DEVELOPMENT OF CELLS. 13 



detected in the rind of the orange and lemon, and in the leaves of Myr- 

 taceae and Hypericacese. When small portions of the fresh leaf of Sohinus 

 Molle are thrown on water, the resinous matter, by its rapid escape, 

 causes them to move by jerks, and the surface of the fluid is covered 

 with the exudation. In the bark of the Fir tribe there are cavities 

 with thick walls containing turpentine. In the fruit of Umbelliferse, 

 canals occur called vittce (yitta, a head-band, from surrounding the fruit), 

 containing oil. 



Air-Oblls, or cavities containing air, consist either of circumscribed 

 spaces surrounded by cells (fig. 43), or of lacunae formed 

 by the rupture or disappearance of the septa between a 

 number of contiguous cells, as in grasses, Equisetum, 

 Umbelliferous plants, and pith of Walnut. They are 

 often large in aquatic plants, and serve the purpose of 

 floating them, as in Pontederia, Trapa, Aldrovanda, and 

 sea- weeds. The air-cells of Limnocharis Plumieri are Kg. is.' 

 beautiful objects. 



3. — Development and Functions of Gells. 



The subject of Cell-development, or Gytogenesis (xiroj, a cell, and 

 yhiaig, origin), has given rise to great diversity of opinion among 

 physiologists. We have already noticed that in the interior of grow- 

 ing cells there is a mucilaginous matter called protoplasm, which con- 

 tains granules. The first lining of the cell-wall arising from the 

 protoplasm, is the primordial utricle. It forms a sort of fil'm around 

 the protoplasm, and in certain cases it may supply the place of the 

 proper cell-membrane. In the protoplasm cavities are sometimes seen 

 filled with a watery sap, and called vacuoles. In the interior of the 

 young cell may be seen a nucleus or cytoblast (ziiros, a cell, and 

 ^Ka.gTog, a germ), (fig. 33), composed of protoplasmic matter, and con- 

 taining granules, called nucleoli. 



The nucleus often becomes attached to one side of the utricle. It 

 is sometimes, however, retained in the centre of the cell by means of 

 cords of protoplasm, which ultimately form the boundaries of vacuoles, 

 or spaces containing fluid. Most physiologists think that the cyto- 

 blast is not specially concerned in cytogenesis, but only takes part in 

 the various chemical and other changes which occur in the contents of 

 the cell during its growth and nutrition. 



It is supposed by some that cells may be formed by the simple 

 aggregation of granular matter, which becomes enveloped in a mem- 

 brane, and thus forms a cell with granular contents. Dr. Bennett 

 advocates a molecular view of cell formation. He traces cytogenesis 

 to the presence of histogenetic (ifTo;, veil, web, or tissue, and yheeis, 

 origin) molecules, which unite together to form the cell- wall. New 



Fig. 43. Air-ceUs in Eanunculus aquatilis. ' 



