478 BOTANICAL GAZETTE [December 



' -\ 



becomes transformed into isolated individual chromosomes, the 

 granules and fibrils being entirely used up in the formation, in the 

 reduction mitosis the granular parts become transformed into 

 fibrils, and consequently during the presynaptic stage there are 

 only thread structures within the nucleus. These chromatin 

 threads, at their beginning irregularly thickened and branched, 

 become much evener, and the transformation continues until long 

 continuous threads are formed, which run freely throughout the 

 cavity. The threads thus formed, from the beginning of their 

 transformation to their completion as continuous structures, have a 

 single nature. Entering the synaptic condition, the single threads 

 then shorten and thicken, and become either eccentrically grouped 

 as a loose tangled mass at one side of the nuclear cavity, or are 

 variously scattered all over the membrane. The threads eventually 

 form loops by repeated folding. The number of loops is 24. Each 

 loop folds together at its bent end so that the bent arms come into 

 contact with each other in the culmination of synapsis. As they 

 emerge from synapsis, there are present 24 bivalent chromosomes 

 which become detached from the nuclear membrane, moving toward 

 various parts of the nuclear cavity. 



The relationship of the chromatin threads in prophase, the loops 

 in synapsis, and the bivalent chromosomes of postsynapsis have 

 been clearly traced. A pair of bivalent chromosomes corresponds 

 to one of the loops in synapsis, the loop being formed by a folding 

 back of the chromatin thread, so that a loop in synapsis should be 

 considered as composed of two sporophytic chromosomes associated 

 end to end. The two elements of the bivalent chromosomes 

 separate from each other at the metaphase of the first reduction 

 division, thus effecting what may be regarded as a qualitative 

 reduction. 



The origin and behavior of bivalent chromosomes as described 



(73) and Zanardinia (76) agree perfectly in essentials 



>le, figs. 161, 



1 



with 



exam 



162, and 172, illustrating looping threads in synapsis and chromo- 

 somes in late prophase in the zoospore mother cell of Aglaozonia, 

 when compared with figs. 43, 44, and 49 illustrating similar stages 

 of Fucus (73), demonstrate at once their similarities. 



