108 



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 four 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 spiremesegment and second by 

 transverse division of the halves. 



3. Period of 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 Ptem and Fig. 20 for Adianttctn, while 

 Fig. 1 1 shows the completion of the spindle in Ptej'is and the 

 definite formation of the nuclear plate. In this stage the tetrads 



