b. The " rod type." The tetrad begins as before with the short and somewhat thickened double spireme-segment, but here no separation of the two parts of the segment takes place (Fig. 5 a). The chromatin segregates at the two ends in four swellings (Fig. 6 b). These swellings enlarge, become more definite and the segments become shorter by the gradual drawing together of the ends. The ends finally round out and tetrads are formed by what would seem to be the simplest method possible.
There are some modifications of this type. In some cases the two halves of the spireme segment slide along on each other until, in an extreme case, the opposite ends may become contiguous (Fig. 6 d, e). The resultant tetrad does not differ essentially from one formed in the simpler manner. There is the same segregation of chromatin at the four ends, the same shortening of the segment and finally the same end result, although at first the tetrad is some what distorted. In the rod type, therefore, the tetrad originates first by a longitudinal division of the spireme, and second, by transverse division of the halves and is equivalent in all respects to the tetrad of the " ring type." c. The " cross type." In this type the halves of the double spireme segment, instead of separating in the centre as in the " ring type," or of remaining parallel to each other as in the " rod type," become separate at the two ends but remain attached to each other in the centre. (Fig. 6 a, 1., etc.) These ends sepa rate farther and farther until each half segment forms a loop which lies in contact with the other half segment only at the center of the convex side (Fig. 6 c). It is the opposite of the ring type. In some cases the segregation of the chromatin begins at an early period (Fig. 6 a), and, as separation continues, the segregation becomes more marked, until finally there are four dis tinct swellings lying at right angles to each other (Fig. 6 c, 19 a). The loops meantime become shorter and shorter, until finally the four parts of the chromosome are brought together, and a tetrad is formed similar in all respects to those of the " ring " and rod " types (Fig. 6 k).
Like the " rod type," the " cross type " shows some modifica tions. After the ends have begun to diverge as in the normal cross type, one of the loops may swing around through an angle of 90 degrees on the point of attachment as a pivot (Fig. 6 f ). It thus comes to lie in a plane at right angles to its original posi tion. Segregation of the chromatin gives rise to the fou'r parts of the chromosome as before. Various other modifications of this type are found (Fig. 5 x), but in all of them the result is the same. Here, therefore, as in the other types the tetrad originates first by a longitudinal division of the spireme-segment and second by transverse division of the halves.
3. Period Reduction.
It is in this period of spore development that reduction of the chromosomes actually takes place. It begins with the arrange ment of the mature tetrads into the nuclear plate of the primary sporocyte spindle. Before this arrangement the tetrads are dis tributed throughout the nucleus (Fig. 7). The nuclear membrane disappears, and after this, for the first time, it can be clearly seen that the nuclear space is filled with almost parallel spindle fibres (Fig. 8). The latter at this stage could not be traced to definite points at the poles. The tetrads lie in various positions on the spindle fibres (Fig. 9), but they gradually collect at the equator of
the spindle. The migration towards the equator of the spindle is clearly shown in Fig. 10 for Pteris and Fig. 20 for Ad lanhtm , while Fig. 11 shows the completion of the spindle in Pteris and the definite formation of the nuclear plate. In this stage the tetrads are closely packed, and are so numerous that counting is impossi ble. In the early stages, however (Figs. 8 and 10), it can be seen that the number is about sixty.
The compact arrangement of the tetrads in the nuclear plate leaves no chance for orientation. It is impossible, therefore, to tell from this division whether the tetrad divides through the line of original cleavage, or through the secondarily acquired trans verse cleavage. In other words, it is impossible to tell whether the division of the primary sporocyte is a reducing or an equation division. There is, however, good reason to regard this as an equation division, and the division of the secondary sporocyte as transverse, and, therefore, as a reducing division. The second mitosis follows closely on the first, but in the short interval the two parts of each dyad, which at first appear like two small balls closely pressed together (Figs. and 21), now become drawn out in the direction of their common axis, which is probably the original longitudinal axis of the spireme-segment (Figs. 13 and 14). It is immaterial in the final spore cells whether the first or the second division is a reducing division in the Weisrnann sense. That one of them must be is shown by the method of tetrad formation ; but, from the manner in which the dyads elongate, the probability is certainly strong that reduction is effected by the second mitosis. The change in shape of the chromosomes in the secondary sporocyte-spindle makes the general appearance of the nuclear plate conform more nearly with that of the somatic cells (cf. Figs. 13 and IS), although they are fundamentally different.
4. The spore.
The cylindrical shape of the daughter chromosomes as they come from the division of the dyads in the secondary sporocytes is retained until late in the anaphase (Fig. 14). The resulting four daughter-nuclei lie freely in a single cell which, until the cell plates are formed, is a syncytium. In the division of multinuclear cells it has been frequently noted that the nuclei are connected by spindle fibres. This occurs in ferns, and long after division, and even as late as the telophasc after the cell-plates are formed and the nuclei have gone into the resting stage, fibres can still be seen connecting each nucleus with all the others (Figs. 14 and 15). While the cell-plates are forming, the chromosomes gradually dis integrate and pass into the reticulum although their outlines can be dimly made out even after the reticulum is well formed and the nucleoli have reappeared (Fig. 16).
5. The centrosome.
It is extremely difficult to stain, and correspondingly hard to find the centrosomes in this material ; even at the spindle-poles its identification is not easy. I was able, however, to make it out in two different stages (Figs. I I, 12 and 13). One of these was in the mitosis of the primary sporocyte, the other in that of the secondary sporocyte. In the first of these the centrosome at the spindle-poles was double (Figs. I i and 12), in the second it was single (Fig. I3).