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 ab initio, 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 

 (i) 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 halves 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, n, 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 



