216 THE THEORY OF THE GENE 



contain an X-, and each sperm a Y-chromosome. After 

 fertilization the sporophyte will have 16 chromosomes 

 (including one X and one Y). When the spores are 

 formed, reduction takes place, the X and the Y separat- 

 ing. Half of the haploid spores so formed will contain an 

 X and give rise to a female prothallium, and half will con- 

 tain a Y and give rise to a male prothallium. 



More recently still, Wettstein has made some critical 

 experiments with dioecious mosses, experiments that 

 carry the analysis further. By utilizing a discovery of the 

 Marchals, he produced gametophytes that contained both 

 the male and the female groups of chromosomes (Fig. 125 

 to the left). For example, following the Marchals' method, 

 he cut off pieces of the spore-bearing stalk (whose cells 

 are diploid). From the fragment a gametophyte devel- 

 oped, also diploid. In this way he obtained FM game- 

 tophytes. 



Then in another way he made diploid male and female 

 moss plants that were double females (FF) and double 

 males (MM). This was accomplished as follows: 



By treating the protonema threads with chloral hy- 

 drate and other drugs and reagents, he brought about the 

 suppression of a cell division in an individual cell after 

 the chromosomes had already divided. In this way he 

 could produce in these dioecious species, diploid giant 

 cells that were doubled in their female or else in their 

 male elements, chromosomes, for example. From such a 

 diploid cell a protonema or moss plant was produced. 

 By artificial means Wettstein then brought about several 

 new combinations, some triploids, others tetraploids. 

 Some of the most interesting of these combinations are 

 shown in the diagram (Fig. 125, to the right). 



A diploid cell from a female thread gives a diploid 

 moss plant, FF, that produces diploid egg-cells. Similarly 

 an MM plant is produced from a diploid male thread. 



