498 T. H. MORGAN 
total number of one species is double that of the other. If this. 
is the case the loss of the two smaller z’s might take place in 
those eggs of the stem-mother that give rise to the male pro- 
ducers and its loss might not be apparent unless its mate were 
sufficiently reduced in size to make its loss visible. It has not 
been possible to make out the size relations of the chromosomes 
in P. fallax with sufficient clearness for the evidence to be of any 
value one way or the other. The question for this species must 
be left unanswered, but it should at least be noticed that the two 
large lagging chromosomes of P. fallax behave like the single 
lagging chromosomes of P. caryaecaulis, and not like the large 
X and the small one of the latter form. 
