it has more cells. Now it is a very important fact for us that animals have a more or less constant size of their cells. They do not differ from one another by a difference in the size of their cells; the bigness of an animal does not depend upon the size, but upon the number, of its cells. We can, therefore, in studying the changes of size, to which I shall next direct your attention, omit altogether these details, and speak of the cells in a general way safely as having a certain uniform or standard size. This will save us a great deal of time, for we learn, as we study cells, that their size increases with the age of the animal. The animal, when it is young, has cells with a small amount of protoplasm. And that, you will perceive from the pictures which have been thrown upon the screen, is an absolutely necessary corollary of the discovery that differentiation is mainly a function of the protoplasm. If there is to be a large degree of differentiation it is necessary that the quantity of protoplasm in the single cells should be increased, so that there may be the raw material on hand out of which the differentiated product can be manufactured. If there is not such a preliminary increase of the protoplasm, then the differentiation can not occur. In order that perfection of the adult structure should be attained, it is necessary that the mere undifferentiated cells, each with a small body of protoplasm, should acquire first an increased amount of protoplasm, and that then from the increased protoplasm should be taken the material to result in differentiation, in specialization.
An undifferentiated cell performs all the fundamental functions of life. An amœba, or any unicellular organism such as I have presented to you upon the screen, does everything which is indispensable to life. It takes food; it forms secretions and excretions; its activity depends upon chemical alterations going on in the food in the interior of its body: it is capable of sensation and of locomotion. It is probable that every living cell has all of these fundamental properties of protoplasm. When a cell becomes differentiated, however, though it does not necessarily give up any of its vital properties, it becomes different from other cells because one of its properties is made conspicuous. And in order to acquire that conspicuousness, that excess of development of one function of the cell, a modification in the structure is necessary. The apparatus in the interior of a cell to produce the exaggeration of the function must be developed, so that to effect the complex physiological machinery of the adult body, this differentiation, of which I have so often spoken, is indispensable. A nerve cell carries on all the vital functions, but it has in addition a special series of modifications of its protoplasm which enable it to accomplish the transmission of the nervous impulses with greater efficiency than ordinary protoplasm can do, probably at a higher speed and with a more perfect adjustment of communication between the various parts of the body than is possible with any machinery of pure protoplasm. So
