740 THE AMERICAN NATURALIST. [VOL. XXXVI II. 



less fusion, the multipolar structure becomes a bipolar spindle 

 (multipolar polyarch) generally just previous to the period of 

 metaphase. The formation of cell plates and the disappearance 

 of the spindle fibers have been discussed in Section II under the 

 title " Cleavage by cell plates." 



Mention should be made of some irregularities in the division 

 and distribution of the chromosomes that are conspicuous in cer- 

 tain spore mother-cells and which have been the cause of much 

 discussion. The subject has especial reference to certain older 

 views of the reduction phenomena in plants. Chromosomes split 

 once longitudinally in all typical mitoses and the halves are drawn 

 apart in a symmetrical manner which is very easily understood. 

 This division is really determined by the longitudinal fission of 

 the spirem thread. But appearances during the first nuclear 

 division in the spore mother-cell of many forms have puzzled 

 investigators for many years and have given rise to a number 

 of interpretations. It seems to be pretty clearly established 

 now that in these types there is a double longitudinal splitting 

 of the chromosomes at the time of this mitosis. The first divi- 

 sion takes place during prophase and the second follows closely 

 after the first and is generally clearly seen at metaphase or dur- 

 ing anaphase. Therefore the chromatic bodies which appear at 

 the nuclear plate during the first mitosis are in reality divided or 

 about to be divided into quarters and they separate after this 

 mitosis as pairs of granddaughter chromosomes instead of simple 

 daughter elements. These pairs are either firmly united at one 

 end into a V or irregularly drawn out so that the bodies have 

 very unusual and sometimes bizarre forms. Nuclear figures of 

 this irregular appearance were originally described by Flemming 

 for the first mitosis in the spermatocyte of Salamandra and named 

 by him " heterotypic." These in the spore mother-cell of plants 

 are of similar character and the designation " heterotypic " has 

 been adopted by botanists for this condition. The pairs of chro- 

 mosomes that enter the daughter nuclei after the first mitosis 

 fuse end to end to form a spirem thread which breaks up again 

 during the second mitosis, without longitudinal fission, into pairs 

 of chromosomes which are believed to be identical with those 

 that entered the nucleus after the first mitosis. Since there is 



