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each other in the centre. (Fig. 6a,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.6c). 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 spireme-segment 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 Pvers and Fig. 20 for Adiantum, while 
Fig. 11 shows the completion of the spindle in Preris and the 
definite formation of the nuclear plate. In this stage the tetrads 
