4 THE AMERICAN MONTHLY [Jan 



the thirteen (more or less) spots at the periphery of the 

 cell cap : to this it may be replied that anyone making 

 such a statement is bound to prove that the interference 

 ghost image of thirteen (more or less) spots in a circle is 

 the same as that of a quincunx pattern ; which of course 

 could not be done. The crushing argument is, however, 

 this : that there are structures precisely similar in every 

 respect, both as to size and arrangement of spots, etc., 

 which do not posssess this tertiary structure. The ter- 

 tiaries cannot, therefore, be interference effects arising 

 from the primary or secondary structures, otherwise they 

 would be seen in all cases where the conditions were 

 similar. This brings down the matter to two points : 

 viz., that this tertiary structure is what it appears, or is 

 an interference ghost of some as yet undiscovered quin- 

 cunx membrane inside the polygonal cell. Against this 

 latter supposition we have the dissimilarity in the pat- 

 terns and number of tertiary spots. Fig. 3, is drawn 

 from another diatom to show irregularity in pattern. 

 There are, it will be noticed, six, five, four, and three dots 

 round a very faint and ill-defined central spot. This 

 central spot may or may not be an out-of-focus image of 

 the eye spot on the lower membrane; anyhow, this is of 

 no importance, for the argument rests on the peripheral 

 dots. No one will contend for a moment that such an 

 irregular pattern is merely an interference effect, and 

 not the image of an entity; its very irregularity nega- 

 tives the idea. Moreover, it should be noted that this 

 is a coarse structure. 



Now, with regard to the meaning of this tertiary 

 structure, my theory is that it is a stage in the evolution 

 of the central perforated membrane; the large spots at 

 the periphery become divided up and thrown into the 

 central portion, while other large ones are formed to be 

 broken up in their turn until the valve has reached 

 its adult size. 



