290 GENERAL ZOOLOGY 



Some interesting experiments bearing on the problem of 

 conjugation in Paramoecium were carried out in the years 

 1901 and 1902 by Professor G. N. Calkins, of Columbia 

 University. He found that as the generations approached 

 ninety to one hundred and seventy in number, the individuals 

 become smaller in size and appear to lose some property in 

 their protoplasm. He was able to " renew the youth " of his 

 series of animals by artificial means, twice by change of 

 food, once by mechanical agitation, and once by a rise in 

 temperature. After each stimulation the Paramcecia divided 

 frequently, and in other respects imitated the phenomena 

 subsequent to conjugation. 



Results strikingly similar to these have been obtained by 

 Professor Jacques Loeb, of the University of California, in 

 stimulating by mechanical and chemical means the eggs of 

 sea-urchins to develop to advanced stages. The term " artifi- 

 cial parthenogenesis " (see parthenogenesis, p. 34) has been 

 used in describing the results obtained by Professor Loeb. 

 It is interesting to notice in connection with these experiments 

 that in the eggs of sea-urchins stimulated to develop naturally 

 by the spermatozoon, and artificially by mechanical and chem- 

 ical means, and in Paramcecium stimulated to frequent divi- 

 sion naturally by conjugation, and artificially by mechanical 

 and chemical means, we have a result the identity of which 

 can be expressed under the name rejuvenescence. Although 

 these experiments have value in interpreting the phenomena 

 of reproduction, it is well to keep in mind the fact that the 

 artificial method fails after a few cycles in Paramoecium, and 

 before the end of the larval period in the sea-urchin. 



When the water in which Paramoecia live shows indica- 

 tions of drying up, the animals encyst themselves by secret- 

 ing a film of gelatinous substance from their ectoplasm. The 

 encysted animal retains its usual form, and remains inactive 

 until it is brought into water again. 



