Tetradesmus, a new four-celled coenobic alga 
GILBERT MorGAN SMITH 
(WITH PLATE 1) 
METHODS AND MATERIAL 
The alga described below was cbtained during a study of 
various algae in pure cultures. The isolation of the different algae 
was by means of a medium consisting of 0.2% Knop’s (9) solution 
and 2.0% agar in Petri dish cultures. By this method cultures 
were obtained which were the descendants of a single cell. Great 
care was used in the isolation, and only those cultures that were 
entirely free from other algae and bacteria were used in this study. 
After the isolation, cultures were made in sterile Knop’s solution 
in 200 c.c. Erlenmeyer flasks. A complete description of the 
methods used will be given in a forthcoming paper. 
The details of the cellular structure were studied in material 
which had been fixed in Flemming’s weak osmic-acetic-chromic 
acid mixture diluted with an equal volume of water. For the 
washing and dehydration of the material a modification of Oster- 
hout’s (12) method was used. After the material had been al- 
lowed to stand in the killing solution for 24 hours, as much as 
possible of the solution was removed from the vial by means of a 
pipette, and then the vial was filled with distilled water. A 
celloidin film was prepared by pouring a celloidin solution on a 
clean surface of mercury and allowing it to harden so that it could 
be lifted up and placed over the mouth of the vial. The membrane 
was then allowed to harden still further while on the mouth of the 
vial. The material was then washed by simply placing it in a 
dish containing running water. For the purpose of dehydration 
the vial was transferred to 70 per cent. alcohol for 12 hours and 
then to two successive portions of 95 per cent. alcohol, at intervals 
of 12 hours. By means of this method the loss of material through 
decantation was obviated. The material was imbedded in paraffin 
and cut on a microtome into sections of from 3 to 5 # in thickness. 
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