56 INTRODUCTION. 



number persists, and whether in the egg-cell this number obtains was 

 not determined. 1 



As is well known, two views are held concerning the manner in 

 which the reduction in the number of chromosomes is accomplished. 

 One of these views, which has been given prominence by Weismann, 

 holds that the chromosomes are qualitatively different, and that reduc- 

 tion is accomplished during the maturation divisions in animal cells 

 and in the first two divisions taking place in the spore mother-cells of 

 higher plants. For example, in the second maturation division of the 

 animal egg it is maintained that the daughter chromosomes do not arise 

 as a result of a longitudinal splitting, but by a transverse division, or 

 what is known as a qualitative division. The nuclei of the four cells 

 thus resulting, whether representing the egg and its polar bodies or 

 those which develop directly into spermatozoa, ai'e hereditarily different 

 in character, and it is upon this assumption that hereditary variation is 

 based. 



The other view, which is now very generally accepted by botanists, 

 is that, in plants no qualitative division exists, but the chromosomes 

 of each mitosis arise in every case by a longitudinal splitting. The 

 reduction takes place in the resting nucleus or during the early pro- 

 phase of the first, or heterotypic, mitosis in the spore mother-cell of 

 higher plants. The fact, as shown in preceding paragraphs, that 

 during this first mitosis a double longitudinal splitting of the chromo- 

 somes occurs, probably as a preparation for the two divisions, has led 

 to much confusion, because these divisions were supposed to have been 

 rather the instrument of reduction than a consequence of reduction. 



Assuming the persistent individuality of the chromosomes, we may 

 conclude on good grounds that the reduction represents the actual and 

 complete fusion of the chromosomes of both parents, which have 

 remained separate in the sporophyte until the formation of the spore 

 mother-cells. There is no visible evidence that a qualitative difference 

 exists between the chromosomes in plants, and our assumption here is 

 that they are hereditarily similar, because of the fact that every indi- 

 rect nuclear division is preceded by a longitudinal splitting of the 

 chro matin. 



Since the nucleus is the unquestionable bearer of hereditary char- 

 acters, fusion of sexual nuclei in fecundation has for its purpose 

 the blending of two lines of descent and possibly the restoration 



\J. Lloyd Williams in a recent paper (Studies in the Dictyotaceae, Ann. Bot., 18 : 141-160, 1904) 

 observes facts that seem to point to the conclusion that the plantlets developing from the tetraspores, 

 with their reduced number of chromosomes, may become gametophytes, and that the fecundated egg 

 cells probably develop into tetraspore plants which have been shown to possess the increased number 

 of chromosomes. If this be true, an alternation of generations exists in Dictyota. 



