174 THE PHENOMENON OF 



some application only in systematic Algology, as generic 

 characters,* be taken more and more into account. 



Among the essential parts of the organisation of the 

 cell, is also the cell-nucleus or cytoblast, as it were a cell in 

 the cell, mostly containing in its interior again a nucleolus 

 (nuclear corpuscle, kernkorperchen}. Nageli's extensive 

 researches! nave demonstrated its occurrence in all 

 divisions of the Vegetable kingdom ; only in particular 

 families of the Algae, as, for example, in the PalmellaceEe, j 

 Chroococcaceae, Oscillatorineae, and Nostochineae, as also 

 in the large-celled Cladophora, and the unicellular 

 Algae, with unlimited growth of the cell (Vaucheria, 

 Codium, Caulerpa), no trace of a nucleus has yet been 

 discovered. Schleiden was the first to direct attention 

 to the importance of the nucleus and its relation to the 

 original formation of the cell. It undoubtedly originates 

 before the cell is outwardly defined ; it is thus, in the 

 truest sense, a central organ, around which the vital 

 circle of the new cell is drawn. || Originally it is always 

 situated more or less in the centre of the protoplasm ;^[ 

 by the subsequent development of a cavity in the in- 

 terior of the cell, it mostly becomes parietal, i. e., 

 imbedded in the layer of mucilage forming the internal 



* See especially the family of the Zygnemaceae, as also the Desmidiacese, 

 in which latter Nageli has founded the more accurate diagnosis of the 

 genera in part upon the condition of organisation of the cell-contents, 

 ('Einzell. Algen.,' p. 100, t. vi-viii). 



f ' Zeitschrift. fur Wiss. Botanik,' 1844, pp. 34-68. Transl. in Ray 

 Society's publications, 1845, pp. 215-246. 



{ In the genus Chlamidococcus, at least allied to the Palmellaceae, (see 

 above, pp. 138 and 158), I have found in the centre of the cell a vesicle filled 

 with fluid, doubtless corresponding to the nucleus, surrounded and con- 

 sequently hidden by the oily (?) red colouring matter of this plant. In 

 most of the true Palmellacese there is a chlorophyll-vesicle in the centre of 

 the cell. 



'Beitrage zur Phytogenesis/ 'Muller's Archiv,' 1838. Transl. in 

 Scientific Memoirs, vol. i, p. 281. 



|| Vide, in reference to this especially, Nageli, ' Ueber Entwicklung des 

 Pollens,' (1842), and Hofmeister, 'Enstehung des Embryo der Phanero- 

 gamen,' (1S49). 



H H. von Mold, ' Bot. Zeit.,' 1846, p. 76. ('Ann. Nat. Hist.,' xviii, 3. 

 A H.) 



