somes the nuclear membrane has disappeared. The chromosomes soon reach their maximum staining capacity, and appear usually as a collection of rods or bands of deeply staining substance lying free in the cytoplasm.

"While this is taking place in the nucleus, another series of changes has been gone through by the centrosome and the cytoplasm immediately surrounding it. We have already indicated the presence of the centrosome as a minute spherical structure lying at one side of the nucleus. This body assumes an ellipsoidal form, constricts transversely into a dumbbell-shaped figure, and divides into two daughter centrosomes, which at first lie side by side but soon move apart. Around each of them is gradually developed a stellate figure composed of a countless number of delicate fibrils radiating out in all directions from the centrosome as a center. This 'aster' or 'astrosphere' is at first small in extent, but grows in size progressively as the two centers move apart, apparently being derived from a rearrangement and modification of the threadlike network of the cytoplasm under the influence of the centrosomes.

Between these two asters, which lie a short distance apart and at one side of the nucleus, a spindle-shaped system of delicate fibrils may often be made out, stretching from the center of one aster to that of the other. This fusiform figure is termed the 'central spindle.' The two asters, together with the central spindle, form what is termed the 'amphiaster' or the 'achromatic' portion of the karyokinetic figure. The two series of changes in nucleus and cytoplasm, which have thus far gone on apparently independently of each other, now become closely interrelated in that, as the nuclear membrane disappears, a system of fibrils grows out from each astrosphere, which attach themselves to the individual chromosomes. These 'mantle fibers' insert themselves along the chromosomes in such a way that each segment receives a series of fibrils from each pole of the amphiaster, the two series being attached along opposite sides of the chromoUnder the influence of these fibers, probably by direct pulling, the chromosomes, now bent into V- or U-shaped loops, tend to place themselves in a circle around the center of the spindle, transversely to its long axis, and form the 'equatorial plate.'

somes.

"The changes thus far constitute the 'prophases' of the division. The 'metaphases' following these consist primarily in the longitudinal splitting of each chromosome and the moving apart of the halves. This longitudinal splitting of the chromosome into two equivalent parts forms the most important act of the whole cell division, and is

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FIG. 145.-Cell fission in the salamander: A, Resting nucleus stage, centrosome partly developed; B, skein stage, chromatin visible as a convoluted band, the centrosomes having separated; C, the nuclear membrane having disappeared, and a few of the chromosomes lying free in the cytoplasm; D, central spindle complete, the chromosomes on splitting being drawn to the spindle; E, metaphase; F, anaphase, the chromosomes being drawn to the poles. (After Drüner.)

of the greatest theoretical significance. By it the chromatin substance of the original nucleus is equally distributed between the two daughter nuclei, so that each receives a half of each original chromosome. The elaborate mechanism and consequent expenditure of energy involved in this careful longitudinal division of each chromosome, rather than a simple mass division, such as might be brought about by far less complicated means, indicates clearly that the distribution of the definite organization of the chromatin to the daughter cells is of primary importance, a conclusion which is further strengthened by much evidence too extended to be entered upon here.

"In the 'anaphases' and 'telophases,' which include the closing stages of division, the daughter chromosomes migrate along the fibers of the central spindle toward its poles, perhaps through the direct contraction of the mantle fibers under the influence of the centrosome, though this and many other points regarding the forces at work must be left for future investigation to decide. Arrived at the poles, V-shaped chromosomes become grouped in a star-shaped figure, the 'aster,' their outer ends become again joined together in the form of a tangled skein, the individual chromatin granules separate somewhat along the threads of the linin network, their deeply staining quality is decreased, and a new nuclear membrane develops around each group of chromosomes. Simultaneously with this the cytoplasm constricts across the middle of a somewhat elongated cell, resulting in complete division in the equatorial plane of the spindle, and two separate daughter cells result. Each of these is made up of cytoplasm containing a centrosome and a nucleus, similar in all respects to the parent cell from which it has arisen.

"A simple tabulation of the changes just described is as follows:

"It is readily seen that the culmination of the process lies in the splitting of the chromosomes and the separation of their component halves to form the two new daughter nuclei."

The obvious distinction in capacity of development shown by the various cells which compose an animal's body leads us to ask whether we can distinguish differences associated with these different potentialities in the fine structure of the cells themselves, and especially in their behavior during the process of multiplication. For the fate or future character of any cell must largely depend on the nature of its origin, the character of its inheritance. Now in some cases this difference in potentiality of the undifferentiated dividing cells is plainly shown by differences in the details of the process of division. A conspicuous and important instance of this, and one bearing directly on our subject of the relation of the structure and character of the germ plasm to the fully developed organism, is the distinction, usually easy to make, between the body or so-called somatic cells and the reproductive or germ cells of any organism.

"Every multicellular organism arises by a process of division from a single cell, the fertilized germ or egg cell, which in turn has been cut off from the cells of a preëxisting individual. Out of the group of cells which result from the continued division of the germ cell and its descendants are differentiated the various tissues and organs of the body through which the vital functions are carried on. Those tissues and organs which perform functions pertaining directly to the existence of the individual have been termed 'somatic,' and their constituent cells the somatic' or body cells, in contradistinction to the reproductive tissues or cells whose function concerns the continuance of the species. In some forms these groups of cells, the somatic and the reproductive, become isolated from each other quite early in development; in one case, indeed, the differentiation of reproductive cells from the somatic ones has been traced by Boveri back to the first division of the egg. This case of Ascaris megalocephala is so striking and of such fundamental theoretical importance that it must not be passed without notice, for in it we find marked differences between the somatic and reproductive cells in their nuclear structure, their relative amount of chromatin, and mode of division. The egg of Ascaris has been the classical object for cytological studies on account

of its small number of chromosomes (two in variety univalens, four in bivalens), their large size, and the diagrammatic clearness of the

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FIG. 146.-Reduction of the chromatin in the cleavage of the egg of Ascaris megalocephala var. univalens.

changes which take place in division.

(After Boveri.)

In the division of the fertilized

egg cell we have two (in univalens) long chromosomes handed over to each daughter cell. As these two cells in turn divide, a striking