286 T. H. MORGAN 



known to produce large eggs, and all of the offspring of other 

 stem-mothers to produce small eggs. It follows, for this species, 

 either that there must be two kinds of stem-mothers, or else that 

 some external factor must determine that one kind of stem-mother 

 produces daughters all of which contain large eggs, and another 

 kind of stem-mother produces daughters all of which contain 

 small eggs. If such effects are produced by the environment 

 they must be wrought before the mother is mature, because all 

 the daughters of a single female are alike in regard to the size of 

 the eggs that they carry. On the other hand in P. fallax some 

 of the offspring of one stem-mother contain large eggs while other 

 offspring of the same mother contain small eggs. In this species 

 there must be only one kind of stem-mother, and either external 

 conditions, or some difference that arises when the polar body 

 of the stem-mother's egg is extruded, must determine whether a 

 given egg becomes a large egg carrier or a small egg carrier. 



In either case it became necessary to find out what occurs 

 when the polar bodies of the egg laid by the stem-mother are 

 given off before any further advance in the analysis was possible. 



During the last two years I have studied these stages, and I 

 am now in position to give the complete history of the chromo- 

 somal cycle. And I can now also give certain additional facts 

 connected with the chromosomes at the time of extrusion of the 

 polar body of the male egg of P. fallax. This new evidence 

 makes possible the interpretation of the entire life cycle of the 

 phylloxerans ; an interpretation that has a wider interest and 

 application than relates to the life cycle of this group alone. 



Before taking up the history of the chromosomes, I may re- 

 call the salient features in the life cycle of the two species that 

 are to be considered. 



The life cycle of P. caryaecaulis is shown in figure 92 in my 

 book on ''Heredity and Sex." The stem-mother that hatches in 

 the early spring produces a gall on the leaf of the hickory. As 

 soon as she is mature she begins to deposit her eggs within the 

 gall. From these eggs winged daughters hatch; all those in one 

 gall contain large eggs, all those in another gall contain small 

 eggs. The winged forms leave the galls and deposit their eggs 



