VROTOCOCCUS PLUVIALIS. 525 



surrounds the contents with a broad, pecuHarly refrac- 

 tive ring, (Fig. 33.) In cases where this tunic does not 

 exhibit a double contour Une, it is manifested by the 

 dark and sharp border surrounding the contents, and 

 which, especially on the transition of the motile into the 

 still form, is the first and most certain criterion of the 

 incipient change. Thus it resembles the membrane of 

 many Protococcacese and other Algse. It never secretes 

 true thickening layers on the surface. Although this 

 cell-membrane exhibits all the optical characters of one 

 composed of cellulose, I have found it impossible to 

 demonstrate the presence of that principle by means of 

 iodine and sulphuric acid ; it is not coloured by those 

 reagents even after the contents of the cell have been 

 expressed. But this does not show that the membrane 

 is not of a vegetable nature, because other Protococcacese 

 behave in the same way ; nor can cellulose, by those 

 reagents, be shown to exist in Oscillatoria, Nostoc, 

 Merismopadia and other unicellular Algse. 



The still cell always assumes a deep, almost black 

 colour, upon the application of a watery solution of 

 iodine ; but this does not depend upon the coloration of 

 the cell-membrane itself, but upon the circumstance that 

 the nitrogenous contents, coloured by the iodine, are 

 seen through the transparent tunic. 



The contractility of the contents when exposed to the 

 action of acids or alcohol, proves that they are endowed 

 with the same properties as the primordial utricle of Mohl, 

 the amylid-cell of Kiitzing. But there is no evidence of the 

 existence of any special layer of the contents, which could 

 be exclusively designated as a " primordial sac ;" it would 

 much rather appear as if the entire contents of the cell 

 {endochrome of authors), in most cases, were to be regarded 

 as essentially homogeneous, and that it is only the outer- 

 most, for the most part densest peripheral portion of them 

 which possesses the faculties, which in other plants, par- 

 ticularly in the large-celled Algae, appertain to an also 

 optically distinct layer. ( Vide ' Ueber die Rotation des 



