Sex Determination 187 



conditions. Then by using some remarkable technique 

 he grafted parts of one strain on to the other, and the 

 result was a neutral strain. This seems surprising for the 

 moment, but when it is remembered that Mucor is coeno- 

 cytic the result is what might be expected, for the two 

 sets of nuclei can mingle freely. 



LIVERWORTS. A great deal of experimental work 

 has been done with this group, but only one conclusive 

 result has been obtained. In dioecious liverworts, as 

 Marchantia, the sexes appear in about equal numbers. 

 It is natural for cytologists to conclude from this that 

 sex is determined at the reduction division in the forma- 

 tion of spores. Thus when a mother-cell produces a 

 tetrad, two of the spores would produce male gameto- 

 phytes and the other two female gametophytes. This 

 of course was an assumption, for no sex chromosome was 

 discovered, nor was it known that the spores of a tetrad 

 always divide in this 50-50 way as to sex. 



The assumption was strengthened by some work by 

 NOLL (10). He found that gemmae from male game- 

 tophytes always produce male gametophytes, while 

 gemmae from female gametophytes always produce 

 females. This established the fact that in the mature 

 gametophyte sex has been fixed, but it did not indicate 

 where it had been fixed. 



It remained for STRASBURGER (13) to answer this 

 question. He selected Sphaerocarpus, which is pecul- 

 iarly favorable material for such work. It is dioecious, 

 like many liverworts, but a remarkable feature is that 

 the spores hang together in the tetrad. Ordinarily 

 when spores are mature the tetrads are no longer dis- 

 tinguishable. Sowing such free spores one may get the 



