12 MOVEMENTS OF CELLS. 



have acquired a, move or less dodecahcdral form (as is frequently the 

 case in plants, see fig. 2), or where, by groAvth of young cells beneath 

 them, tliey become flattened out and forced towards a free surface, as 

 probably happens in the case of the stratified epithelia. Examples of 

 the latter are observable in the ramification of the cells of the nervous 

 and connective tissues, and in the elongation of cells to form muscular 

 fibres. 



3. Movomcnis of Crlls. — Many cells undergo spontaneous movements, 

 leading to temporary changes in their form. If we watch carefully under 

 a high power of the microscope any cell which is exhibiting these pheno- 

 mena — a pale blood-corpuscle, for example — we observe, in the first place, 

 at one point of its circumference, a protrusion of a portion of its proto- 

 plasm, which is commonly at first clear and hyaline, but into which 

 granules arc soon seen to flow. After a short time this process may be 

 i*etracted, and another similarly protruded at another point, and again 

 withdrawn, and so on for a considerable time, the corpuscle remaining 

 all the while perfectly stationary. Occasionally, however, especially if the 

 corpuscle be maintained at the temperature of the bod}', the part protruded 

 remains fixed, and the cell itself is drawn towards tlie extremity of the 

 process. Should this occur a number of times in the same direction, a 

 slow progressive motion of the whole cell is the result. In this way 

 cells such as we are now considering may undergo very considerable 

 changes of form and place within a relatively short time. Thus, under 

 certain conditions, the pale blood-cori)uscles may some of them make 

 their way out of the blood-vessels and move freely in the surrounding 

 tissues : hence the term " migratory cells " ( Wanderzellen) applied to 

 them. 



The movements which we have just been describing as occurring in 

 cells are quite similar to those which are exhibited, but in a more 

 vigorous manner, ]jy the connnon fresh-water amoeba, and are hence 

 designated "amoeboid." They are more marked in cells in the young 

 state, such as tliose of the embryo, but are not altogether absent in 

 some which persist in the fully-developed tissues, as, for example, in the 

 connective tissue corpuscles. The contractile property of the proto- 

 plasm, to which its movements are due, would seem to be quite com- 

 parable to the contractility of muscular substance ; for it is found that 

 the substance of these protoplasmic cells contracts under the electric 

 stimulus, whether this be directly applied, or, as observed by Kiihnc in 

 the cornea, indirectly through the medium of the nerves.'* 



In the cells of the Valllmcrla, (liaru, and various other plants, when exposed 

 under the microscope, the green colom-ed grains (of chlorophyll) and other small 

 masses and corpuscles contained in the cavity, are seen to be moved along the 

 inside of the cell-wall in a constant and determinate direction. This phenomenon 

 appears to be of very general occurrence in the vegetable kingdom, although the 

 niovement docs not always go on with the same regularity as in the instances 

 cited. It is obviously due to a layer of protoplasm on the inner sm-face of the 

 cell-wall, which enters into a peculiar flowing or imdulating motion and trails 

 the passive chlorophyll gi-anules along with it ; but how the motion of the pro- 

 toiilasm itself is produced is not at all understood. 



To the same class of phenomena are probably to be refeiTcd the remarkable 

 movements obscn-e.l in tlie pigment-cells of the frog's skin, wliich were carefully 

 investigated by Lister.f In these ramified cells the dark particles of piginent are 



* Untersuchungen iiber das Protoplasma und die Contractilitiit. ISGi. 

 t rhil. Trans., 1858. 



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