310 T. H. MORGAN 



same year I published a full account of my work. In 1912 I 

 obtained evidence that showed how the male-producing egg sent 

 out a whole undivided chromosome (or two whole chromosomes) 

 into the polar body. The number of chromosomes being di- 

 minished, a male develops. 



Another paper by Von Baehr appeared in 1912 in La Cellule. 

 A detailed account of the spermatogenesis of Aphis saleceti is 

 given, but no new facts of any importance were added. Certain 

 details in this paper will be referred to in other connections. 



In my 1908 paper I pointed out that the male in the phyllox- 

 eran had one less chromosome than the female, in contradiction 

 to Stevens' earlier conclusion that the same number of chromo- 

 somes is present in the male and the female. I added ''It seems 

 probable that the change takes place in the formation of the 

 single polar body given off by the parthenogenic egg." Von 

 Baehr also described one less chromosome in the male than in 

 the female, but, in his earlier paper, he like Stevens and myself, 

 had made no observations to show where or how the loss takes 

 place. In von Baehr 's full paper in 1912 also, there are no criti- 

 cal stages to show how the elimination is brought about. Ste- 

 vens found in 1910 two polar 'plates' in the male-producing egg, 

 containing one less chromosome than in the parthenogenetic fe- 

 male. She suggested that two chromosomes had united in an- 

 ticipation of the reduction division when the polar body forms. 

 Whether this union does really occur in the aphid has not yet 

 been shown, nor has the presence of a lagging chromosome been 

 seen as yet, but from analogy with the phylloxerans there can 

 now be little doubt that a whole chromosome is lost in the polar 

 body of the male-producing egg, and that a preliminary union of 

 the two X chromosomes may take place. In P. fallax I have 

 already recorded two cases in which four chromosomes met. to 

 produce two pairs ('09, p. 246, figs, u, w), and from the shifting of 

 the chromosomes, or rather from the size relations, I inferred 

 that such a union must also take place in P. caryaecaulis. 



Stevens' identification of the synapsis stage in her 1905 paper 

 I hold to be erroneous, because what appears to be the true sy- 



