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THE POPULAR SCIENCE MONTHLY. 



them back to life. These blood-cells that I have described are called 

 " human amoebae" or the amoebae of man. Those that are found in 

 the blood of other animals are somewhat different, but there is a kind 

 of amoeba, or crawling-cell, which grows on the top of stagnant water, 

 that has a greater difference (Figs. 21, 22). Take a little of the scum 

 that rises on ponds in hot weather, and put it under the microscope. 

 In it you will find these jelly-lumps of a much larger size. They are 

 from ytjVo to yfg- of an inch across, and move about by the same kind 

 of queer-looking feet. But the border does not look at all the same. 

 First, on the outside you see a clear, glassy-looking rim ; .inside of this 

 is a thicker, darker ring filled with little grains granules. 



Fig. 21. Pond Amceba. 



Fig. 22. Pond Amceba. 



Fig. 23. 



The centre of the cell is quite clear, and contains a thin space or 

 vacuole, like that you saw in the yeast and green mould. The outer 

 border or rim is called the ectosarc, which means outer flesh. The 

 inner is called the endosarc, or inner flesh. Near the clear outer rim or 

 ectosarc you will find the kernel or nucleus a roundish, solid-looking 

 little body which does not change its form. If you look closely you 

 will see a small round, clear space in this outer rim or ectosarc which 

 has motion, something like the beating of the heart. Indeed, by some 

 it is thought to be the simplest form or beginning of a heart. It is 

 called by a long name, the " contractile vesicle," or " contractile space." 

 All amoebae do not have this heart, nor do they all have a kernel. 

 This contractile space or heart is very important, because it seems to 

 be doing a work of its own. This is the first time we have found one 

 part of a cell doing something entirely different from another part. 

 The jelly or protoplasm of the yeast and the green mould is " maid-of- 

 all-work." But the amoebae family seem to be looking up in the world, 

 and are trying to pattern after those establishments that keep a ser- 

 vant for each kind of work. 



Inside of some of these large pond amcebae you will often find the 

 green protococcus-cells, little diatoms, desmids, and all kinds of cells 



