late figures are developed about the respective poles of the central spindle. The appearance presented is known as a diaster. Our knowledge of the part taken by the amphiaster or achromatic spindle in metakinesis is not above controversy. It would appear, however, that certain cytoplastic fibers, which arise from the centrosphere and hang over the central spindle and chromosomes, designated as mantle fibers, assist in drawing the daughter chromosomes toward the poles of the central spindle.

(c) Anaphases.-After the formation of the diaster, the loops belonging to each stellate figure are joined together to form a skein, thus forming the dispirem. The chromatin threads of the two skeins gradually assume the disposition found in the resting nucleus. This process takes place in such a way that the threads of the

skeins (or the single thread) send out lateral processes. interlace, and little by little reproduce the network of the resting nucleus; at the same time the nuclear membrane and the nucleolus reappear. In this stage the changes that lead to the division of the cell-body are observed. In some cases the division of the cell-body is ushered in by an equatorial differentiation of the connecting threads of the central spindle. Chains of granules, arranged in double rows, are seen to appear in this region. The cell now begins to contract at its equator, the contraction extending between the two chains of granules until the cell is completely divided. At this time, also, the threads of the amphiaster disappear or are drawn into the nucleus. The centrosomes, with centrospheres, again lie by the side of the daughter nuclei.

According to the opinion of C. Rabl (85), there remains in the nucleus, even after it has fully returned to a state of rest, a polar arrangement of the chromatin loops—that is, an arrangement of the axis of the loops in the direction of the centrosphere. The area toward which the crowns of the loops point is known as the polar field.

The equatorial differentiation of the connecting threads of the central spindle, above mentioned, was first observed in vegetable tissue, and is known as the cell-plate. (Fig. 27.) In animal cells such a plate is relatively rare, and, when seen, is found developed in a rudimentary form (v. Kostanecki 92, I).

(d) Telophases (M. Heidenhain 94).-In these phases of mitosis the cell divides completely. The daughter nuclei and centrospheres, which do not yet occupy their normal position in the daughter cells, show movements that result in their assuming their normal positions.

From our description it is seen that the anaphases represent the same stages as the prophases, only in an inverted sequence. In the latter case, the result is the resting nucleus, while the prophases lead to the metaphases.

The fertilized ovum also divides by indirect nuclear division. (Figs. 20-27.) From it are derived, by this process, the segmentation cells, or blastomeres, from which the whole embryo is developed.

(e) The Heterotypic Form of Mitosis.-The above-described type of indirect or mitotic nuclear division (homeotypic mitosis) is the usual one. Variations, however, occur, as, for instance, in the so-called heterotypic form of division (Flemming 87), which occurs in certain cells of the testes (spermatocytes). In this form the first stages are lacking, the nucleus possessing from the beginning a skein-like structure. The longitudinal splitting and division of the chromatin threads take place during the first spirem stage, after which there is a phase in which the figure may be compared with an aster of ordinary mitosis, although the free ends of the threads in this case are seldom observed. The latter is due to the fact that after the longitudinal splitting, the ends of the chromosomes remain united, or, if entire separation occurs, they are again joined. In this way closed loops are formed extending from pole to pole. Later the threads break at the equator and move toward the poles, again dividing to form the daughter stars.

2. AMITOSIS.

Very different from the indirect form of nuclear division is the direct or amitotic. It appears to occur seldom as a normal process, and is only exceptionally followed by a subsequent cell-division (vid. Flemming, 91, III). As a consequence, this process, in most cases, results in the formation of polynuclear cells (polynuclear leucocytes, giant-cells, etc.). The complicated nuclear figures of

indirect division are here entirely absent. The nucleus merely contracts at a certain point and separates into two or more fragments (direct fragmentation, Arnold); often the nucleus first assumes an annular form and then breaks up into several fragments, which remain loosely connected (polynuclear cells). Centrospheres are also present, and appear to take a prominent part in the whole process, although the exact relationship between the achromatin and chromatin has not as yet been determined.

Arnold (83) gives the following comparison of indirect and direct nuclear division: (1) Segmentation. Division of the nuclei in the equatorial plane into two or more equal parts; (a) direct segmentation without, (b) indirect segmention with, increase and changed arrangement of the chromatic substance (mitosis). (2) Fragmentation. Contraction of the nucleus at some point, forming two or more equal, but oftener unequal, nuclear fragments; (c) direct fragmentation without, (d) indirect fragmentation with, increase and changed arrangement of the chromatic substance.

'D. PROCESS OF FERTILIZATION.

The sexual cells form a special group among cells in general. Before the division of the egg-cell leading to the development of the embryo can take place, the ovum must be impregnated (the socalled parthenogenetic ova are an exception to this rule). Fertilization is produced by the male sexual cell, the spermatozoon.

The process of fertilization consists in a conjugation of two sexual cells, and in this process certain peculiarities in the behavior of both cells must be mentioned.

The cell forming the ovum and the one forming the spermatozoon must pass through certain stages before fertilization can be accomplished. These consist in the loss of half their chromosomes by the nuclei of both sexual cells. In this way are produced the matured sexual cells (ova and spermatozoa), which retain only half of the number of chromosomes of a somatic (body-) cell. In the conjugation of the male and female sexual cells their nuclei unite to form a single nucleus, known as the segmentation nucleus. Consequently, this nucleus contains the same number of chromosomes as does that of a somatic cell.

In its earlier developmental stages the ovum is an indifferent cell, the nucleus of which is known as the germinal vesicle. As the ovum matures the germinal vesicle approaches the periphery, and a peculiar metamorphosis, which may be regarded as a double, unequal division of the egg-cell, takes place. One portion, in the case of both divisions, is much smaller than the other, and is known as a polar body. At the close of these divisions, during which the chromosomes have been reduced to half the original number, there are, therefore, two polar bodies and the matured ovum, which is now ready for impregnation.

Figs. 29-31.-Diagrams of the process of fertilization, after Boveri (88). Figure 29, the ovum is surrounded by spermatozoa, one of which is in the act of penetration. Toward it the yolk is pushed forward in a short, rounded process. Figure 30, the tail of the spermatozoon has disappeared. Beside the head is a centrosome with polar radiation. Figure 31, the pronuclei approach each other.

The development of the male sexual cell in its earlier stages is similar to that of the ovum. They are derived from cells known as spermatogones. These divide into equal parts, forming the cells of a second generation, the spermatocytes. From a further division of the spermatocytes, during which division the chromosomes are reduced to half the number, the spermatids are produced. These latter are then changed directly into spermatozoa. The reduction division of the egg-cell and that of the spermatocytes is in principle the same, except that in spermatogenesis all cells become matured sexual cells (spermatozoa). In short, there is here an absence of structures analogous to the polar bodies, which degenerate after maturation of the ovum.

The spermatozoa are flagellate cells. The head consists principally of nuclear substance, to which is added a smaller middlepiece containing, according to the investigations of Fick, the centrosome. These two portions of the male sexual cell, the head- and

Figs. 32-34.-Diagrams of the process of fertilization, after Boveri (88).

Figure 32, from the spirems in the pronuclei, chromosomes have been formed. The centrosphere has divided. Figure 33, the double chromosomes of the two pronuclei lie in the equatorial plane of the ovum. Figure 34, the ovum has divided. Chromosomes from the male and female elements are seen in equal numbers in both daughter nuclei.

middle-piece, are the most important, and are exclusively concerned in fertilization, the flagellum or tail playing no part in this process.

The spermatozoon usually penetrates the ovum after the first polar body has been extruded. The tail disappears during this process, being either left at the periphery of the egg or dissolved in the protoplasm. From this time the head represents the so-called male pronucleus, and the middle-piece the centrosome. From this stage the male pronucleus undergoes changes, the first of which consists of a loosening of the chromatin. Chromatin granules are formed, which later arrange themselves in the form of chromo

somes.

After the second polar body has been extruded, the chromatin remaining in the ovum is transformed into the female pronucleus. The latter then approaches the male pronucleus, the membranes of both nuclei disappearing. The chromosomes of