CELL. 



[ 120 ] 



CELL. 



writers attribute great importance in the de- 

 velopment of cells. Its nature is not well 

 defined, but in the best observed cases it 

 consists of a small globular or lenticular 

 mass, apparently composed of protoplasm in 

 a condensed and granular (solid) condition. 

 It mostly exhibits one or more bright gra- 

 nules or points in its substance, which are 

 called nucleoli. It is well known that Schlei- 

 den considers this body as of the first import- 

 ance in cell- development ; but as we are by 

 no means satisfied as to the character of its 

 agency, its peculiarities and its relations to 

 the cell are spoken of separately under the 

 head of Nucleus (PI. 38. figs. 8, 9 ?i). 



All development of new cells depends 

 upon the division of the primordial utricle 

 of existing cells into two or more portions, 

 which, becoming independent centres of life, 

 produce new cellulose membranes, and be- 

 come new cells. The phenomena in which 

 this law is manifested are far more varied 

 than would be imagined from this simple 

 statement. The numerous subordinate mo- 

 difications may, however, be arranged 

 under three principal heads : — 1 . Cell-divi- 

 sion, sometimes called merismatic cell- 

 formation ; 2. Cell-division with libera- 

 tion of the new cells; 3. Free cell-forma- 

 tion. 



1 . Cell-division is the process which occurs 

 in all reproduction of cells connected with 

 vegetative growth or increase of the mass of 

 existing structures. This is the manner in 

 which the cells are multiplied in the growth 

 of the thallus of the inferior plants, and in 

 the growth of the stems, leaves, roots, and 

 other organs of the higher plants. It occurs 

 also in the formation of the hasidiospores or 

 stylospores of Fungi, the spermatia of these 

 and Lichens, of gonidia in the Lichens and 

 conidia in the Fungi. The essential fact 

 observed in all the cases is, the division of 

 the primordial utricle of the parent-cell into 

 two distinct primordial utricles, each of which 

 secretes a layer of cellulose over its whole 

 surface ; and thus when the two are in appo- 

 sition, a partition is formed dividing the pa- 

 rent-cell into two parts. The form of the 

 daughter-cells depends of course on that of 

 the parent-cell at the time of division. In 

 the case of cellulose tissues, such as those in 

 the imnctum vegetationis of the buds of the 

 higher plants, in cambium , &c., the division 

 is ordinarily into two halves, which respect- 

 ively grow until equal in size to the ])arent ; 

 and either both or only one of these divides 

 again in the same way, and so on, until the 



whole structure is completed. It is evident 

 that the external forms of all cellular struc- 

 tures must depend greatly upon the laws of 

 division of the cells of plants ; for example, 

 supposing we start from a single square cell, 

 when this divides into two halves, and these 

 grow to equal the parent-cell, we have an 

 oblong figure; if the half-cells divide again 

 in the same direction, we shall in time get a 

 long filament ; and if both new cells divide 

 again each time, the filament will grow much 

 longer in a given time than if only the end- 

 cell continually divided, leaving one new cell 

 behind it at each division. If the pair of 

 cells produced by the first halving divide at 

 right angles to the first division, a square 

 group of four cells results ; and if this law 

 continues to act, a flat plate of cellular tissue 

 will result. Further, if the cells also divide 

 by horizontal partitions (in the third direc- 

 tion of space), the mass of cells wdll gradually 

 acquire thickness or height as well as length 

 and breadth. Lastly, if the cells of particular 

 regions cease to divide sooner than others, 

 irregular or complex but definite structures 

 will be produced ; as those parts where the 

 cell-division goes on, will emerge from the 

 general mass, in the Cellular plants as lobes, 

 and in the higher plants as conical bodies 

 which are gradually developed under similar 

 laws into the organs. The diversities of 

 internal organization depend also to some 

 extent on the same laws, but less on these 

 than on the laws regulating the forms which 

 the cells acquire when full grown. 



Cell- division may be observed most easily 

 in the lower Cellular plants, or in the simpler 

 structures (such as hairs) of the higher plants 

 (PI. 38. figs. 8, 9). The Conferva afford 

 exceedingly favourable opportunities, as do 

 also the filamentous or thalloid structm'es 

 of germinating Mosses, Ferns, microscopic 

 Fungi, &c. The behaviour of the parent- 

 cell before division exhibits some diversities. 

 If a simple filament is increasing by cell- 

 division, the cylindrical parent-cells merely 

 elongate a little before dividing transversely. 

 If the filament is to branch, the wall of the 

 parent-cell bulges out gradually at the point 

 where the branch is to appear ; the bulging 

 soon becomes a pouch, and this pouch is 

 soon shut ofi^ by the formation of a partition 

 at its base. Bead-like rows of cells likewise 

 divide by budding in this way, as may be 

 observed, for instance, in the Yeast-plant, 

 the new cell first appears as a little 'bubble' 

 on the side of the parent, with its cavity 

 continuous, and after it has acquired a cer- 



