FIG. 35.--THE NUMBER OF THE CHROMOSOMES: (_a_) Cell of the asexual generation of the cryptogam _Pellia epiphylla_: the nucleus is about to divide, a polar ray-formation is present at each end of the spindle-shaped nucleus, the chromosomes have divided into two horizontal groups each of sixteen pieces: _sixteen_ is the number of the chromosomes of the ordinary tissue cells of _Pellia_. (_b_) Cell of the sexual generation of the same plant (_Pellia_) in the same phase of division, but with the _reduced_ number of chromosomes--namely, _eight_ in each half of the dividing nucleus. The completed cells of the sexual generation have only _eight_ chromosomes. (_c_) Somatic or tissue cell of Salamander showing _twenty-four_ =V=-shaped chromosomes, each of which is becoming longitudinally split as a preliminary to division. (_d_) Sperm-mother-cell from testis of Salamander, showing the _reduced_ number of chromosomes of the sexual cells--namely, _twelve_; each is split longitudinally. (From original drawings by Prof. Farmer and Mr. Moore.)] A few words must be said here as to the progress of our knowledge of cell-substance, and what used to be called the protoplasm question. We do not now regard protoplasm as a chemical expression, but, in accordance with von Mohl’s original use of the word, as a structure which holds in its meshes many and very varied chemical bodies of great complexity. Within these twenty-five years the ‘centrosome’ of the cell-protoplasm has been discovered (see fig. 34), and a great deal has been learnt as to the structure of the nucleus and its remarkable stain-taking bands, the chromosomes. We now know that these bands are of definite fixed number, varying in different species of plants and animals, and that they are halved in number in the reproductive elements--the spermatozoid and the ovum--so that on union of these two to form the fertilized ovum (the parent cell of all the tissues), the proper specific number is attained (see figs. 35 and 36). It has been pretty clearly made out by cutting up large living cells--unicellular animals--that the body of the cell alone, without the nucleus, can do very little but move and maintain for a time its chemical status. But it is the nucleus which directs and determines all definite growth, movement, secretion, and reproduction. The simple protoplasm, deprived of its nucleus, cannot form a new nucleus--in fact, can do very little but exhibit irritability. I am inclined to agree with those who hold that there is not sufficient evidence that any organism exists at the present time which has not both protoplasm and nucleus--in fact, that the simplest form of life at present existing is a highly complicated structure--a nucleated cell. That does not imply that simpler forms of living matter have not preceded those which we know. We must assume that something more simple and homogeneous than the cell,