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 may 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 division 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 G and 7), but breaks into only 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 types 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 amis of the loops into which 
the spirem is thrown (Figs. 7, 8 and 9). As they arc 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 becomes 
much more striking in organisms like the hyacinth with numerous 
chromosomes of diverse sizes. 
In the metakinesis stage of the first reduction division (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 poles 
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 alike 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 chromosomes, all of them 
being mixed as to crosses and dots. When the number of chro- 
