8 ESSENTIALS OF ZOOLOGY 



common, especially in the growing stages of animals and plants, 

 and is known as karyokinesis or mitosis, Wilson found that 

 small masses of chromatin are given off from the nucleus. 

 These are called chromidia and are liberated in bud-like out- 

 growths of the cytoplasm. This method of reproduction has 

 been witnessed in other forms, and may be described as repro- 

 duction by budding involving nuclear material. 



After a series of multiplication processes carried out by cell 

 division and by budding, large numbers of descendants are 

 produced during the summer when temperature is favourable 

 and food plentiful. And it is possible in this case, as in others 

 to be considered, that a change results from successive divisions 

 which can only be remedied by conjugation or syngamy. 



Amoebae are liable to desiccation, and in this state may 

 be carried and widely dispersed by winds. They are also 

 spread by streams and rivers and sea currents. It is not to 

 be wondered at, therefore, that they are so widely distributed. 



The Amoeba is very adaptable and fits well into its varied 

 environment. All Amoebae have been derived from pre- 

 existing Amoebae, and the survivors may be said to be the 

 relatively unchanged descendants of all past ages. They 

 represent a primitive and probably very ancient form of life, 

 for their near allies, the Foraminifera and the Eadiolaria, 

 which form limy or siliceous skeletons, have been found in all 

 geological formations from the beginning, and some of them 

 form to-day a skeleton not very different from their prede- 

 cessors of the Cambrian. 



The Amoebae which are found even under the same con- 

 ditions are not all exactly similar in form and structure. 

 They differ in shape and in the form of the pseudopodia. 

 The latter may be blunt and lobular, or they may be quite 

 thin and pointed. We are not always certain that some of the 

 changes are not due to differences in history under different 

 conditions, that is, due to variation. But the differences 

 have led to a number of species being described under the 

 generic name Amoeba, as Amoeba proteus, A. princeps, A. 

 Idbosa, A. Umax, and other species have been arranged under 

 other generic names. These again are grouped under families, 

 and the families are all brought together to form the order 



