220 THE PROTOZOA 



Karyogamy is widely spread throughout the Infusoria, where 

 conjugation in the different species is characterized by very similar 

 features. Two individuals unite, usually by the anterior ends, with 

 the mouth openings apposed. A protoplasmic bridge is formed 

 between the two, through which there is an interchange of micro- 

 nuclei. This interchange is followed by final separation of the 

 conjugants, each of which regenerates the parts lost during the 

 period of conjugation (oral cilia, macronucleus, etc.). After careful 

 observations upon many different species of Infusoria, Maupas ('88, 

 '89) found that certain conditions are apparently necessary in order 

 that conjugation between two individuals can take place and lead to 

 fertile results. These conditions are: (i) Sexual maturity, that is, 

 the individuals must be removed by some generations from the last 

 conjugating pair. Maupas established the fact that, in LencopJuys 

 patula, only individuals of the three hundredth to the four hun- 

 dred and fiftieth generation could be reinvigorated by conjugation; 

 in Onychodromus only individuals between the one hundred and 

 fortieth and the two hundred and thirtieth generation ; and in Stylo- 

 nychia piistulata only individuals between the one hundred and 

 thirtieth and the one hundred and eightieth generations. If these 

 individuals, when thus mature, are restrained from pairing, they 

 become over-mature, after which, if they conjugate, the union is 

 without result, and the individuals finally succumb to what Maupas 

 calls " senile degeneration." 



(2) A second condition is scarcity of food. Maupas also shows 

 that an over-abundance of food causes the individuals to die from 

 senile degeneration without developing the "sexual hunger." 



(3) A third condition is diverse ancestry. Maupas arrived at the 

 conclusion that two individuals from the same ancestor would not 

 conjugate. " In many pure cultures of nearly related individuals," 

 he says, " the fast to which I subjected them resulted either in their 

 becoming encysted, or in their dying of hunger." " It was not until 

 after senile degeneration had already begun to make inroads in the 

 culture that I noticed that the conjugation of nearly related indi- 

 viduals occurred in the experimental cultivations. However, all such 

 conjugations ended with the death of the Infusoria which had paired, 

 but which were unable to develop further, or to reorganize themselves 

 after they had fused. Such pairings are, therefore, pathological 

 phenomena due to senile degeneration." * 



The latter conclusion, drawn by Maupas, has not been entirely 

 sustained. Butschli was one of the first to question it, 2 and recently 

 Joukowsky ('98) observed fertile conjugations among descendants of 



1 Loc. at., p. 411. 2 ('88), p. 1638. 



