314 GARY N. CALKINS 



after as the parachute nucleus. It is analogous in its period of 

 formation, and in its subsequent stages, to the crescent nucleus 

 of Paramecium (caudatum, aurelia, and bursaria). 



The 'chromosomes' are the chromatin granules formed by seg- 

 regation of chromatin of the denser net-work derived from the 

 mass of the micronucleus. A small granule, alone, remains as a 

 reminiscence of the original dense micronucleus, and this gran- 

 ule forms the second pole of the mitotic figure (fig. 34). 



The second pole of the mitotic spindle ultimately formed starts 

 as a projection, again, from the, now granular, chromatin ag- 

 gregate. A second vesicular region, traversed by fibers and sur- 

 rounded by the nuclear membrane, is thus formed. The two 

 poles of the spindle, therefore, are not developed simultaneously, 

 although formed in the same way by migration of a granular 

 constituent of the original chromatin mass. In the center, mid- 

 way between the granule-holding poles, lies the group of chro- 

 matin granules which, by further segregation of the chromatin 

 substance from the net-work, now lie as independent 'chromo- 

 somes' in and among the spindle fibers (figs. 35 and 36). The 

 finished spindles measure from 8 m to 10 m in length and from 2| ii 

 to 4 /^ in diameter at the widest part. 



The granules of chromatin forming the nuclear plate of the 

 metaphase stage undoubtedly correspond to the chromosomes 

 of metazoon nuclei. But, of these there are two distinct types 

 in these first maturation spindles (figs. 47 and 48) . In one type, 

 owing to their very minute size and to their usually crowded ar- 

 rangement, it is impossible to count them with any degree of 

 accuracy. A few favorable specimens (figs. 35, 36, and 37) show 

 conclusively, however, that they are more numerous than twenty, 

 and my best counts average twenty-four. In this type of mitosis 

 the granules apparently pass undivided to the daughter nuclei 

 (figs. 38 and 39). 



In the second type of the first maturation spindle the chromo- 

 somes are larger, more compact, and form a definite nuclear plate 

 (fig. 40 to 44). Here they are distinctly eight in number and 

 eight daughter chromosomes pass to each pole of the spindle. 

 An early anaphase stage is shown in figure 40, from which it might 



