Where the hexagonal surface epithelium has been removed, there will be seen stained connective-tissue corpuscles, with numerous fine processes branching and anastomosing with each other.

C. PIGMENT-CELLS.

Pin out on a frog-board one of the webs between the toes of the same frog, and observe first with a low and then with a high objective. There will be seen large corpuscles loaded with dark pigment, and possessing numerous branched processes. In some places the pigmented cells will appear as round dots, the processes in this case having been retracted. Every intermediate stage between these two states may be observed by watching from time to time.

D. FAT-CELLS.

: 1.

Cut out a small piece of the omentum or subcutaneous tissue of a mammal, from a part containing comparatively little fat. Spread it out on a slide, and cover with normal saline solution and a cover-slip. Examine with a low objective. Observe the groups of highly refractive fat-cells.

2. Examine with a high objective. Observe the individual fat-cells, their variable size, the apparent absence of a nucleus, and the fibres of connective tissue passing between and over them.

3. Take a small piece of adipose tissue, which has been kept for some time in alcohol, and place it in hæmatoxylin until it has become well stained. Wash it with spirit, place it on a glass slide, and tease it out if it is too thick. Remove as much of the spirit

as possible with blotting paper. Cover the tissue with a mixture of carbolic acid (1 part) and turpentine (4 parts), and let it remain until it is completely transparent, changing the fluid and gently warming if necessary. Remove the excess of turpentine and cover the tissue with a drop of Canada balsam and put on a cover-slip.

Observe the groups of cells from which the fat has been thus removed. Note the shrunken outlines of the cells, and the presence in each cell of a nucleus and of a small amount of protoplasm, both of which are stained with hematoxylin.

4. Snip off a small portion of the gelatinous material from the orbit of the rabbit (Hdb. p. 44, Figs. 47, 48), and examine the transitional forms between connective-tissue corpuscles and fat-cells.

E. CONNECTIVE TISSUE with CARTILAGE-CORPUSCLES: FIBRO-CARTILAGE.

Imbed in paraffin B a small piece of mammalian intervertebral cartilage which has been hardened in chromic acid. Cut sections and stain them with hæmatoxylin. Observe

a. The fibrous connective-tissue, sometimes collected into bands, sometimes forming a meshwork.

b. The ordinary connective-tissue corpuscles which occur in parts.

C.

The cartilage-corpuscles; large cells with a distinct nucleus; they are often seen in groups.

LESSON VI.

STRUCTURE OF CONTRACTILE TISSUES.

1. Scrape the mucous membrane of the throat or roof of a frog's mouth, which has been treated with osmic acid 1 p. c. for twenty-four hours. Tease out the flakes so obtained in a drop of water, and examine with avoidance of pressure. Groups of ciliated cells, intermingled with goblet-cells will be observed. Neglect the latter; note in the former

a. The shape of the cell; it is more or less conical, often with a branching or irregular apex.

b. The cilia on the base. Seen in profile, they appear to form a row, but when a cell is obtained in such a position that it is viewed from above, the cilia, foreshortened into mere dots, are seen to be spread over the whole surface of the base.

c. The nucleus, the granular protoplasm, and the hyaline border just below the cilia.

2. Scrape the same membrane in a recently killed frog, and gently tease the scrapings in normal saline solution.

Note

a. The movements of the cilia, watching especially

those in which the action is becoming faint.

b. The results of ciliary action: granules and bloodcorpuscles are driven along. Detached cells may also be seen carried about by the action of their own cilia.

c. The contracted, almost globular form of the cells which have been set free.

3. Tear off the smallest possible strip of a frog's muscle, preserved in alcohol, add a drop of acetic acid, and tease it out as finely as possible in water. Note a. The varying size of the elementary fibres.

b. The striation of the fibres: alternate dim and bright cross-bands passing through the whole thickness of the fibre.

c. The breaking up of the fibre into fibrillæ, the latter also striated. Try to obtain as fine fibrilla as possible. The fibres sometimes split transversely into discs; the surfaces of the discs. appear dotted.

4. Remove carefully, with sharp scissors, one of the thin muscles from the neck of a frog, and mount it in normal saline solution, avoiding pressure. The fibres will for a brief period be seen in a normal condition. In many, the alternate dim and bright strie will be very distinct; in others the transverse markings will be more or less obscured by an appearance of longitudinal fibrillation. Occasionally fibres are met with, having only a confused granular aspect. If the preparation has been successfully made, waves of `contraction may, at times, be seen to travel along the fibres. At the contracted part, the fibre is, for the time being, thicker, and the striæ set closer together. 5. Irrigate with distilled water; the fibres will become

more opaque, and after a while, a pale homogeneous membrane, the sarcolemma, will bulge out at one spot or another.

6. Irrigate with dilute acetic acid. Observe the elongated nuclei disposed lengthways, with lines of granules proceeding from their ends.

7. Make a thin transverse section of muscle hardened in chromic acid.

Observe the cut ends of the fibres, and their arrangement in bundles.

8. Repeat § 3 on mammalian muscle. The fibres are much more uniform in size, but the same general features may be observed.

9. Tease out in water a strip of plain muscular tissue, taken from a stomach or intestine preserved in potassium bichromate. Note

10.

a. The isolated fibre-cells. These are long, and, when not broken off in the preparation, fusiform masses of hyaline or faintly granular protoplasm. They appear very much thinner when seen in one direction than in another; that is to say, they are much flattened. In the middle of each fibre an oval nucleus may with difficulty be seen. In the neighbourhood of the nucleus, especially at its ends, granules are more abundant in the protoplasm.

b. The arrangement of the fibre-cells in bands and sheets: the fibres may frequently be seen projecting like a palisade from the torn end of a

band.

Examine a strip of plain muscle from a perfectly fresh intestine, having teased it out very gently in