REPRODUCTION AND SEX 195 



it in these cases. Parthenogenetic ova do not need the 

 stimulus of conjugation, and in the unfertilised eggs of 

 various animals — conspicuously, for instance, in those of 

 sea-urchins — it has been found possible, by the use of 

 certain solutions of salts and other substances, to provide 

 a stimulus which causes division and thus brings about 

 artificial parthenogenesis. 



(2) Another theory of the meaning of conjugation 

 supposes that by its means the vitality . of the protoplasm is 

 renewed. This view is based upon certain facts that are 

 supposed to have been observed in cultures of Paramecium. 

 It is said that conjugation between the members of a 

 depressed culture (p. 137) will avert depression in the 

 descendants of the conjugants. This, however, is doubtful, 

 and even if it be true gives no explanation of the normal 

 function of conjugation. Depression is an abnormal event, 

 at present only known in cultures in which it has been 

 produced by artificial conditions. It is not, as we have 

 seen, an inevitable senility. Conjugation, on the other 

 hand, is a normal process which occurs without any 

 apparent connection with depression. Whether, as some- 

 times happens, it takes place repeatedly at very short 

 intervals (in which case it cannot be necessitated by 

 depression), or thousands of generations succeed one 

 another before it occurs, in the vast majority of cases it 

 happens in a stock which shows no sign of depression. It 

 is also very important to remember that in Paramecium the 

 process loosely called " conjugation " includes, not only true 

 conjugation or the union of the gametes, which concerns 

 the germ-substance alone, but also the loss by destruction 

 of the meganuclei (and perhaps also of some part of the 

 cytoplasm) of the conjugants. Now the meganucleus 

 represents,- as we have seen, the nuclei of the body-cells of 

 the frog. It may well be that, just as the body of the 

 frog is subject to natural death, so the meganucleus of 

 Paramecium (and perhaps also some part of the cytoplasm) 

 becomes in time effete, that in both cases the germ- 

 substance needs no revitalising, but is immortal when once 

 it is separated from the effete part of the body (which 

 happens in the frog by the germ- cells leaving the body and 

 in Paramecium by the destruction of the meganucleus), and 



