FIG. 10.-Diagrams illustrating the fertilization of the egg.

A, egg surrounded by spermatozoa; on the right, one has just penetrated the egg membranes and is entering the egg cytoplasm; egg nucleus in the centre. B, egg nucleus with chromatin reticulum on left; on right, the sperm nucleus preceded by its centrosome and attraction sphere. C, egg nucleus on the left, sperm nucleus on the right of the centre of the egg; stage immediately preceding the division of the centrosome. D, the centrosome has divided, the two attraction spheres separate to form the first cleavage spindle; the chromosomes of the egg and sperm nuclei clearly visible and indistinguishable (in the figure the egg chromosomes are black, the sperm chromosomes shaded). E, the first cleavage spindle, with splitting of chromosomes. F, completion of first cleavage; two-celled stage, each nucleus contains four chromosomes-two from the egg and two from the sperm. (After Boveri.)

generates. A few moments after the sperm has entered a system of radiations appears around the middle piece, which develops into an aster surrounding the centrosome of the sperm (Fig. 10, B). The sperm nucleus swells up and rapidly increases in size, its chromatin changing from the compact condition in which it is arranged. in the sperm head to a reticulate condition (Fig. 10, C). The chromatin reticulum of the egg nucleus becomes also more clearly visible. Sperm aster and sperm nucleus now move in toward the egg nucleus, the aster usually preceding. As the nuclei approach the sperm nucleus increases still more in size until it becomes indistinguishable from the egg nucleus (Fig. 10, C). The chromatin network of each now breaks up into a number of chromosomes, one half of the number found in the somatic cells, and the nuclei come into contact, fusing together in some cases. In the sea urchin, Echinus, the number of chromosomes is eighteen, nine would therefore be found in the germ nuclei; for the sake of clearness and simplicity but two are represented in the diagram, those of the sperm nucleus being slightly shaded while those of the egg nucleus are black. The centrosome divides together with its aster Cleavage. (Fig. 10, D), the two daughter centrosomes move apart to opposite poles of the egg, and the typical amphaster of cell division is formed (Fig. 10, E), the nuclear membranes disappearing and the chromosomes being drawn together into the equatorial plate where each splits longitudinally. The halves are drawn by the mantle fibrils toward the opposite poles and the egg divides transversely into two cells (Fig. 10, F). This process of division is repeated continuously in each of the resulting generations of cells, and from the mass of cells thus formed develops the new organism. Each cell in the two-celled stage has received half of its

chromosomes from the egg nucleus and half from the sperm, thus containing equal amounts from each parent. The centrosome, which, as we have seen, is to be regarded as the dynamic centre of the cell division, comes from the spermatozoon alone; the egg, on the other hand, furnishes the yolk and practically all of the cytoplasm.

The reduction of the chromosomes.

After this preliminary outline of the facts of fertilization we are in a better position to understand the details of a process which occurs in the development of both egg and sperm cells, namely, the reduction of the chromosomes. The necessity for such a reduction is evident from a moment's reflection. We have seen that the number of chromosomes in the nucleus is a constant and typical one for each animal and plant species so far as known. As fertilization consists in the union of two cells into one, from which the young organism develops, it is plain that, were there no reduction, the number of chromosomes would be doubled in each succeeding generation. However simple this necessity for reduction may appear, the minutia of the processes through which it is brought about, and the theoretical significance of these facts, form the most involved problem of biology to-day. In a few forms, especially among the lower Crustacea, the facts of the reduction are clear and relatively simple; in other forms they thus far stand in direct contradiction, and, for the present, a comprehensive explanation applicable to all forms must be left to further investigation.

The significance of reduction turns upon the conception of a definite organization and individuality in the chromosomes and the assumption that they represent the physical basis of heredity-i. e., that they influence and determine into what the fertilized egg shall develop.

Theories as to structure and significance of the chromosomes.

Fifteen years ago Wilhelm Roux showed with convincing clearness that the complicated facts of nuclear division, the careful longitudinal halving of the chromatin thread and its equal distribution between the two daughter cells, can only be explained on the basis that the chromosomes possess different structure in different parts of their extent, and that these structures, representing tendencies in development, are distributed in definite ways to the daughter cells. Were this not the case a simple direct mass division of nucleus and cytoplasm instead of the complicated process of Karyokinesis with its consequent much greater expenditure of energy would serve all purposes.

The theories of Weismann are all based upon an extension of Roux's ideas. Briefly, he assumes a definite architecture of the chromatin filament, each nuclear rod or idant being composed of a number of "ancestral germ plasms or ids, the vital units of the third order. Each id in the germ plasm is built up of thousands or hundreds of thousands of determinants, the vital units of the second order, which in turn are composed of the actual bearers of vitality or biophors, the ultimate vital units. The biophors are of various kinds, and each kind corresponds to a different part of a cell; they are therefore the bearers of the characters or qualities of cells. Various but perfectly definite numbers and combinations of these form the determinants, each of which is the primary constituent of a particular cell, or of a small or even large group of cells— e. g., blood corpuscles."

"These determinants control the cell by breaking up into biophors, which migrate into the cell body through the pores of the nuclear membrane, multiply there, arrange themselves according to the forces within them,

and determine the histological structure of the cell. But they only do so after a certain definitely prescribed period of development, during which they reach the cell which they have to control." (Weismann, The Germ Plasm, pp. 75, 76.)

The ultimate vital units.

Cell division, then, is a process of qualitative analysis through which the determinants, in virtue of possessing a certain definite location in the architecture of the chromosome, are distributed ultimately to that portion of the body which they are to direct. Weismann has developed this theory to a most elaborate degree of complication in explaining the various phenomena of heredity -to a degree, it need hardly be remarked, which passes far beyond our present knowledge of the facts of cytology. Just as the chemist and physicist, however, are forced to the assumption of the existence of ultimate atoms and molecules to explain the phenomena of nonliving matter, so the biologist must in some form or other assume the reality of ultimate self-propagating vital units, be they called "biophors" with Weismann, "micella" with Nægeli, "pangenes" with De Vries, "plasomes" with Wiesner, or "physiological units" with Herbert Spencer

In the light of this probable individuality and morphological organization of the chromosomes the method of their reduction in number, preparatory Significance of to the fusion of the germ cells, becomes reduction. of the greatest significance; to those who may deny this individuality and definite architecture, the phenomena can have no great importance save as concerns a general mass reduction in the amount of the chromatin present in the germ nuclei. It may be assumed as true, in the majority of cases now accurately known, that the reduction takes place somewhere.