ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 275 



contains the central portion of the chromatophore. Two pieces, 

 together about equal in length to each of the middle pieces, lie 

 separately on the lower shell, while the ends which enter the apices 

 of the cell turn to the girdle-hands of the cell-wall, where also the 

 pieces which start from the upper and under shells unite. The 

 function of the chromatophore is therefore distributed nearly equally 

 to both sides of the protoplasmic body of the cell. The median line 

 of the chromatophore coincides, as in Navicula, with that of the 

 girdle-bands ; but the portions which project upon the adjoining 

 cells are not arranged symmetrically in relation to the plane of 

 division. The middle piece of each chromatophore, which lies on 

 the upper shell, on both sides of the raphe, incloses, in the typical 

 arrangement, the central cell-nucleus with a semicircular opening 

 inwards. 



Pleurosigma balticum also contains two chromatophores whose 

 median line coincides with that of the girdle-bands, and which project 

 on the shells on both sides ; but they are not band-shaped and folded, 

 as in Pleurosigma angulatum, but plates of a somewhat complicated 

 structure. Pleurosigma Hippocampus has chromatophores of a similar 

 form, but narrower. 



Nitzschia Sigma has only a single chromatophore, which is, how- 

 ever, completely divided by the cell-nucleus; not so completely in 

 other species belonging to the same group of the genus. It is plate- 

 shaped, and is applied to that girdle-band which is opposite the two 

 points of the keel. On its median line, in each of the two halves, lie 

 five or more round or oval pyrenoids, bodies which are not unfre- 

 quently present in diatoms. They appear here not to be of such 

 simple structure as in other cases. They are coloured more or less 

 dark by nigrosin, and are surrounded by a light border, which, with 

 very high powers, exhibits a differentiation into small bright dots, the 

 structure being therefore similar to that in the Chlorophycese. 



Division of Synedra Ulna.* — G. Schaarschmidt has found this 

 diatom in an active state of division ; he fixed the specimens with 

 picric acid or absolute alcohol, coloured them with haematoxylin or 

 eosin, compared them with living specimens, and gives the following 

 as the chief results obtained. When division commences the breadth 

 increases by the girdle-bands becoming separated to a greater distance ; 

 but the lamellae of endochrome retain their position, and their margins 

 scarcely project in this condition over the girdle-bands ; while, in 

 individuals that are not dividing or only preparing to divide, they 

 reach the end of the cells ; but in those which have just divided or are 

 dividing repeatedly, they are about 1/6 of the length of the cell 

 shorter than the shell. The strongly refractive colourless nucleus is 

 in the central mass of protoplasm which often lies only on one shell ; 

 no nucleoli can be detected in it. In cells which are about to divide 

 it moves into the middle of the cell ; its enveloping protoplasm, 

 through which small mucilaginous particles are scattered, then 

 lengthens towards the ends of the cell, where the protoplasm then 



* Magy. Nov. Lapok, vii. (1883) pp. 49-58 (1 pi.). See Bot. Centralbl., xvi. 

 (1883) pp. 198-9. 



