CELL

the unit ma of living structure, so called because in vegetables the ti ue units are enclosed in definite walls. In animal cells the outline is often not so clearly circumscribed. The simplest a organisms may be composed of a sil cell (Protozoa and Protophyta), and male and female sex elements (game are single cells. Cells may vary in; from something smaller than the mic scope can show, to the dimensions of ostrich’s egg, which is but a single ¢ The reason for the large size of the fer reproductive unit in birds and rept is found in the large store of nutri material which it contains for the us¢ the embryo during the incubat period. Simple cells are spherical shape, but cells may be faceted their neighbors, and numerous cells specially modified in shape in accord a: with the requirements of their spe function. The e ential element of a cell is protoplasm or cytoplasm, which | usually a foam-like structure, apparen due to the presence of a support framework which contains a sof jelly-like substance in its meshes. the cytoplasm there may be granules pigment or stored food. There is gen ally a nucleus, which is enclosed in membrane, and is composed of a netw« which readily takes up dye-stuffs, he1 called chromatin, and a nuclear flu In the nucleus there may be one or m« bodies which do not react to stains the same way as the remainder. Thi are nucleoli. The cell membrane vegetables is composed of cellulose, substance allied to starch but ve resistant to the human digestive juic In animals there may be a membra of chitin, but in most animal cells t membrane is very fine, and contai fatty substances. In growing cells time arrives when the bulk of the c is so great in proportion to its surfz that nutrition becomes increasingly df cult. In those circumstances the e¢ must either die or divide. The divis i may take place in one of two ways. amitotic division’ a line of cleavage a pears in the nucleus, then the c¢ simply divides into two. This sim] proce is unusual, and the comm proce of division is that known mitosis (thread-work). A body (cent i some) which in resting cells lies near t nucleus divides, and the two née bodies take up positions at oppos i poles of the nucleus. The nucle chromatin forms a long skein and th breaks up into a number of loo (chromosomes). The number of these constant for each animal species, al in sexual cells the number becomes duced to one half, so that the cell mu be fertilized by chromatin from a ¢ of the opposite sex before repro duct it can take place. The chromatin split and the halves appear to be d apart by a spindle of threads w ‘CELL REACTIONS forms between the two centrosomes. After they have separated some distance, the split loops join to form a skein, the skein becomes a network, and ‘a new nuclear membrane is formed round each. The protoplasm then divides and two new ceils now replace the original larger cell. The proce of division occupies about half an hour.