MOVEMENTS OF SIMPLE ORGANISMS. 39 



the dust, which are dissolved in the melted snow. They grow and at last divide 

 so as to form daughter-cells, usually four in number but often six or eight and 

 less frequently two only (figure 25a, /, g). As soon as the division is accom- 

 plished, the daughter-cells, so produced, free themselves, assume an oval shape, and 

 display at their narrower extremity two rotating cilia by means of which they 

 move about in snow-water with considerable vivacity. The interstices of the still 

 unmelted, but now granular, snow, are filled with water from the melted parts, and 

 through these the red cells swim away and are thus diffused over the snow-field. 

 At the moment of escape and first assumption of movement the cell-body appears 

 to be uninclosed. But it soon clothes itself with an extremely delicate, though 

 clearly discernible skin, which, curiously enough, does not lie close to the proto- 

 plasm, which is withdrawn slightly and inclosed as in a distended sac (see 

 figure 25a, e). Only in front, where the two cilia carry on their whirling motion, 

 does the skin lie close to the body of the cell; and it must be presumed that the 

 cilia, which are simply extensions of the protoplasmic substance, are projected 

 through the envelope. The swarm-spores afford an example of an unusual type of 

 protoplasts, namely of those that move about singly in the water by means of cilia 

 and at the same time carry their self-made cell-membranes with them. 



How long the motile stage lasts under natural conditions has not been deter- 

 mined for certain. On the mountains of central and southern Europe, where hot 

 days are followed, even in the height of summer, by bitterly cold nights, causing 

 the melted snow which has not run off' to freeze again in the depressions of the 

 snow, the movement no doubt is often interrupted. On the other hand, in high 

 latitudes, where the summer sun does not set for weeks together, such interruption 

 would be exceptional. In any case, however, the locomotion of the red cells with 

 their hyaline cell-membranes is not limited to so short a period as is that of naked 

 ciliated protoplasts. Moreover they have the power of nutrition and growth like 

 the red resting-cells from which they originate, and they have been observed, in a 

 culture, to increase in size fourfold within two days. When at last they come to 

 rest they draw in their cilia, assume a spherical shape, thicken their cell-membrane, 

 which now once more lies close to the protoplasmic body, and divide anew into two, 

 four, or eight cells (figure 25a, f,g). The fusion of the protoplasts of the red cells in 

 pairs, and their sexual propagation, which has been observed in addition to the 

 above-described asexual multiplication, will be the subject of discussion later on. 

 At present we need only add with reference to this remarkable plant that it was 

 named Sphcerella nivalis by the botanist Sommerfelt, and that not only in mode 

 of life, but also in form and colour, it most closely resembles a kind of blood-red alga, 

 which makes its appearance in Central Europe in little hollows temporarily filled 

 with rain-water in flat rocks and slabs of stone, and also inside receptacles exposed 

 to the open. This alga has received the name of SphcB^^ella pluvialis, and also 

 that of HcBmatococcus pluvialis. 



Lastly, we have to consider the mysterious movements exhibited by many 

 Diatomacese, and by the filamentous species of Zonotrichia, Osoillaria, and 



