D. B. WARD, M. D. 69 



are minute globules of oil. What function the oil per- 

 forms is not known. The outer surface of the diatom 

 moreover is covered with an exceedingly transparent 

 delicate membrane of protoplasm. 



Now with regard to the valve itself. The two valves 

 are usually symmetrical and exactly alike, but in some 

 species they are always different, for example, some 

 being fiat and the opposite valve very convex. They are 

 of every conceivable shape — round, square, triangular, 

 polygonal, boat shaped, star shaped, and in fact you can 

 scarcely imagine any symmetrical form which is not rep- 

 resented by some of the species. The markings occur 

 on the surfaces of these valves. One of the most fre- 

 quent methods of ornamentation is by fine parallel lines 

 or striae. The striae are found when seen under a high 

 power to be formed of little dots arranged in rows so as 

 to look under lower powers like continuous lines. The 

 explanation of this was very simple and satisfactory 

 until quite recently, viz. : that the dots were minute ele- 

 vations or thickenings of the silex for the purpose of in- 

 creasing the strength of the valve at the least possible 

 expense of material. It was assumed that they were 

 elevations, but within the last three years it has been 

 demonstrated that they are depressions and the striae 

 accordingly instead of adding to the strength of the 

 valve really weaken it. This discovery was made by 

 Mr. Gill, in England. He precipitated various chemical 

 substances on the surfaces of the valves and then washed 

 them thoroughly. Prussian blue, metallic silver and 

 sulphide of mercury were used for this purpose. That 

 deposited on the surface of the valve was easily washed 

 off, while the little cup-shaped cavities retained the sub- 

 stances and proved that they were depressions, as they 

 would be the first to wash clean if they were elevations. 

 There is much yet to learn about the structure of the 

 diatom valve, and many disputes have arisen regarding 



7 



