12 FUNCTIONS OF CELLS. 



In such cells, a body similar to a nucleus may be afterwards formed, 

 and may assume the function of the cytoblast of Schleiden, as far as the 

 subsequent endogenous development of new cells is concerned. Some 

 physiologists maintain that the cytoblast is never concerned in cyto- 

 genesis, but only takes part in the various chemical and other changes 

 which occur in the contents of the cell during its growth and nutrition. 

 Mohl and Henfrey state that new cells are produced by the division of 

 the primordial utricle (T 14), which gradually folds inwards about 

 the middle, forming an annular constriction, and ultimately a complete 

 separation of the utricle into two parts. Each of these afterwards be- 

 comes covered by a permanent cell-wall. Henfrey has supported this 

 view by observations made on the hairs of Tradescantia and of Achimenes 

 grandiflora, in which he has traced the gradual formation of a septum. 



24. Naegeli maintains that new cells are produced by the division 

 of the primordial utricle, or mucilaginous sac, as he calls it, and its 

 contents into two or four portions, each of which encloses a free nu- 

 cleus. From each of these portions, a cell, with its outer layer of 

 cellulose, originates, while the parent cell becomes dissolved and disap- 

 pears. The outer layer of the new cells is formed, according to him, 

 round, and by the separate portion of the divided utricle. The mode 

 of division he does not explain. This view does not appear to differ 

 much from that adopted by Unger, who traces in Alge the develop- 

 ment of new cells, by ajissiparous (Jissus, split, andjoano, I produce), 

 or merismatic (^gj/o-^of, division) separation of the old ones into four 

 divisions, in the same way as occurs in pollen grains. In some of the 

 most simple plants, multiplication takes place by a sort of sprouting of 

 new cells from old ones, like buds from a stalk. 



25. The various theories of cell-development may be therefore re- 

 duced to the following: 1. The Endogenous formation within a parent 

 cell; 2. the Exogenous (<!%&>, without), without, or on the outside of 

 the cell; 3. Merismatic, or by division of cells; and, 4. Isolated, or 

 the independent formation of cells in a protoplasm.* 



26. The formation of cells from nuclei, and their fissiparous division, 

 are by some attributed to different electrical currents excited by the 

 chemical actions going on in the cell. Cells are produced with great 

 rapidity, especially in the case of fungi. Lindley calculates that the 

 cells of Bovista gigantea have been produced at the rate of more than 

 sixty-six millions in a minute, and Ward has noticed a similar occur- 

 rence in Phallus impudicus. In warm climates, at the commencement 

 of the wet season, the production of cells in the higher classes of plants 

 proceeds with astonishing rapidity. 



* For a full view of the subject of the development and growth of cells, the following works 

 maybe consulted: Schleiden on Phylogenesis, and Mohl on the Structure of the Vegetable 

 Cefi, translated in Taylor's Scientific Memoirs, Vols. II. and IV. ; Naegeli on Vegetable Cells, 

 Ray Society's Reports, 1846; Sharpey, Anatomy; M. Barry, Physiology of Cells, &c., in Philo- 

 sophical Transactions, 1840 ; and on Nucleus of Cells, in Jameson's New Philosophical Journal for 

 September, 1847; Carpenter's Physiology. 



