W. E. Agar 189 



Three conditions of the environment seem to be specially significant 

 in interpreting the ahnost uniform lack of sexuality, and the total lack 

 of any tendency to degeneration observed in these three lines under long 

 continued parthenogenesis. 



Fii'stly, with the few exceptions of those individuals fed with the 

 protophyte culture, they were all fed with the same food, and this food 

 supply, though it may have fluctuated from day to day, probably did 

 not do so over long periods, as the water in the Lepidosiren tank (which 

 had been established more than three years before the experiment began) 

 was changed weekly. Hence any cyclical change in the food supply was 

 probably a weekly one, and there was little chance of a progressive 

 deterioration in the culture medium lasting over weeks and months 

 which might have caused sexuality and degeneration to set in. 



Secondly, practically every individual was isolated in a separate 

 tube within 48 hours of birth. The only excep)tions were so rare as to 

 be negligible, and moreover in only one case where more than one 

 individual was kept in the same tube was such a sj)ecimen used as the 

 ancestor of any considerable number of generations. In the light of 

 Grosvenor and Smith's results as to increased sexuality in overcrowded 

 animals, this is a significant point. 



Thirdly, the water was changed regularly in all the tubes every 

 second day — with again an insignificant number of exceptions. 



General considerations. 



It is obvious that it is no longer necessary in the present state of 

 our knowledge to discuss the extreme Weismannian hypothesis that the 

 parthenogenetic or sexual mode of reproduction is determined entirely 

 by internal changes which aie an integi-al part of the physiology of the 

 animal and independent of environment. The idea of an internal cycle 

 or rhythm still persists however, and it is undoubtedly the most generally 

 accepted view to-day among workers on Cladocera that the change from 

 parthenogenetic to sexual reproduction is determined by such a cycle, 

 with the limitation that this change can be accelerated or delayed by 

 particular conditions of the environment. This is expressed by Hertwig 

 in the following sentence (p. 29): " Fortgesetzte Parthenogenese fiihre 

 schon ah solche' in der Beschaffenheit der Tiere zu Veranderungen, 

 welche die Entwickelung der Geschlechtsgeneration veranlassen. Diese 

 Umformung der Zellen konne durch einen entgegengesetzt wirkenden 

 Faktor, wie die Warme, vielleicht dauernd zuriickgedriingt werden." 



1 My italics. 

 Journ. of Gen. iii 1^ 



