62 TERATOLOGY OF DIATOMS. 



" In the examples which I have placed in Class II. and III. above, 

 we see the distortion occurring by the miscarriage or interruption of 

 the vital force which tends to produce the symmetrical sculpture of 

 the shell of the diatom. In Class I. the cause of the irregularity 

 would seem to be external and mechanical. It results, apparently, from 

 the conditions of growth within a strong silicious box. When fission 

 occurs the envelope of the new cells is at first flexible, and is 

 often found very slightly silicified, but it acquires rigidity as the 

 deposition of the silex goes on (') 



" Under these conditions, if we suppose that the diatom is better 

 supplied with nutriment than common, and its circumstances are such 

 as to make more vigorous growth, the valves of the new cell may 

 tend to grow larger than the parent in all directions, and being con- 

 fined by the strongly silicified coat of the mother cell, its own more 

 flexible wall may fold upon itself and make a wavy irregularity of 

 outline, such as is shown in some of the examples above noted. 



" The example of irregular wavy outline suggest also the conclusion 

 that if, in the circumstances last supposed, the diatom were one whose 

 shell presented alternate strong and weak points in its circumference, the 

 indentations resulting would also be regular, and we should have a 

 crenulate form of a shell normally smooth in outline. My own obser- 

 vations have led me to think it probable that crenulated forms are 

 thus originated. Once produced, further fissiparous division would tend 

 to perpetuate the peculiarity, since the growing force in the new cells 

 would force the flexible walls outward to fill the matrix. 



" It would not be till the process of conjugation is reached that the 

 resumption of the normal type could reasonably be expected ; conse- 

 quently it would be permature to conclude that the crenulation is a 

 specific distinction, or even the mark of a lasting variety, until it is 

 proven that after conjugation the new series of diatoms retains that form. 



" Considerations of this sort may assist us in reducing the enormous 

 catalogue of species in the Diatomacece, a consummation devoutly to be 

 wished." 



But not only may the valve undergo modifications and alterations, but 

 the living frustule is liable to be attacked by certain diseases, which are only 

 at present known to us by the alterations that can be observed in the 

 endochrome, and it may moreover be attacked by parasites both 

 externally and internally. 



( J ) See Study of Isthmia nervosa by the Author, in Am. Journal of Microscopy, Vol. III., 

 p. 97 (1878). 



