CYTOLOGICAL DISTINCTION 47 
This absence of strict criteria distinguishing between the two alternating 
generations of Archegoniate Plants has given rise to much discussion, 
and the differences of opinion have centred round the question of their 
origin. Were the two generations distinct ad cmitfo, or were they merely 
phases differentiated from a common source? “Under the “homologous” 
theory of alternation the two generations were held to have been similar 
in origin, and the alternation to have originated by a secondary modification 
arising in a pre-existent and independent organisation. The adherents of 
the “‘antithetic” theory held that the sexual generation was pre-existent, 
and that a new organisation arose, derived by amplification from the 
zygote: the sporophyte was thus originally not a result of change in a 
pre-existent organisation, but it arose as a newly expanded phase, distinct 
in its origin from other phases of the life-cycle. The difference of opinion 
entailed in these two theories is essentially one of history, and of method 
of origin. 
In the absence of strict criteria of distinction, such discussions are apt 
to be long and inconsequent. It seemed accordingly to be a welcome 
advance when facts were gradually disclosed, showing that a cytological 
difference exists between the two generations. This appeared to raise 
the whole doctrine of alternation in Archegoniate Plants to a higher plane, 
and to relate the origin of the two alternating phases intimately with the 
existence of a sexual process. In order to understand the nature of 
this new criterion of distinction it is necessary to be acquainted with 
the main features ‘of nuclear division. When a nucleated vegetable cell 
divides, the nucleus takes the initiative, and goes through a series of phases. 
as shown in Fig. 31, which is quoted from Strasburger, to whom the 
discovery of the details is chiefly due. Without describing these at 
length it may be stated that the chromatin, that constituent of the nuclear 
body which stains most deeply, distributes itself in the linin: the body 
thus formed changes from a network of fine fibrils in the resting nucleus 
(1) to a thicker convoluted thread, which then divides transversely into. 
segments—the chromosomes (3, 4). These segments then divide longitudin- 
ally (6, 7, 8), and the halyes of each, separating from one another, pass. 
to the opposite poles of the nuclear spindle, which has meanwhile been 
formed (8, 9, 10): they there reconstitute the chromatin-system of the 
two new nuclei (10, 11, 12). An essential part of this process is found to be 
that the number of chromosomes is definite, and though in different plants 
and groups of plants it may vary within wide limits, still in the species. 
or individual the number is (with some exceptions) strictly maintained. 
But this is so normally only in the cells of the one or of the other 
generation; for it has been found, in cases which are constantly becoming 
more numerous as observations extend, that there is a numerical difference 
in the chromosomes of dividing nuclei in the two alternating generations. 
of the same plant: in the sporophyte the number is twice that in the 
gametophyte: the former has accordingly been styled the ‘*diploid,” the 
