columnar epithelium from the small intestine of man: a, Isolated cells; b, surface view; c, longitudinal section.

Simple ciliated columnar epithelium is found in the oviduct and uterus, central canal of the spinal cord, and smaller bronchi.

(d) Pseudostratified Columnar Epithelium. This type is one in which all the cells rest on a basement membrane, but they are so placed that the nuclei come to lie in different planes. Thus, in a longitudinal section the nuclei are seen to be placed in several

rows.

Fig. 39.-Diagram of pseudostratified columnar epithelium.

The development of this type from the simpler forms occurs when the cells are too crowded to retain their normal breadth. As a result, they become pyramidal, alternate cells resting their bases or apices on the basement membrane. As the nucleus is usually situated at the broader portion of the cell, the result is that there are two rows of nuclei simulating a stratified epithelium. Occasionally there are spindle-shaped cells wedged in between the pyramidal cells, and as the broad portion of these cells is midway between the basement membrane and external surface, a third row of nuclei is seen midway between the other two. Such epithelia usually possess cilia (portions of the respiratory passages).

2. STRATIFIED EPITHELIUM.

Should the increase of the cells forming the last type of simple epithelium proceed to such an extent that all the cells no longer rest on the basement membrane, an epithelium is formed having dis