278 THE AMERICAN MONTHLY [December, 



hoops made by cutting a couple of circles from pill boxes of the same 

 respective sizes as the first two. The original pill boxes are thus still 

 united together by these hoops or bands, one of which encloses the other 

 or ratlier overlaps it more or less. Imagine this to go on, each of these 

 bands growing wider, and at the same time imagine the new inner 

 valves to grow until they form, respectively, the counterparts of the 

 outer valves. In other words, suppose two pill boxes to have formed 

 inside the original ones and each a trifle smaller than the pill box in 

 which it forms. Now pull your two original pill boxes apart ; that is, 

 slide the larger oft^ the smaller, and imagine the inner boxes to meet 

 your eye, each like the outer one in which it has grown except that it 

 is reversed and a trifle smaller, and you have the idea of the method by 

 subdivision. That is to say, a frustule or diatom has two valves joined 

 together, one of which, however, is a trifle larger than and slightly over- 

 laps the other. And in each of these valves a new^ valve is formed 

 which is a trifle smaller than the one it is. formed in and the reverse of it. 



Prof. H. L. Smith states, however, that in some genera the connecting 

 membrane of the valve does not overlap that of the other like the caver 

 of a box, but that the two edges of this membrane are in contact and 

 of the same size on each valve. (Proceedings of Amer, Soc. of Micro- 

 scopists, 18S7, pp. 68, 69.) Still he admits that in the greater portion 

 of the diatoms one membrane overlaps the other. 



To get the correct idea of the process of subdivision therefore one 

 ought to make himself perfectly familiar with the structure of the diatom 

 frustule, and, I may add, with the growth of Desmids, especially such 

 as Mlcr aster ias and Euastrutn. 



Now, as to the first point, the books usually consulted are not as clear 

 as they might be. Thus the Micrographic says, " The individual cells 

 of the diato?nacece are called frustules, and are furnished with a coat 

 of silica. * * * This consists of two usually symmetrical portions 

 or valves comparable to those of a bivalve shell, which are in contact at 

 their margins with an intermediate piece (the hoop) variable in breadth 

 according to age. * * * When this is very narrow it forms a mere 

 junction line, and is called the line of suture. 



* * * During the process of multiplication by division * * * the 

 narrower or broader band or hoop undergoes an increase of width, 

 and the two valves are removed some distance apart (see pi. xv, figs. 7 

 and 45.) " Fig. 7 is Navicula cusfidata, Eig. 45 is Stauroneis 

 pulchella. " Sometimes it consists of two pieces, one overlapping the 

 other." Now, that whole statement is, in my opinion, wrong and mis- 

 leading. For (i) the hoop is not an intermediate piece between the 

 Valves at their margins nor do the examples cited prove it; and (2) it 

 does not sometimes or ever consist of two pieces overlapping each 

 other. In such cases there are two hoops, and not one hoop composed of 

 two pieces : at least this is so in Tricei'atiufn, Isthtnia, Aiilacodiscus, 

 Biddulphia, and others, as I can show by slides, and Carpenter states 

 this generally. Moreover, in the plates of the Micrographic there are 

 over 200 figures of diatoms and 50 of the front view. But there is not 

 a single figure of them all which show s the student this most important 

 fact respecting the nature of the hoop. Among them are figures of 

 the diatoms mentioned above. In not one of these figures is the draw- 

 ing correct. In each and every case the diatom is represented as ap- 



