354 DONNELL BROOKS YOUNG 



Hole, during the summers of 1919 and 1920. I wish to take 

 this opportunity to thank Doctor Calkins for the interest which 

 he has shown and for the help and advice which he has given. 

 I am indebted to my wife, Helen Daniels Youne, for her pains- 

 taking preparation of the figures. 



MATERIAL AND METHODS 



Uronychia is a common marine hypotrich found at Woods 

 Hole. Practically every salt-water culture examined during the 

 summer was found to contain it, although all the forms which 

 were used for experimentation were collected from two limited 

 regions. It soon became evident that more than one species 

 was present in these cultures, and the first problem was to make 

 an accurate study of them so that they could be distinguished 

 from each other. This led to a study of the genus as a whole 

 with the results given in the next section. 



No satisfactory culture medium was found, so that it was not 

 possible to keep isolation cultures going for more than a few 

 generations. However, for the purposes of this work an infusion 

 made of eel-grass, flour, and a very little malted milk in sea- water 

 was found to serve the purpose. Mass cultures were kept on 

 hand, and the forms to be used for the various experiments were 

 selected from these. 



The method of procedure was as follows: the mass cultures 

 were examined under the low power (10 X oc. and 56 mm. obj.) 

 of a binocular microscope, and those individuals which were 

 seen to be in the process of division were picked out by means 

 of a small pipette and isolated in culture dishes in a few drops of 

 the culture medium mentioned above. Such dividing forms 

 can be identified quite easily because of the cirri which form 

 precociously on the daughter cells. Usually there was but little 

 difficulty in finding as many such individuals as could be used 

 at one time. These were kept under observation and the time 

 of division noted. Then at the desired time after division one 

 of the daughter cells would be isolated in another culture dish 

 and cut. It was found that after a little practice the cutting 

 could be done free-hand, in any plane desired, with a small sharp 



