Examples of Pupils' Work 183 



begin to feed, and recover their original dimensions. Each 

 possesses the original unaltered macronucleus and a large 

 micronucleus called the combination nucleus. Immediately 

 after separation the original macronucleus undergoes 

 changes which lead to its final disappearance. 



Meanwhile the combination nucleus in each ex-gamete 

 undergoes three successive mitotic divisions, and there are 

 eight products in the combination nucleus, arranged in 

 two groups of four, one group at the anterior end and 

 another at the posterior end of the body. Of the posterior 

 group three members disappear, one surviving as micro- 

 nucleus. The four nucH of the anterior group increase 

 in size and become the macronuclei of the progeny of the 

 ex-gamete. Twenty-four to thirty hours after separation 

 the ex-gamete divides in such a manner that two of the 

 macronuclei pass into one product of division and two 

 into the other, while the micronucleus divides mitotically, 

 one of its products entering each of the two daughter 

 Paramecia. A second division quickly follows, accom- 

 panied by the mitotic division of the micronucleus, while 

 the macronuclei are again passively distributed among the 

 products of division. The result is that each ex-gamete 

 has produced four normal Paramecia containing each a 

 macronucleus and a single micronucleus, both derived 

 from the combination nucleus. These Paramecia feed 

 and multiply by transverse division at the rate of two or 

 three divisions in the twenty-four hours. 



e. Death. After four or five days the offspring of a 

 Paramecium, some four thousand in number, become 

 smaller and begin to show signs of decay. Eventually, 

 if they are unable to conjugate, they undergo degeneration 

 and die. Similarly, if the food supply runs short, the 

 members of a swarm of Paramecia become weak and 

 diminished in size, but under normal circumstances the 

 onset of degeneration is arrested by conjugation. Decay 

 and death are the natural accompaniments of existence. 

 The Paramecium may also be killed by bacteria and the 

 application of chemicals. 



