PSEUDOPODIA OF DIATOMS. 619 



finer parts. Alcoholic satranin just stained the pseudopodia ; 

 but all alcoholic stains are apt to fail with them. 



The evidence of the stains, therefore, joins with the other 

 tests in pointing to the presence of protoplasm with or without 

 a very fine cuticle of uncertain nature. The thick pseudopodia, 

 with their variation in density (fig. 5), also point to a very fine 

 cuticle with protoplasmic contents. There is no evidence in 

 any of my slides of a layer of protoplasm normally present out- 

 side the diatom shell. The pseudopodia or their bases spring 

 straight from the shell. Most probably there is a fine layer of 

 protoplasm outside the shell, for ImhofF has shown that the in- 

 ternal protoplasm reaches the surface, but it is not thick enough 

 to be visible in optical section. 



In the Kew gathering I have met with two or three speci- 

 mens surrounded by a thick layer of what looks like a gelatinous 

 substance, not granular. The pseudopodia, however, are inde- 

 pendent of this, and are clearly traced through it up to the 

 margin of the shell of the diatom. 



I have also seen granular or fluffy substance round Cyclo- 

 tellas, which might be protoplasm ; but this is not at all 

 common. I have seen it occasionally round other forms which 

 have no pseudopodia. 



Next we come to what has been said or suggested as to the 

 meaning of the pseudopodia. 



The first suggestion was that these were not diatoms at all, 

 but the identification of a well-known species, Cyclotella 

 Kiitzingiana, with pseudopodia has settled that point. Next 

 it was suggested they might be extensions of the gelatinous 

 layer which Professor H. L. Smith has shown to surround 

 many diatoms. 



But there is no evidence here of any kind of normal gela- 

 tinous envelope, and the stiffness of the pseudopodia and the 

 permanence of the bases tell against this theory, which is 

 further negatived by the fact already stated, that the pseudo- 

 podia penetrate the gelatinous layer when this is present. 



A third hypothesis, based on the remarkable slides where 

 the diatoms and Archerina are mixed in countless profusion, is 



