486 THE AMERICAN NATURALIST. [Vol. XXXIX. 



tation proceeds the numbei" 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 

 (:05), 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, i. 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 

 Gr^goire ('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 



