May, 1913.] A Cytological Life Cycle. 143 



the whole plant complete in all its parts may be reproduced from 

 any small slip which can be made to grow. In some cases e. g. 

 the leaves of Bryophyllum even single cells ma}^ be made to 

 propagate the plant which of course would be impossible unless 

 they contained all of the hereditary characters. This type of 

 division continues then until the reduction di\'ision occurs and 

 the familiar nonsexual spores so frequently found on fern leaves 

 are produced. 



In the reduction division the spirem is formed and divides 

 in the same manner (Figs 6 and 7), but breaks into onh' half 

 as many pieces as in the ordinary mitosis (Fig. 9). Thus each 

 piece really corresponds to two of the divided chromosomes seen 

 in the metaphase of ordinary mitosis. This pairing or "syn- 

 apsis" of the chromosomes is the essential difference between 

 the two t3-pes of mitosis, for all of the subsequent difference 

 of the reduction chromosomes is the necessary consequence of it. 

 Before they pull apart these paired, doubled chromosomes become 

 definitely associated together forming the variously shaped 

 tetravalent chromosomes or "tetrads" characteristic of the 

 reduction division. In their early stages they may be seen to be 

 formed by the association of the two arms of the loops into which 

 the spirem is thrown (Figs. 7, .S and 9). As they are pulled apart 

 they may retain the form of the original loop or may appear as 

 crosses or rings depending on their length and the manner in 

 which they are attached to the spindle fibres (fig 10). Curiously 

 enough the pairs are always made up of chromosomes of exactly 

 the same size. This is indicated in the diagrams but l^ecomes 

 much more striking in organisms like the hyacinth with numerous 

 chrojTiosomes of di\'erse sizes. 



In the mLctakinesis stage of the first reduction di\-ision (Fig. 10), 

 the pairs of chromosomes which fused or rather failed to separate 

 in the early stages, are pulled apart so that one goes to each of the 

 daughter nuclei (Fig. 11). Immediately after the first mitosis 

 the spindles of the second mitosis organize at each of the iDoles 

 and the doubled chromosomes separated in the first mitosis are 

 divided along the line of the early longitudinal split (Fig. 12), 

 giving rise to the nuclei of the four nonsexual spores. Each spore 

 thus contains one of the four parts of each of the tetrad chromo- 

 somes of the first reduction division. It will be observed that 

 they are not aliJce in the chromosomes they bear. One set of 

 spores bears only those designated by circles and dots while the 

 other bears only those designated by crosses. If it had so hap- 

 pened that one of the tetrad chromosomes of the first mitosis had 

 been turned the other side up as is indicated in the alternative 

 Figure 10a, it is clear that the resultant nonsexual spores would 

 have borne a different combination of chromosonies, all of them 

 being mixed as to crosses and dots. When the number of chro- 



