107 



some cases the chromatin portion is thin and the opening com- 

 paratively large (Fig. 4 c, 5 c, 19 c). In no case is the ring thin 

 and delicate as in Hetcrocoperobusta (RiJckert, 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. 6 J, 19 c and 20 c). 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. 



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 



