THE PROTOZOA 123 



the whole of the cell at the same time, a process of growth known as 

 growth by intussusception, which is extremely characteristic of living 

 things, and entirely different from that in inorganic substances. A 

 crystal grows, as noted previously, but only by the deposition of new 

 material on its surface ; a method known as accretion. Furthermore, 

 a crystal only grows in a solution of the same chemical composition 

 as itself, and it is quite unable to synthesise such a compound from 

 other substances. If any of the digested food remains after the 

 waste of the body has been made good and growth provided for, it is 

 formed into the reserve material, scattered as we have already noted 

 in the form of granules throughout the cell. 



After a more or less prolonged period of growth the animal 

 reaches its maximum size, and if the food is still plentiful it divides 

 into two by a simple process known as binary fission. It is not quite 

 clear exactly what determines the point at which division occurs, 

 but some light is thrown on the matter by a consideration of the 

 relation of Amaba to its environment. The respiration of the animal 

 takes place all over the general surface, and so in a small specimen 

 can readily be carried on. As growth proceeds we shall find, as in 

 all solid objects, that while the volume varies as the cube of the 

 diameter the surface only varies as the square. Roughly speaking, 

 then, when the Amoeba has grown to eight times its original bulk it 

 has only four times its original surface. If we assume that the 

 need for gaseous interchange depends, in the main, on the volume 

 of the protoplasm, then it becomes increasingly difficult to satisfy 

 as the animal grows larger. Division into two cells would, of 

 course, restore the proper ratio again. Other factors, such as the 

 relative size of the nucleus to the cytoplasm and the surface tension 

 of the protoplasm, are also concerned in the process of fission. 



The actual process of division in some species of Amoeba is very 

 simple. The nucleus elongates, becomes dumb-bell shaped, and, 

 finally, divides into two, a good example of direct nuclear division. 

 Closely following this a similar division of the cytoplasm takes 

 place, and so there result two daughter Amaba. 



In A. proteus the proceedings are slightly more complex. The 

 chromatin granules in the nucleus become rearranged and take up 

 a median position transversely to the long axis of the elongating 

 nucleus. They then divide up into two groups, one going to each 

 end of the nucleus, so that when the two daughter nuclei are pro- 

 duced each contains a set of granules which are subsequently 

 scattered about as in the original nucleus. This furnishes a very 

 simple example of indirect nuclear division, a process that becomes 

 much more elaborate in the higher animals, and is by far the more 

 common method of nuclear division met with in living beings. 



