No. 479] 



FINER STRUCTURE OF ARCELLA 



801 



When the test was seen in optical section the reason for the view 

 that there are two membranes was apparent, for the limiting upper 

 edges of the raised network give the appearance of a wall covering 

 in the top. This appearance seems to be merely the effect of re- 

 fraction of light. The basal membrane may be clearly seen. In 

 general the height of the raised network above the basal membrane 

 is about equal to the width of the hexagonal areas. 



Besides the further comphcation of structure in the test, another 



