486 THE AMERICAN NATURALIST. | [Vor. XXXIX. 
tation proceeds the number of loops becomes fewer and the free 
ends more numerous. Allen finds the breaking apart of the 
arms of the loops, whose heads are at the periphery of the 
nucleus, as described by Schaffner (97) and Farmer and Moore 
(105), but cannot accept the interpretation of these latter 
authors (to be described presently). Allen’s studies show that 
the loops are the points of separation of adjacent split chromo- 
somes and not a point where the spirem bends on itself to form 
a pair of chromosomes.: The ends of the split chromosomes 
when properly stained are seen to be distinct even though they 
may be in contact or apparently fused. At the time of the seg- 
mentation of the split spirem the two threads are generally 
twisted about one another. : 
The split chromosomes now shorten and thicken, the num- 
ber of twists is reduced and the pairs of elements take on 
the many forms characteristic of the heterotypic mitosis and 
described as I's, J's, X's, Y's, V's, and O's. These chromo- 
somes of the heterotypic mitosis are of course pairs of chromo- 
somes, 7. e., bivalent chromosomes or dyads. They are believed 
to represent morphologically the full number of sporophytic 
chromosomes (24) now associated in pairs forming the reduced 
number (12) of bivalent chromosomes. The two threads which 
fuse are believed to represent two spirems of maternal and 
paternal origin and the chromosomes in the pairs are derived 
from different parents. 
Shortly after the segmentation of the spirem the sporophytic 
chromosomes of each bivalent element or dyad may show evi- 
dence of a second longitudinal fission, first recognized by 
Grégoire (99), Guignard (99), and Strasburger (: 00) which is 
completed during the metaphase of the heterotypic mitosis. 
The evidence consists in the appearance of a double row of 
granules in each sporophytic chromosome, the result of the 
division of the chromomeres. However, these chromomeres 
soon become indistinguishable from the linin and the chromo- 
somes appear homogeneous from now on. 
While the spindle of the heterotypic mitosis is being organ- 
ized the position of the sporophytic chromosomes shifts with the 
development of the spindle fibers until they are brought to the 
