107 
some cases the chromatin portion is thin and the opening com- 
paratively large (Fig. 4c, §c,19c). Inno case is the ring thin 
and delicate as in Heterocope robusta (Riickert, 1893, Fig. 23). The 
ring stage begins with a lateral bulging of the two halves of the 
spireme segment (Fig. 6 i); this is followed by the appearance of 
a furrow at the center. This furrow enlarges until it forms a cir- 
cular space, and, the ends of the segment remaining attached, the 
chromatin forms a closed ring (Fig. 19 c). The chromatin then 
begins to accumulate in four parts, each half of the originally 
double spireme forming two (Figs. 6J,19 cand 20c), These parts 
become more and more distinct and individualized; more com- 
pact and tightly packed together, until finally the tetrad is com- 
pleted (Fig. 7). The tetrad is, therefore, derived first, by a lon- 
gitudinal splitting of the spireme segment, and second, by the 
transverse splitting of the two halves. 
6. 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. 
5a). The chromatin segregates at the two ends in four swellings 
(Fig. 6b). 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. 6d, e). The resultant tetrad does not differ essentially from 
one formed in thesimpler 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 
