INHERITANCE 749 



are of the same sex, it is clear that sex segregation does not occur at the 

 reduction division. 



Moewus determined by careful experimentation at exactly what stage 

 in the development of the haplont sex determination occurs. It is at the 

 first division of the single nucleus of the svv'armer that is dividing to 

 produce a haplont. If at this time the conditions are acid, the swarm 

 cells later produced from the haplont are plus; if the conditions at that 

 time are alkaline, the later swarmers are minus. 



In this case, therefore, the sex is determined by an external condition, 

 and this sex is inherited by all the descendants of the cell in which it is 

 thus determined. The consequences induced by an environmental condi- 

 tion are inherited. 



In his article of 1938, Moewus sums up the conclusions to which his 

 investigations lead as to inheritance and determination of sex, and par- 

 ticularly as to the constitution of the sex chromosomes in the different 

 sex conditions. In pure dioecious races, each sex chromosome carries but 

 one sex factor, P or M, and sex is segregated at the reduction division. 

 In different dioecious races, the strength or valence of the sex factors may 

 be diverse. In monoecious races, the sex chromosome of each individual 

 carries both sex factors, P and M, in equal strength; which of these shall 

 prevail in determining the sex of the gamete is decided by external 

 conditions. In the subdioecious cases, in which any clone is prevailingly 

 of one sex but produces also a few cells of the other sex, the sex chromo- 

 somes are held each to contain both sex factors, P and M, but every cell 

 contains an additional factor which inclines it toward one sex or the 

 other. The prevailingly plus clones contain a factor which inclines them 

 toward the plus sex, the prevailingly minus cells a factor which inclines 

 them toward the minus sex. But the tendencies of these additional fac- 

 tors are in some cases overcome by special outer conditions, so that a 

 few cells of the opposite sex are produced. 



In addition to these three types, there are other types in which the sex 

 chromosome carries both the sex factors, P and M, but one of these is 

 stronger than the other, so that it prevails and determines alone the sex 

 of the clone. If P is the stronger, the sex of the clone is plus; if M is 

 stronger, the sex of the clones is minus. Each clone is therefore pure for 

 sex, and the race is dioecious. Such are called by Moewus complex dioe- 

 cious races. They are not found in nature, but occur as a result of crossing 



