292 THE CELL 



gation. We expect by this means to increase our knowledge by 

 the study of the lowest organisms chiefly, since in them the indi- 

 vidual cells possess an absolute, or at any rate a large, degree of 

 independence, and are not, as in the higher organisms, related 

 to and dependent upon the other cells of the body. Hence in 

 them the fundamental vital phenomena are more clearly to be 

 recognised. 



The facts which we know at present may be summed up under 

 the following heads : 



(1) The need for fertilisation occurs periodically during the life 

 of the cell ; (2) it invariably lasts only a short time ; (3) it de- 

 pends to a certain extent upon external conditions ; and hence (4) 

 in many cases it maybe suspended and transformed into partheno- 

 genesis and apogamy. 



That the need for fertilisation is a phenomenon occurring periodi- 

 cally in the life of the cell may be demonstrated experimentally 

 through the study of Ciliata. Maupas (VII. 30) has carried out 

 a large number of very instructive experiments upon this subject. 



During the life of one of the Ciliata, two periods can be dis- 

 tinguished an asexual one and one of sexual maturity or need of 

 fertilisation. The first commences after two animals have ferti- 

 lised one another and moved apart ; multiplication then occurs by 

 the rapid and repeated division of the cells. During this period,, 

 individuals from different cultures may be brought together, and 

 the most favourable conditions for conjugation be provided, and 

 yet pairing never occurs. However, after a considerable time r 

 they again experience a need for fertilisation. If at this time 

 individuals from two cultures are brought together under suitable 

 conditions, pairing occurs to a considerable extent for a few 

 days. 



Thus Maupas has established the fact, that in Leucophrys jpatula 

 only individuals of the 300th to 450th generation after the act 

 of fertilisation has taken place can reproduce themselves sexually. 

 In Onyclwdromus this sexual period occurs between the 140th and 

 230th generations, and in Stylouichia pustulata between the 130th 

 and 180th. 



The second law runs : This condition of " need for fertilisation ' 

 is invariably of short duration. If cells capable of fertilisation are 

 not fertilised at the right time, they soon perish. This may be 

 demonstrated with Ciliata, swarm-spores of Algse, and animal egg- 

 cells. 



