10 PROTOPLASM AND CELLS 



constantly changing. Their average diameter is about 

 100-250 /xi. 



Lobes, pseudopodia, are constantly being thrown out from 

 any part of the surface of the body and then withdrawn. The 

 very thin outer surface coating of the minute creature is 

 clear and free of granules, but the great bulk of the animal is 

 slightly opaque and is full of granules of various sizes, some of 

 which are food particles. The Amoeba will slowly crawl towards 

 any small organism and around this it will thrust a couple 

 of lobes or arms, forming a bay. The tips of the arms will 

 unite, and then we have a small lake, in the centre of which 

 the engulfed food-particle is now floating. Digestive fluids 

 are passed from the protoplasm into this food-vacuole, and 

 when the food is digested the refuse passes out of the Amoeba 

 by a reverse process. The fluid surrounding the engulfed 

 particle is at first acid and later it becomes alkaline, just 

 as the contents of our stomach is acid and that of our 

 intestine alkaline. This fluid dissolves the food so that it 

 can be incorporated in the surrounding protoplasm. The 

 pseudopodia of an Amoeba at times exert a surprisingly great 

 force. They are even capable of nipping a Paramoecium in 

 two, engulfing one half and leaving the other half outside, 

 and a Paramoecium is a pretty tough organism. 



Embedded somewhere near the centre of the little animal 

 is a more solid ball which takes up stains more freely than 

 the rest of the protoplasm; this is the nucleus. During life 

 the nucleus is almost invisible, and it is larger in active, busy 

 cells than in quiescent, inactive cells. Its functions include 

 both the control of the building up of the food into proto- 

 plasm, assi7nilation, and the control of reproduction. If an 

 Amoeba be cut in two, one half Avith, the other \vithout, the 

 nucleus, the nucleated fragment behaves as a normal Amoeba. 

 The non-nucleated fragment also behaves quite noTmally except 

 that it cannot divide and cannot digest its food, though food 

 particles may be ingested. The life of such a non-nucleated 



^ A /x is the thousandth part of a milHmetre. The smallest thing you 

 can see with an ordinary microscope must have a diameter of 0-14/x 

 and with an ultra-microscope about 0G05/x. The smallest bacterium 

 which has yet been seen has a diameter of about 0-5/i. 



