DUPLICATIVE SUBDIVISION IN DIATOM ACE M. 283 



FIG. 81. 



plished by Mr. Wenham, the appearance represented at c is 

 obtained ; which is in all respects comparable with that presented 

 under a low power by the valve of Triceratium or Isthmia. At the 

 upper part of this figure, which represents a portion of the object 

 that was accurately in focus, the hexagonal arese are seen to be 

 light, and the intervening spaces dark ; the reverse is the case with 

 the lower portion, which was out of focus ; and a curious transition 

 from one condition to the other is seen in the intermediate part. 1 

 176. The process of multiplication by self-division takes place 

 among the Diatomacese on the same general plan as in the Des- 

 midiacese, but with some modifications incident to the peculiari- 

 ties of the structure of the former group. The first stage con- 

 sists in the elongation of the cell, and the increase in the breadth 

 of the "hoop," which is well seen in Fig. 81; for in the newly- 

 formed cell e, the two valves are 

 in immediate apposition, in d 

 a hoop intervenes, in a this 

 hoop has become much wider, 

 and in b the increase has gone 

 on until the original form of 

 the cell is completely changed. 

 At the same time, the endo- 

 chrome separates into two halves 

 so that its granules form two 

 layers, applied to the opposite 

 sides of the frustule ; the nu- 

 cleus also subdivides, in the 

 manner formerly shown (Fig. 

 67, G, H, i) ; and (although the 

 process has not been clearly 

 made out in this group) it may 

 be pretty certainly concluded 

 that the primordial utricle folds 

 in, first forming a mere con- 

 striction, then an hour-glass 

 contraction, and finally a com- 

 plete double partition, as in 

 other instances ( 165). From 

 each of these two surfaces a 

 new siliceous valve is formed, 

 as shown at Fig. 81, A, <?, just 

 as a new cellulose wall is gene- 

 rated in the subdivision of other cells ; and this valve is usually 



1 The Author does not think it necessary to go more in full into the discussion of the 

 nature of these markings, which some have represented to be due to hemispherical 

 elevations on the valves; as he thinks that no argument is Jikely to convince those, 

 whose minds are prepossessed with a conception which influences their interpretation 

 of what they see. To those who come fresh to the question, he would strongly recom- 

 mend the education of their judgment upon the larger and more closely marked Diatoms, 

 as above described, before they commit themselves to an opinion on either side. A 

 fuller statement of the question will be found in the " Micrographic Dictionary," "Intro- 

 duction," p. xxxiii, and Arts. " Angular Aperture" and " Diatomaceae." 



Biddulphia pulchetta ; A, chain of cells in 

 different states ; a, full size ; b, elongating 

 preparatory to subdivision; c, formation of 

 two new cells; d, e, young cells, B, end 

 view; c, side view of a cell more highly 

 magnified. 



