HARDWICKE'S SCIENCE-GOSSIP. 



!°5 



What a Diatom is. 



The little microscopic organism of which we here 

 propose to say a few words is distinguished from 

 the unicellular algae, properly so called, as well as 

 from the animals of the class Infusoria and the 

 Rhizopods, by very distinct characteristics. They 

 constitute one of those numerous links between that 

 which one is apt to consider (very empirically) as 

 forming two distinct series in nature, known as the 

 Animal and Vegetable kingdoms; grand divisions that 

 are only separated one from the other by the traditions 

 of the first fathers of biological science. In their time 

 the studies of anatomy, biology, physiology, and 

 morphology, did not exist, and the philosophical 

 idea, relating to life, was presented to us under a 

 very dim veil, of which the microscope has only very 

 recently succeeded in raising the corners. 



An isolated diatom reduced to its simplest form is 

 composed, according to our view : — 



Fig. 73. Section of a diatom undergoing deduplication : A A, 

 new nucleus and nucleoli ; B B, protoplasm ; b' b', double 

 primordial utricles ; c c, divided endochrome ; D D, central j 

 cavities ; f', external mother-valve ; F, internal ditto ; f /' , 

 new young valves ; G G, connectives. 



1st. A central nodule, or nucleus, more or less 

 apparent, colourless, formed of living matter of dense 

 protoplasm in the interior of which a nucleolus may 

 generally be detected (fig. 71, A). 



2nd . A mass of the protoplasm less dense ( fig. 7 1 , B B), 

 surrounding the nucleus, very finely granulated, which 

 form a certain number of prolongations, which extend 

 up to the limits of the internal cavity of the diatom, 

 sometimes under the form of two large bands, some- 

 times under that of radiating filaments, which some- 

 times anastomose and are of various diameters. 



Under favourable circumstances an amoeboid cir- 

 culation, analogous to that seen in the cells of Spiro- 

 gyra and in the hairs of the Urticaceae. 



3rd. A closed vesicle, more or less dense, transpa- 

 rent, and of the same composition and texture as that 

 of the mass surrounding the nucleus, and that may be 

 considered as an effusion of the branchings above 

 mentioned. It is the primordial utricle (fig. 71, B'B'). 

 Its thickness is considerable at the two ends of those 

 diatoms whose axes are lengthened. 



4th. A material of a golden-yellow or brown colour, 



known under the name of diatomine, or a mixture of 

 chlorophyl and phycoxanthine, and which constitutes 

 the endochrome (fig. 71, CC). This material is placed 

 in the substance of the primordial utricle, which 

 entirely surrounds it. It often forms (according to 

 the species) one or more bands, continuous or inter- 

 rupted, sometimes the granules are more or less 

 rounded and scattered, or radiating. 



The endochrome is subject to periodic changes of 

 position, at certain epochs in the life of the diatom, 

 dragged by the movements of the entire substance 

 of the primordial utricle. The diatomine is evidently 

 analogous to the chlorophyl of ordinary plants, of 

 which it possesses many of the chemical and most of 

 the spectroscopic characteristics. 



5th. A central cavity (fig. 71, DD), surrounding the 

 nucleus, and limited exteriorly by the primordial 

 utricle, and which contains — 



(a) A limpid, colourless fluid. 



Fig. 75. The same with two 

 valves and one connective. 



Fig. 74. Diatom (Isthmia) 

 formed of four valves and 



two connectives. 



(b) Drops of an oily nature, coloured or colourless, 

 and highly refractive (fig. 71, EE). 



(<) Granules of a nature not determined. 



6th. An external protecting envelope formed of 

 cellulose strongly impregnated with silica, secreted 

 by the primordial utricle (which it invests) in a pecu- 

 liar manner during the life of the diatom. 



This envelope is never simple, as in ordinary cells, 

 but is composed of two distinct opposed valves (fig. 71, 

 FF), representing the lids of a little pill-box, and of 

 one or two rims which adapt themselves to these 

 valves, without, however, forming an intrinsic part of 

 them, in a similar manner to the sides of the pill-box. 

 These rims have received the name of connectives'" 

 (fig. 71, GG). The valves and connectives form what 

 is called the frushde. 



The living diatom must be considered as an or- 

 ganism in which the interior is in conformity with 

 many other unicellular organisms, but in which the 

 protecting envelope is siliceous and formed of several 



* This is the connecting zone, cingulum, or Gurtelband of 

 various observers : we shall here use M. Deby's term. 



