240 LECTUEES ON BIOLOGY 



and male germs. Only gradually is it perceived that these 

 many-shaped organisms are but different developmental stages 

 of one organism ; but even with that discovery little progress 

 has been made, for these numerous stages have apparently no 

 connection. One knows neither beginning nor end of the chain. 

 Yet patient research never flags ; what remains hidden to one 

 is discovered by another investigator, and gradually untiring 

 observation and combination forge link to link in right succes- 

 sion, until at last after long laborious work and many errors the 

 cycle of evolution is perfect. 



I have so far omitted one important fact in the process 

 of the reproduction of the protozoa. It happens frequently that 

 multiplication commences with a period of rest. In such cases 

 the organs of locomotion in the protozoa usually become de- 

 generate, the organization becomes more simple, the plasmic 

 body assumes a globular form and secretes a protective cover 

 a cyst. They remain in this state often for a considerable time 

 without exhibiting any perceptible signs of life. Protected by 

 the cyst they are almost unsusceptible to all injurious external 

 influences, and in many cases, when the water in which they live 

 dries up, when winter comes, or food becomes scarce, it is only 

 this ability to enter into a stage of rest which can keep death 

 away. Reproduction takes place usually before the encysted 

 protozoans leave their shells. 



The faculty of proceeding under certain circumstances to 

 cyst-formation we find developed already in the lowest proto- 

 zoans. The beautiful Amoeba proteus is found in our ponds, 

 clinging to water plants or floating in the water. With its 

 life-history we are already acquainted, because it is identical 

 with that of other aquatic amoebse to which I referred in the 

 beginning of this lecture. But at certain times we can observe 

 striking changes in the body of Amoeba proteus. The pseudo- 

 podia are retracted, the difference between the hyaline ectoplasm 

 and the granulated endoplasm disappears, and the amoeba 

 assumes the shape of a ball which soon commences to rotate 

 slowly around its axis. In the course of its movements the 

 amoeba secretes a covering; at the end of five days this has 

 become a complete cyst, and the revolving movement ceases. 

 In the meantime the nucleus has undergone certain changes, 



