The manner of movement common to amebas has attracted the attention of biologists ever since the discovery of ameba by RÖsel v. Rosenhof in 1755. In his description of “Der kleine Proteus” he records the observation that the various form changes which the ameba undergoes are associated with the streaming of the endoplasm. This observation marks the very beginning of the investigation of ameboid movement. And this investigation also possesses the distinction of being the most important single observation that has thus far been recorded in this special field, for it is now generally understood that by ameboid movement is meant movement due to the streaming of protoplasm.

The phenomenon of ameboid movement as discovered by v. Rosenhof, was an isolated phenomenon. It attracted attention mainly because of its uniqueness, for it was the only instance of the kind that was then known. It could not be compared with any other form of movement; and the animal itself, considered apart from the streaming of the protoplasm, was unique also, because of its remarkable form changes which it alone, of all the animals then known, exhibited.

But when Corti in 1774 discovered streaming protoplasm in the cells of chara and various other plants, the ameba could no longer be said to occupy this position of isolation. Although streaming is not accompanied by locomotion in chara, it had been observed that movement in the ameba was always accompanied by streaming, so it came to be generally accepted that the really fundamental feature of ameboid movement was the streaming of the protoplasm.

The ameba came to be of especial interest to the physiologists later on when the finer structures of the larger animals were studied more carefully. Thus when the normal movements of the white blood corpuscles were discovered, no one failed to be struck with their ameboid characteristics in almost every detail of movement, feeding habits and gross structure. The great importance of the functions that have been ascribed to leukocytes, and their very widespread occurrence in the higher animals has served to give rise to the belief that ameboid characteristics were not unique among animals, but common to many of them. The discovery of ameboid movements among plant zoospores, among animal ova, in the endoderm cells lining the digestive tract of a great variety of animals, in the nuclei of some animal cells, in the wandering cells of sponges and other animals—all these instances of ameboid movement occurring in such widely different tissues inevitably placed it among the most important phenomena known to occur in organisms.

Out of the discovery that ameboid movement may be exhibited in some form or other in so many different kinds of organisms, grew the theory that even muscular movement as known in man and the higher animals is at bottom a specialized sort of ameboid movement; not merely phylogenetically, but as it is now known. As we shall see however in the following pages, this theory of muscular movement cannot be based specifically on the streaming process per se, but it is very probable, on the other hand, that the same process which underlies contraction of the ectoplasm in the ameba also underlies contraction in muscular tissue.

But this remarkable story of the development of a single unrelated observation into a widespread biological phenomenon is not yet complete. With its further development the following pages are concerned. It will be shown that the movement of the surface film of the ameba is analogous to that of some blue-green algae, diatoms and crawling euglenas, in which organisms the surface film seems to be the vehicle of movement. Thus the ameba finds itself related to these organisms by new ties. More important still is the significance of the wavy path of the ameba, which may possibly be due to the same fundamental mechanism that controls, under suitable conditions, the direction of the path in man and many other animals and motile plant cells. Thus the phenomenon of ameboid movement born in nakedness and utter isolation, has become attired, in a brief space, with the Victorian garb of a Fundamental.