402 ZOOLOGY. 



6. Multiplication takes place much as in Amoeba. 

 First the micronucleus and then the meganucleus divides 

 the former by karyokinesis, but not the latter and 

 the animal becomes divided into two in a transverse plane. 

 Each half by slight changes soon comes to have the form of 

 a perfect Paramecium. 



When well fed, Paramecium will multiply in this way 

 very rapidly, dividing once or twice in a day. Careful 

 observations on the descendants of a single Paramecium, 

 carefully isolated, have shown that this fission may continue 

 for several months. By the end of that time, however, the 

 character of the Paramecia was found to be changed ; their 

 vigour was diminished, their nuclei and protoplasm showed 

 signs of decay, and eventually death became inevitable. 

 Thus the power of continued division is not unlimited. 



7. Conjugation. Under normal conditions, this con- 

 dition of "old age" is avoided by means of a process termed 

 conjugation, which is of the greatest interest as affording 

 a means of comparison between the higher Protozoa and 

 the Metazoa (multicellular animals). Conjugation in 

 Paramecium consists in the temporary union of two indi- 

 vidual Paramecia, during which certain internal changes 

 take place. The exact details of these changes are very 

 difficult to observe, and somewhat different accounts are given 

 by different observers. But about the essential facts there 

 can be no doubt, and to these we will confine ourselves. 



Firstly, the internal changes concern the nuclei, and 

 especially the micronucleus, which is very active; the 

 meganucleus of each individual is at first unaffected, but 

 eventually becomes disintegrated and disappears (possibly 

 being digested by the protoplasm). The micronucleus of 

 each individual, on the other hand, increases in size and 

 repeatedly divides by karyokinesis ; some portion of the 

 products of division is exchanged between the two individuals, 

 which then separate ; fusion of the native and imported 

 micronuclear material occurs in each one, and from the 

 product a new meganucleus and new micronucleus are 

 formed. The two Paramecia are now "rejuvenated" and 

 at once proceed to divide in the ordinary way. 



