a.

a.

I.

2.

b.

Examine, with a low power.

The organ is chiefly made up of tortuous tubules, which are seen cut in various directions.

Examine with a high power.

a. Note the epithelium lining the tubules: it varies with the season of the year (whether before or after the breeding-time), and is usually extremely granular and ill-defined. The cells are arranged in two or three rows, and at the time of breeding the most superficial layer of cells is transformed into spermatozoa, each cell giving rise to several. These lie side by side at right angles to the lumen of the tubule, which accordingly appears to be lined by them.

C. The spermatozoa (B. 10. a. y).

1. The ovary.

The structure of this organ is easiest made out shortly after the breeding-time. Remove one of the ovaries, place it in water, and make an incision into it: it will be seen to contain a cavity, and projecting upon the walls of this cavity and also upon the outer surface of the ovary are numerous round eminences of various sizes: these are ova in different stages of development, and the large ones will be seen to have become more or less pigmented.

Tease out a bit of ovary in normal saline solution: cover, and examine with a low power.

a. Note the ova, many much smaller than those which were seen (1) with the naked eye: they

b.

appear as granular spherical masses with a clearer central patch.

Examine with a high power a portion of your specimen containing some of the younger and more transparent ova. Note

α. The thin structureless membrane, vitelline membrane, enveloping each.

B. The granular matter (yelk, vitellus) forming most of the ovum. It sometimes appears to

y.

be composed of an outer granular and an inner clearer layer.

The clearer central mass (germinal vesicle) imbedded in the vitellus. The large number of highly refracting masses (germinal spots) within the germinal vesicle.

K. THE PHYSIOLOGICAL PROPERTIES OF MUSCLE AND nerve. Place a frog under a beaker, with a drop or two of chloroform: take it out immediately it becomes unconscious, which will probably be in a few seconds. Now feel with a finger-nail for the depression beneath the skin at the back of the animal's head, which indicates the point of articulation of skull and spinal column: it lies in a line joining the posterior borders of the two tympanic membranes. Divide the skin and muscles at this point until the neural canal is laid open, and then pass a stout wire into the cranium and down the neural canal of the vertebral column. By this process (known as pithing) the frog is rendered totally incapable of further consciousness, though most of its tissues will retain their vitality for some time.

a.

b.

Remove the skin from one leg, so as to lay bare the muscles: send an interrupted electric current through any one of them (or tap the muscle sharply with the back of a scalpel): it will immediately contract, or alter its form in a definite way; it becomes shorter and thicker, and in so doing moves the bones to which it is attached. Very carefully lay bare the sciatic nerve, taking care not to crush or drag it: divide it as high up as possible and, seizing it with a pair of forceps close to its cut end, lay it over the electrodes of an induction-coil. Probably when the nerve is cut the muscles of the limb will contract: whether or not, however, they will contract violently while the interrupted current is going through the nerve.

[If an induction-coil is not at hand a bit of clean copper wire twisted round a strip of zinc, with the points of contact moistened with dilute acetic acid, may be used to stimulate the nerve; smart tapping or pinching with a pair of forceps will also excite it, but by such means the nerve is soon killed.]

The above experiments shew:

C. That the muscle is irritable and contractile: certain external agencies (stimuli) excite some change in it, the result of which is a muscular contraction.

d.

The nerve is irritable: certain external agencies excite some change in it, which in this particular case manifests itself by a contraction of the muscles connected with the nerve.

The nerve possesses conductivity: although it is stimulated at some distance from the muscles, yet the change excited by the stimulus travels along it to them.

APPENDIX.

The various re-agents, mentioned in the "Laboratory work" in the preceding pages, are prepared as follows:

I. Acetic acid, Dilute.

Mix I cub. centimetre of glacial acetic acid with 99 cub. cent. of distilled water.

2. Ammonic bichromate, Solution of.

Dissolve 10 grammes of crystallized ammonic bichromate in a litre of distilled water.

Dissolve the carmine in the ammonia and water; leave in an unstoppered bottle until nearly all smell of ammonia has gone. Afterwards keep in a well-closed bottle. Dilute a small quantity with fifteen or twenty times its bulk of water, when required for use.

4. Chromic acid, Solution of.

Dissolve 10 grammes of crystals of chromic acid in one litre of water. This gives a I per cent. solution, from which weaker ones can readily be prepared when required.