In the body there are two sets of cells deputed to act for the rest in this particular. One set is situated in the membrane lining the nose, over which the air we breathe passes; and these cells examine our gaseous surroundings, and warn us, by what we term ‘smell,’ whether the atmosphere is fit for us or we had better seek a purer. The other set is for the examination of liquids. Against these we are protected by our skin, and, as we do not absorb anything through it, it is devoid of the power of examining the things it touches. But with our food it is different; we must have the power of testing that. Accordingly, there are Customs officers in our mouth in the form of little groups of cells, which report upon the liquids and solids moistened by saliva, and enable the animal to reject pernicious imports. Thus, the stimulation of a small portion of the protoplasm composing a body is transmitted over the whole, and is able to awake in it the necessary response. [Illustration: DIAGRAM 51.] [Illustration: DIAGRAM 52.] [Illustration: DIAGRAM 53.] [Illustration: DIAGRAM 54.] [Illustration: DIAGRAM 55.] [Illustration: DIAGRAM 56.] So much for the chemical sense organs; they are comparatively simple. But between a single cell, which always makes towards or always hurries out of a ray of light passing through the water in which it swims, and an animal with eyes capable of recognising the colour, shape, size, and distance of objects in space, there really does seem to be a wide gulf. It is not, however, too wide to be bridged. After the single-cell stage has been passed, and we have beasts consisting of an inner layer of cells which is digestive in function, and an outer layer which is protective, motor, and sensory, the power of perceiving light is doubtless vested in the outer layer. When we get beasts consisting of three layers progressing along the straight path of development which leads to man, we find the outer layer becoming too opaque for this purpose, and the torch is handed on to the sensory tube derived from it. (See Diagram 5.) As more and more protection is required, the skin thickens, and the neural tube comes to lie deeper, as in Diagram 51. In order not to lose the light altogether, it has to throw out buds, which concentrate in themselves the peculiar faculty of perceiving it, and at the same time little pits are formed in the skin just over them to help the light to reach them. (See Diagram 52.) In Diagram 53 both the nervous elements and the integumentary are developing their possibilities; and in Diagram 54 a large surface has been prepared for the reception of light, and a lens formed to focus the rays upon it. Diagrams 55 and 56 give the concluding stages in the development of the eye: the formation of the cornea and its protecting eyelids. The two cavities are filled with clear liquids, and the whole eyeball supported by connective tissue.