‘ 
ao * ALTERNATING GENERATIONS 
and various Polypodiaceae. In the Bryophytes, the earliest case described 
was that of fallavicinta investigated by Farmer, with four chromosomes 
in the gametophyte and eight in the sporophyte: and the same relation 
between the generations has since been found also in other genera of 
Liverworts. In Mosses few observations have yet been made, but Mr. 
M. Wilson has found that in Mnzum hornum the numbers are 6 and 12 
respectively in gametophyte and sporophyte. Accordingly the difference 
in chromosome-number may with high probability be held as a general 
diagnostic feature between the two generations in normal representatives of 
the Archegoniate series. 
This being so, the recognised limits between the generations will 
naturally be expected to be the points of transition from the one chromosome- 
number to the other. Now it is found that when the sexual fusion of 
the two nuclei takes place, the subsequent divisions of the fusion-nucleus 
show the doubled number of chromosomes: therefore the zygote will be 
the one limit, and this is in accord with the old distinction of the 
generations dating from the time of Hofmeister. In his practice the 
other limit was the spore, since this is the actual body separated as an 
independent germ. But it is found that the actual reduction of the number 
of chromosomes to one half, that is, to the original pre-sexua] number, takes 
place at the tetrad-division of the spore-mother-cell. This cell divides 
twice in rapid succession, and the process is well illustrated in the case 
of the pollen mother-cells of Zz/zwm, in which it has been specially studied 
(Fig. 32). It starts from a cell with a nucleus having the double number 
of chromosomes, as shown by its origin. The nucleus first enters the 
condition of synapsis (Fig. 32. 3, 4), in which a lateral fusion of the 
chromosomes in pairs, respectively of paternal and maternal origin, is 
believed to take place: presently a coiled thread frees itself from the 
tangle of synapsis (Fig. 32 4, 5), which becomes shorter and _ thicker, 
still showing, however, indications of its double nature (Fig. 32. 6, 7), 
and divides into segments, which are half as many as those of the parent. 
nucleus (Fig. 32.7, 8): each individual of the chromosome-pairs then 
moves apart (Fig. 32. 10, 11), one of each pair passing to either pole 
of the spindle which has meanwhile been formed: as each half is an 
original chromosome, the number at each pole is one half that of the 
parent nucleus, and the division is styled the heterotype, or reducing 
division (Fig. 32. 12). The second division in each of the two nuclei 
thus formed follows quickly, and is homotypic, that is, each chromosome 
undergoes longitudinal fission into two, as in a vegetative division (Fig. 32. 
13, 14, 15). The four nuclei thus constituted have also half the number 
of chromosomes present in the nucleus of the spore-mother-cell; but the 
reduction is actually effected, as has been seen, in the first, or reducing 
division. Accordingly, Strasburger has recognised the spore-mother-cell, 
in which the reduction is initiated, as the actual limit between the two 
generations. But it is the spore itself which normally terminates the 
