6 MORPHOLOGY OF INVERTEBRATE TYPES 



Sexual reproduction consists in conjugation. In this process 

 the reorganization of the nuclear apparatus is combined with 

 amphimixis. Two individuals approach each other and bring 

 their ventral (oral) surfaces into close contact. The macronuclei 

 remain for a while undisturbed, but later show signs of degenera- 

 tion. After the separation of the conjugants they break up into 

 small parts which are gradually resorbed by the protoplasm. 

 The micronuclei, on the other hand, play the chief role in con- 

 jugation. They divide in both individuals twice in succession. 

 Of the four micronuclei thus formed, three disintegrate and are 

 resorbed by the protoplasm. The fourth micronucleus of each 

 conjugant divides into a stationary or female nucleus and a mi- 

 grating or male nucleus. The latter leaves the individual in 

 which it was formed and enters the other conjugant. Here it 

 fuses with the stationary nucleus forming a synkarion. When 

 both individuals have exchanged their migrating nuclei and 

 formed synkaria, conjugation is finished. The individuals 

 separate, but their nuclear apparatus is not normal. Now the 

 reconstruction of the macronucleus begins. The synkarion di- 

 vides three times in succession giving rise to eight nuclei. Three 

 of these nuclei disintegrate and are resorbed by the protoplasm, 

 one assumes the shape and function of the micronucleus, while 

 the remaining four grow and change eventually into macronuclei. 

 The exconjugant is now ready for a new division. First the 

 micronucleus divides in two; then two of the four future macro- 

 nuclei pass to the one end, and two to the other end of the di- 

 viding Paramaecium. When the division is completed each 

 daughter cell has one micronucleus and two macronuclei. Yet 

 another division must take place. The micronucleus divides 

 again while of the two macronuclei one passes to one end and 

 the other to the other end of the animal. Thus the balance of 

 nuclei is restored in the third generation and each individual 

 is now in possession of a single micronucleus and a single 

 macronucleus. 



