INFLUENCE OF THE MEDIUM 139 



zcospores, but these were formed in great numbers, and 

 very rapidly, in the weak solutions. Since the formation 

 of zoftsporis is to be regarded as the result of protoplasmic 

 activity, this fact is added evidence that the cosmetic 

 extraction of water reduced the general activity of the pro- 

 toplast. 



c) The influence of external concentration upon irrita- 

 bility. (1) Changes in irritability. That Loeb observed a 

 loss of irritability in Echinoderm sperms when these were 

 placed in a concentrated solution, and a return of it when 

 they were brought back to normal sea-water, has already 

 been noted. Richter 1 states that zoospores of Tetraspora 

 lose their activity in strong solutions, but regain it on being 

 returned to normal sea-water. The writer found that zoospores 

 of Stigeoclonium lose their power of movement in concen- 

 trated solutions. Of interest here is also the observation of 

 Engelmann 2 that the cilia of the epithelial cells which line 

 the frog's oesophagus become much more active in pure 

 water or a very weak solution than in a solution of the same 

 concentration as the fluids of the animal's body. 



Loeb 3 gives a very striking account of the reversal of a 

 tropism by osmotic extraction of water. At ordinary tem- 

 peratures the Iarva3 of Polygordius and certain Copepods 

 are partly positively and partly negatively heliotropic. 

 Above 25 C. they all react negatively, while below 10 C. 

 the response is reversed, and they all become positively 

 heliotropic. If NaCl is added to the normal sea-water in 

 which these animals are living, they all react positively to 

 light; if distilled water is added, they all react negatively. 



i A. RICHTEE, " Ueber die Anpassung der Sasswasseralgen an KochsalzlOsungen," 

 Flora, Vol. L (1892), pp. 4-56. 



2T. W. ENGELMANN, " Ueber die Flimmerbewegung," Jena Zeitschr., Vol. IV 

 (1868), pp. 321-479. 



3 J. LOEB, " Ueber kflnstliche Umwandlung positiv heliotropischer Thiere in 

 negativ heliotropische und umgekehrt," Pflilgers Arch.f. d. ges, Physiol., Vol. LIV 

 (1893), pp. 81-107; IDEM, Physiology of the Brain, New York, 1900, p. 198. 



