The Protozoa 127 



where trie animal formed definite mitotic figures. Very few investigators 

 have observed sporulation, but where it was observed the process lasted 

 from two and one-half to three months. The pseudopodia were first 

 drawn in, and the animal became spherical. A three-layered cyst was 

 then secreted within which the Amoeba rotated for several days. Then 

 all movement ceased. The nucleus divided until there were twenty or 

 thirty nuclei present, all arranged near the surface. This division of 

 nuclei continued until there were from five hundred to six hundred. 

 Walls then appeared at the periphery, cutting off each nucleus with a 

 small amount of cytoplasm. The wall of the cyst became soft and was 

 broken to allow the small Amoeba to escape. Hundreds of these 

 amoebulae, or pseudo-podiospores as they are sometimes called, broke 

 out at one time to become recognizable as Amoeba proteus in from two 

 and one-half to three weeks. No reason for such sporulation is known. 

 Experiments have been made in which specimens were starved, given 

 an excess of food, allowed to dessicate, or where they were transferred 

 to water from different localities, but none of these experiments brought 

 about encystment and sporulation. 



Whichever way the animal may divide it is simply a matter of 

 growth before it is ready to divide again. We have here the interesting 

 fact confronting us that these little single-celled animals are practically 

 immortal. That is, they do not die. One may kill them by boiling and 

 in other ways ; but, left to themselves, they will continue until they 

 have reached their limit of adultship when they divide, each individual 

 becoming two new and separate animals. 



It is important that the fact be grasped that in these little unicellular 

 animals a parent does not give birth to its offspring. The parent itself 

 becomes the offspring. That is, there are no ancestors. Each and every 

 animal carries its complete and total ancestry with it. 1 



BEHAVIOR 



The way in which an animal reacts to a stimulus is called its 

 behavior ; and when that behavior has not been learned, but comes forth 

 without consciousness on the part of the animal, yet is protective to the 

 animal, such behavior or reaction is called instinct. In these lower one- 

 celled animals two words are used in discussing behavior and instincts. 

 These are tropisms or taxis, 2 which merely mean a movement of some 

 kind. To these words one adds the generic name of the stimulating 

 cause, using the words positive and negative to explain one's meaning. 

 For example, usually eight tropisms or taxis are mentioned : 



(1) Thigmotropism, meaning a reaction to contact of some kind; 



(2) Chemotropism, meaning a reaction to a chemical ; 



(3) Thermotropism, meaning a reaction to heat; 



1 An apparent exception to this statement arises if we accept, as a fact, that conjugation in the 

 Infusoria is fundamentally a rejuvenation phenomenon as some biologists contend. Even if this be 

 true, still there is nothing extra added, and the statement remains substantially correct. 



2 In a strict sense "tropisms" mean the growing or bending of an organism, or parts of an 

 organism, in relation to external agents, while "taxis" refer to the active migration of organisms or 

 cells. However, most modern writers are inclined to use these terms interchangeably. 



