29 



This investigation has been carried out in the „Labo- 

 ratory for Microscopical Ànatomy" of the Technical 

 Highschool. Herewith I wish to thank Prof. Dr. G. van 

 Iterson Jr. for ail the help he gave me and for enabling 

 me to do thèse experiments. 



Delft. October 1915. 



