1899] MICROSCOPICAL JOURNAL 6 



Fig. 7, shows this going on, for we have the large peri- 

 pheral secondaries breaking into two dots, and the two 

 dots thus formed becoming an integral portion of the 

 central perforated membrane. It will be noticed that the 

 large peripheral secondaries are in all stages; some are 

 only just beginning to notch, others more advanced are 

 nearly cut in two. 



That something of this kind takes place, the figures in 

 illustration of my paper on the "Formation of Diatom 

 Structure," go to prove; further, Figs. 2, 4 and 9, illus- 

 trate the same thing, and are especially interesting be- 

 cause they are the first stages in the formation of the 

 delicate perforated cap of the Asteromphalus; these may 

 be found forming a graduated series from the elementary 

 triangle up to the Asteromphalus pattern, as generally 

 known. 



The evolution of the peripheral dots from the triangle 

 is interesting, I am now able to give a more complete 

 account of this, owing to the discovery of some inter- 

 mediate forms since my previous paper. 



In the first instance, for a terminus ad quem, we have 

 merely the plain polygonal (practically speaking hexag- 

 onal) structure with the usual eye-spot layer attached. 

 Next we find an equilateral triangle at each intercostal, 

 the apices of the triangle pointing to the centre of the 

 hexagons, and the sides of the triangle cutting the sides 

 of the hexagon at right angles (fig. 2). Next we find the 

 triangle growing larger and the apices of it becoming 

 blunted. Next a large dot is formed between the par- 

 allel sides of two adjacent triangles at a point about 

 half way between the intercostal points. By this means 

 six large perforations are formed in each hexagon, not at 

 the angles of the hexagon, but at the bisection of its 

 sides (Fig. 9). About this period the blunted apices of 

 the triangle became notched, the notches deepen, and 

 eventually become perforations. These perforations 



