NATURAL HISTORY OF THE DIATOMACE^E. 447 



As the siliceous exterior skeleton of the diatom does not permit of its 

 expanding in all directions, the consequence is that when it absorbs nutri 

 tive material, increases in bulk, and proceeds to subdivide, it must mul 

 tiply its bulk in one direction only ; and, as the two valves are capable of 

 being separated one from the other along the line of junction with the 

 connecting membrane, it is in that way that the splitting up of the cell 

 into two new ones takes place. The existence of the connecting mem 

 brane in the perfect diatomaceous individual, before subdivision starts, is 

 denied by many. Under those circumstances, the siliceous skeleton con 

 sists of only two parts, the valves, to which, as soon as subdivision sets 

 in the third portion, the connecting membrane is added. In Palmoglcea 

 we have seen that the perfect cell subdivides by a process which shows 

 itself by the bending in of the cell wall, if there be one, or by the pe 

 riphery of the mass, if there be no limiting membrane. In this way, at 

 first, two united and similar halves, and thereafter, two separate individ 

 uals, are formed exactly alike in all particulars. The nucleus and nucle- 

 olus, if there be one, of the diatom subdivide at the same time ; and, to 

 make room for the increasing cell-substance, the enclosing valves sepa 

 rate from one another, the space all around between them being closed in 

 by the new hoop of siliceous substance, the connecting membrane which 

 now makes its appearance and grows by additions to its two edges, as the 

 accumulation of pabulum by the growing individual goes on. At the 

 same time, two new valves have been forming exactly like the two old 

 ones, except that in consequence of their forming entirely within the con- 

 necting membrane, which has the same diameter as the old valves, they 

 are just so much smaller; and, as this mode of subdivision is repeated 

 again and again, there are always two individual diatoms having one old 

 and one new valve, which latest formed valve is continually being replaced 

 by another still newer. That the cell-contents may be at no time exposed 

 to direct contact with the surrounding water, the connecting membrane 

 is formed of two pieces, by additions next to the two valves ; and, as one 

 valve of the diatom individual is always somewhat smaller than the other, 

 one of these sections of the connecting membrane is smaller than and 

 slides within the other. In some cases, however, it would seem that the 

 connecting membrane is made up of but one piece, instead of two, as 

 described. On account of this gradual diminution in size which must 



