22 PRINCIPLES OF GENERAL PHYSIOLOGY 



spores, unite to form a plasmodium. On the other hand, it appears that the 

 pseudopodia of an individual amoeba, or other rhizopod, never unite with 

 pseudopodia of another individual (Jensen, 1895, and v. Uexkiill, 1909, pp. 16 

 and 38). The reason of this is not clearly understood. When the surface of 

 such an organism comes into contact with food, it appears to soften and 

 become sticky, so that the food substance adheres and is more readily taken 

 in. The same thing seems to happen when a protoplasmic process comes into 

 contact with another part of the same individual, but why it does not usually 

 occur when portions of different individuals come into contact, is not easy to 

 explain. Of course, no two individuals will be in precisely the same state, 

 chemical and physical, at the same time, owing to different states of digestion 

 of food and so on ; but the power of discrimination possessed by protoplasm 

 must be very great to appreciate these differences. There are, indeed, many 

 other reasons for believing that living cells are extremely sensitive to minute 

 changes in their environment. 



When structures consisting of naked protoplasm, such as leucocytes or the 

 streaming substance of vegetable cells, are exposed to an electric shock from 

 an induction coil, their movements cease, and they draw themselves together 

 into spheres or series of spheres as shown in Fig. 19 (see Kiihne's description, 

 1864, p. 30). The way in which this effect is produced is not quite clear. 

 Perhaps the colloids are temporarily sent into the "gel," or coagulated state, 

 but it seems also necessary to assume that some kind of contraction of the 

 surface layer occurs, in order to account for the spheroidal forms produced. 

 If the " gel " state were brought about, it is probable that observations on the 

 Brownian movement of particles in the protoplasm would throw light on the 

 matter. In the "gel " state these movements cease, owing to the particles being 

 held in the rigid framework of the separated solid phase. This fact, as we 

 shall see later, has been used to facilitate the counting of particles in colloidal 

 solutions. Some observations by Kiihne himself (1864, pp. 31, 75, 95) point 

 to the stoppage of these movements on excitation, and I have myself recently 

 seen in protoplasmic structures under dark ground illumination that Brownian 

 movements cease under the action of induction shocks too weak to kill the 

 organism. 



The effects due to the anode and cathode of the constant current can, in the main, be 

 explained by electrolytic changes. Details of these effects are beyond the scope of this book, 

 since they do not appear to throw much light on the problems with which we are concerned. 



SURVIVAL OF CELLS 



It has long been known that various organs of cold-blooded animals will 

 continue their activities for a considerable time when separated from the rest 

 of the body, but the corresponding fact in the case of warm-blooded animals 

 has only been established by experiments of comparatively recent date. If 

 artificial circulation of blood, sufficiently oxygenated and at the correct 

 temperature, be maintained, it seems clear that the only experimental difficulty 

 should be with regard to the lapse of time during which the organ is deprived 

 of oxygen, during the necessary operative procedures. 



An important step was taken when Locke (1901, p. 490) showed that the 

 heart of the rabbit continued to beat for several hours if fed with a warm 

 saline solution saturated with oxygen. The method has also been applied to 

 the kidney and salivary glands, although it has, as yet, been found impossible 

 to preserve all their activities. This will be discussed further when we are 

 considering the mechanism of secretion. Other cases of isolated warm-blooded 

 tissues, more especially smooth muscle, continuing their contractions immersed in 

 similar solutions, will be found under the head of intestinal movements (Magnus). 

 Blood vessels and the uterus can also be investigated by this method. 



Further ^advance was made in 1907 by Ross Harrison (1907, p. 140, and 

 1910, p. 787), who found that cells separated from frog embryos and immoral 

 in lymph continued to grow. Particularly valuable results were obtained 



