742 INHERITANCE 



viduals are plus and others minus is held by Moewus to be due to some- 

 thing in the surrounding conditions; the sex determination within the 

 clone is phenotypic instead of genotypic, as in the dioecious races. But 

 in the "prevailingly plus" clones the constitution is such that minus 

 gametes are not so readily or numerously produced by the conditions as 

 is the case in "prevailingly minus" clones. Moewus found that subjec- 

 tion to dilute formaldehyde or acetone causes the zygotes of the sub- 

 dioecious races to produce gametes of only one sex, that sex which would 

 have been in the majority if these substances had not been used. The 

 precise constitution of subdioecious races, together with that of other 

 types, is considered in a later section. 



Monoecious races. — In monoecious races both sexes occur in a single 

 clone, so that such clones may be spoken of as "mixed" as to sex. This 

 is the situation in certain races of various species of Chlamydomonas ; 

 in Polyto}7ia uvella, and in somes races of P. pascheri and of Protosiphon 

 botryoides. 



In monoecious stocks obviously the fission of a single individual gives 

 rise to both sexes. The determination of sex in such stocks is largely or 

 entirely phenotypic; that is, through external conditions. Such pheno- 

 typic sex determination is not dealt with in the present chapter. 



When from monoecious stocks plus and minus gametes are mated, the 

 descendant clones are all monoecious, that is, mixed as to sex. 



Crosses between dioecious and monoecious stocks. — A number of dif- 

 ferent types of crosses may be made between dioecious and monoecious 

 stocks, as follows: 



1. Plus gamete from a "mixed" (monoecious) clone; minus gamete 

 from a "pure" (dioecious) clone (cross of two diverse races of Polytoma 

 pascheri) . Result, two of the four descendant clones are mixed, two pure 

 minus. 



2. Plus gamete from pure clone, minus gamete from mixed clone 

 (cross of two races of P. pascheri). Result, two clones mixed, two pure 

 plus. 



In these two crosses, two results are notable. ( 1 ) The pure condition 

 segregates from the mixed condition at the reduction division, the two 

 depending apparently on the two different chromosomes of a pair. The 

 difference is genotypic. (2) The "pure" condition emerges with the 

 same sex (plus or minus) as that with which it enters the cross. If the 



