Chap. 18 REPRODUCTION 333 



without response. From time to time, more often in some species than in 

 others, this behavior changes with dramatic suddenness. Mating spreads 

 through the population Mke an epidemic and for hours a lone paramecium is 

 scarcely to be found. Couples swim about for hours, always in the same 

 position with parts of their oral surfaces held together by a bridge of proto- 

 plasm (Fig. 18.2). After preliminary divisions of the micronucleus in each 

 one, two micronuclei of unequal size remain in each individual. The smaller 

 male micronucleus, essentially similar to a sex cell, migrates over the proto- 

 plasmic bridge and fuses its substance with the nonmigrating female micro- 

 nucleus. The female micronucleus becomes a permanent part of each recipient 

 Paramecium. After the exchange is completed, the bridge is gradually with- 

 drawn and the mates (conjugants) separate, each animal carrying with it a 

 new strain of inheritance to be distributed to its descendants. 



The frequency of conjugation varies in different species, environments, and 

 physiological conditions. After conjugation paramecia divide more rapidly 

 as if mating were the rescue from a physiological depression. However, no 

 such rescue is essential. In a famous experiment carried on at Yale University, 

 L. L. Woodruff kept a culture of paramecia (P. aurelia) for over 20 years 

 (12,000 generations) without conjugation simply by changing the water 

 daily and keeping the food and environment satisfactory. 



Special mating types of paramecia were discovered by H. S. Jennings 

 who reared thousands of them from natural pond populations. Among them 

 he found certain ones that would and others that would not mate outside 

 their own type, such as type A and B that mated together and a type C 

 that would not mate with either of them. It seems that type C is not a 

 fixed sex but is only generally sexual; animals of this type have not become 

 limited and settled into the bisexual pattern. Their situation suggests that the 

 development of sexes might not have been restricted to two kinds. If a 

 general sexual type had persisted among higher animals including man, 

 would not social behavior have been complex beyond imagination? 



Endomixis. In some species of paramecia and under certain conditions 

 there is a nuclear reorganization, called endomixis, and this is followed by 

 an invigoration similar to that after conjugation. This process takes place 

 entirely within one individual. 



Sexual Reproduction 



The Plan of the System. The bisexual reproductive systems of multicellular 

 animals consist of the gonads, i.e., testes in the male, ovaries in the female, 

 and a series of more or less elaborate tubes and glands located within the 

 system or in another part of the body. The gonads are the essential organs that 

 produce the sex or germ cells. The tubes and sacs provide for the transporta- 

 tion of the sex cells and the developing young that may originate from their 



