become differentiated off from the remainder, and forms what is known as a nucleus, while the protoplasm forming the extreme outer boundary differs slightly from the rest, being more transparent, destitute of granules, and apparently somewhat firmer than the interior. We may also notice that at one spot a clear spherical space has made its appearance, but that while we watch it has suddenly contracted and vanished, and after a few seconds has begun to dilate so as again to come into view, once more to disappear, then again to return, and all this in regular rhythmical sequence. This little rhythmically pulsating cavity is called the "contractile vacuole." It is of very frequent occurrence among those beings which lie low down in the scale of life.
We have now before us a being which has arrested the attention of naturalists almost from the commencement of microscopical observation. It is the famous Amœba, for which ponds and pools and gutters on the house-roof have for the last two hundred years been ransacked by the microscopist, who has many a time stood in amazement at the undefinable form and Protean changes of this particle of living matter. It is only the science of our own days, however, which has revealed its biological importance, and shown that in this little soft, nucleated particle we have a body whose significance for the morphology and physiology of living beings can not be over-estimated, for in Amœba we have the essential characters of a cell, the morphological unit of organization, the physiological source of specialized function.
The term "cell" has been so Ions' in use that it can not now be displaced from our terminology; and yet it tends to convey an incorrect notion, suggesting as it does the idea of a hollow body or vesicle, this having been the form under which it was first studied. The cell, however, is essentially a definite mass of protoplasm having a nucleus imbedded in it. It may or may not assume the form of a vesicle; it may or may not be protected by an enveloping membrane; it may or may not contain a contractile vacuole; and the nucleus may or may not contain within it one or more minute secondary nuclei or "nucleoli."
Haeckel has done good service to biology in insisting on the necessity of distinguishing such non-nucleated forms as are presented by Protamœba and the other Monera from the nucleated forms as seen in Amœba. To the latter he would restrict the word cell, while he would assign that of "cytode" to the former.
Let us observe our Amœba a little closer. Like all living beings, it must be nourished. It can not grow as a crystal would grow by accumulating on its surface molecule after molecule of matter. It must feed. It must take into its substance the necessary nutriment; it must assimilate this nutriment, and convert it into the material of which it is itself composed.
If we seek, however, for a mouth by which the nutriment can enter into its body, or a stomach by which this nutriment can be digest-
