l 4 BOTANICAL GAZETTE [july 



there can be no possible doubt of the number of chromosomes 

 present. 



As already shown (fig. 20), one or in some cases more pairs of 

 chromosomes may be cut off from the spirem before it undergoes 

 segmentation, and frequently while it is still in the second contraction 

 period. The exact method of origin of these pairs has not been 

 observed, but they invariably, so far as observed, lie with their long 

 axes parallel and connected at one end, from which it would appear 

 that they were successive chromosomes on the spirem. In later 

 stages, when the spirem has constricted into a chain of chromosomes 

 arranged near the periphery of the nucleus, one or more pairs of 

 chromosomes are found separated from the rest. Some of these have 

 doubtless had the origin shown in fig. 20. Others appear to have 

 originated later, as indicated in some of the figures, by successive 

 chromosomes on the chain swinging around parallel to each other and 

 thus pairing. Usually in diakinesis one or two such pairs are found, 

 though occasionally there is no evidence of pairing. The highest 

 number of pairs observed at this stage was five, with indications of 

 pairing among the others (fig. 29) ; which is unusual. Later, in the 

 multipolar spindle stage two distinct pairs are usually found in vary- 

 ing stages of conjugation (figs. 35, 36). A single case was observed 

 (fig. 3J) in which the fourteen chromosomes were all paired. 



As the figures indicate, constriction of the spirem at regular inter- 

 vals proceeds progressively until a chain of chromosomes is formed. 

 When this has taken place, the chromosomes are at first several 

 times longer than broad, and their margins have a very irregular, 

 sinuous outline, like that of the spirem just previous to segmentation. 

 They are not so long, however, that they can be twisted and looped 

 in the confusing manner of many heterotypic chromosomes of plants. 

 This is very gratifying in the study of these stages, since it permits 

 a clearness of interpretation which would otherwise be unattainable. 

 Figs. 22 and 23 show the beginning of contraction, which has pro- 

 ceeded farther in fig. 24, leaving only the so-called linin connection 

 between the chromosomes. The constrictions are all equivalent and 

 the spirem thus segments into the sporophyte number of chromosomes 

 and not into the reduced number of chromosome pairs. If successive 

 chromosomes on the spirem are really the members of a pair, there is 





