356 ANIMAL NATURK OF DIATOME^. 



cell respecting which we have to decide whether it belong 

 to one kingdom or the other, the chemical test is reduced 

 to data so precarious as not to afford the same certainty. 

 If the wall of a cell treated M'ith iodine be, as well as the 

 nucleus, coloured brown, we may pronounce it to contain 

 azote, and to be formed of a quaternary material. If, 

 again, treated with sulphuric acid, and subsequently with 

 iodine, it assumes a violet colour, there can be no doubt 

 of its being isomeric with starch, since it has been changed 

 into that substance. But if these trials fail or prove 

 uncertain, as very often happens, we must of necessity 

 suspend our judgment. With respect to the contained 

 substance, besides the promiscuous character of some 

 materials already mentioned, some also w^hich are exclu- 

 sive, as, for instance, chlorophyll or starch, may lead to 

 errors. The stomachs of a Polygastric Infusorium may 

 be filled with these vegetable substances, and their 

 transparency and minute size may cause them to appear 

 very simple. 



The successive changes in a cell furnish other very im- 

 portant characters to distinguish the two kingdoms, but 

 give rise, at the same time, to new and grave difficulties. 

 Its extension, the excess of one diameter over the other, 

 the variety of forms it can assume, are frequent conditions. 

 The disappearance and reabsorption of the nucleus occur 

 in every cell at some period of its existence ; but in the 

 cells of higher plants the fine inner membrane or primor- 

 dial utricle soon disappears. Here is a difficulty as to 

 distinction to be overcome. This remains permanent, 

 particularly in the Algae, if not in all inferior plants. The 

 enlargement of the wall, which in the animal cell takes place 

 in a homogeneous manner, is effected in the plant by the 

 deposition on its inner surface of successive strata, con- 

 tinuously or variously formed into circles, spiral bands, or 

 other intermediate forms, yet of a ternary substance, and 

 varying little from that before mentioned (C.^^H.-^O.^"). 

 These are produced at a much later period, and then only 

 in the cells form part of a complicated tissue. Then the 



