We now know that the bodies of all animals and plants are constituted of minute units so small that they can not be distinguished by the naked eye, although they can be readily demonstrated by the microscope. These units have long been known to naturalists by the name of cells. The discovery of the cellular constitution of living

Fig. 3. Cells from the Mouth (Oral Epithelium) of the Salamander, to show the phases of cell division or mitosis.

bodies marks one of the great epochs in science, and every teacher who has had occasion to deal in his lectures with the history of the biological sciences finds it necessary to dwell upon this great discovery. It was first shown to be true of plants, and shortly after likewise of animals. The date of the latter discovery was 1839. We owe it to