invention of the microscope. As early as 1651 William Harvey asserts that all living organisms come from eggs. By 1677 the human spermatozoon is described. In 1665 Robert Hooke, examining a section of cork under the microscope, sees that it is made of "little boxes or cells distinguished from one another." It remained for Wolff (1759) to catch the idea that growth resulted from the multiplication of these small units or cells. In 1831 Robert Brown discovered the nucleus in plant cells. In 1835 Felix Dujardin discovered protoplasm which is today called "the material basis of life" and which always exists in cell form. In 1838 M. Schleiden and Theodore Schwann showed that plant and animal cells were similar in structure. In 1861 Max Schultze said that is a globule of protoplasm surrounding a nucleus" and in the same year Gegenbauer showed that the eggs of all vertebrates were in reality single cells. This was found to be true of the spermatozoa in 1865.

a cell

As a result of these discoveries man learned that all forms of life start as single cells and that growth results from their multiplication through a process of division. But many lowly forms separate when they divide or form mere masses or aggregates with the different cells seemingly identical in structure and function. In the higher forms there is a specialization of parts. Thus we find that in some plants roots, stems, leaves and, in many cases a piece of the root separated from the plant, will grow and produce a complete plant again. A bit of begonia leaf under favorable conditions will do the same while a post of willow stuck in the ground often becomes a tree. Man takes advantage of this fact in the growing of many of his choicest plants for he thus increases them more rapidly than he could from seed, and moreover he

knows that the new ones will be exactly like the older. In many cases the parts of the plant seem to have lost the power of reproducing the whole under ordinary conditions at least.

When we reach the animal world a change in this regard is evident. It may be possible to cut some worms in two and thus produce two worms. But the worm is really a colony of ringed animals little dependent on each other. A somewhat higher form of animal like the lobster will grow a new claw if one is destroyed, but a marked difference from the behavior of the begonia is already to be noted. No part of the lobster's body will produce a new lobster. In other words, the higher we ascend the ladder of life the greater the specialization of function. The cells of the human skin will produce skin, but nothing else; nerve cells produce nerve cells, nothing else. The human body is then composed of millions of cells which have the power of reproducing themselves, but which for some reason are unable to reproduce the entire organism. For this function, however, nature has set apart the germ cells.

In 1883 Van Beneden, taking advantage of aniline dyes which had recently come into use, discovered that the nucleus of the cell had a definite and rather complicated structure. Thanks to the steady improvement of microscopes and of methods of photography a good deal has been learned. In each cell there is a nucleus containing a number of thread-like substances or filaments, which can be stained and thus observed, to which the name "chromosomes" (color-bodies) has been given. These chromosomes are so tangled, are so delicate and perishable, that it has proved very difficult to make accurate count of them. This difficulty has been overcome and we now

know that the number of chromosomes varies with the species, but that it is constant for any given species barring an exception which will be mentioned later. In certain worms the number is as low as four, in some bugs, 10 or 12; in the rat, 16; in the frog or mouse, 24; in man, 48.1 The nucleus is surrounded by a drop of liquid and the whole is incased in a delicate membrane or cell wall. Attention has already been called to the rapidity of division of many of the one-celled organisms. Under favorable conditions most cells divide at frequent intervals, and the process is most fascinating. The chromosomes contract and then split lengthwise, a half of each moving to opposite sides of the cell and gathering about a nuclear point. Meantime, the cell wall begins to constrict until finally the separation is complete and there are two cells instead of one, enough food having been absorbed in the process so that the daughter cells are approximately as large as the mother cell. It is to be noted that the division of the chromosomes is such that each daughter cell has the same equipment. There is reason to believe that this does not apply to the rest of the contents of the cell, the cytoplasm, where the division may be unequal and hence a basis may be laid for the later differentiation. What causes this division no one knows and concerning this mystery Bateson has well said: "The greatest advance I can conceive in biology would be the discovery of the nature of the instability which leads to the continual division of the cell. When

I look at a dividing cell, I feel as an astronomer might do if he beheld the formation of a double star; that an original act of creation is taking place before me. Enigmatical as the phenomenon seems, I am not without 1 GUYER, M. F. Being Well-Born, pp. 34-41.

hope that, if it were studied for its own sake, dissociated from the complications which obscure it when regarded as a mere incident in development, some hint as to the nature of division could be found." 2

sexes.

This description of cell-division, or mitosis, as it is called, holds true for the body cells of the higher animals as well as for the one-celled forms. The mitosis of germcells is more complex, and differs somewhat in the two The common opinion that they come into existence when the animal reaches physical maturity is mistaken. As a matter of fact, they appear at a very early stage in the life of the individual and in some of the lower their history can be traced from the very first divisions of the fertilized egg. Even in mammals they may sometimes be found in the walls of the digestive tract in early embryonic development when they move into the appointed organs as they appear. The significance of this is that they are not produced by any of the specialized tissues of the body but are derived only from the germ plasm.

From the time these germ cells first appear they continually divide in the manner above described and thus increase in number. The sperm cell of the male divides much more rapidly than the ovum of the female and becomes more numerous and much smaller. By the time the chicken is hatched or the child born it is believed this division period is stopped in the female and hence that the body contains all the eggs it will ever have. The germ cells then enter into a period of growth which in the female lasts till the end of the reproductive period. During this time of growth "the chromosomes form a closely wound coil of long chromatin threads, and when these threads uncoil later it is seen that the chromosomes have 2 BATESON. W. Problems in Genetics, p. 41.

united in pairs; this process is known as synapsis, or the conjugation of the chromosomes, and there is evidence that one member of each synaptic pair is derived from the father, and the other from the mother." 3 The line of junction is plainly seen. Finally there come what are known as the "maturation" or "reduction" divisions two in number. In one of these divisions (probably the first) the chromosomes which had previously united separate and the parts, instead of splitting lengthwise as in ordinary mitosis, go into one or the other of the daughter cells, each of which will therefore have but half of the former number of chromosomes. Then follows a division of the common sort. From each of the original cells we now have four cells each containing half of the standard number of chromosomes. The germ cells have now reached the end of their cycle and unless united with one from the other sex are either absorbed or thrown out of the body. In this maturation process a difference between the two sexes becomes apparent. In the male all the four cells thus formed are alike and may function. In the female, however, in the first of the two divisions one cell is of normal size, the other is small and is known as a polar body. At the second division this divides as does the normal ovum, the latter producing again one polar body and one normal ovum. As a result of the two divisions in the female we have one perfect egg and three polar bodies. Nature is apparently producing a cell with the requisite number of chromosomes and at the same time conserving the valuable cytoplasm for the one perfect

ovum.

The ovum is spherical in form containing much foodstuff or yolk. The sperm "is among the smallest of cells

8 CONKLIN, E. G. Heredity and Environment, p. 131.