526 
R. RUGGLES GATES 
besides those I have mentioned. These for the most part await dis- 
covery or investigation, although the cytological Hterature abounds 
with cases which are probably of this nature. One such instance, 
recently described, may be referred to here. In one of the grass- 
hoppers, Tettigidea parvipennis, Robertson^ found that certain indi- 
viduals possess an abnormally long chromosome mated with a short 
one, while in other individuals the corresponding pair are of equal 
length. The explanation appears to be that a portion from the end 
of one chromosome became attached endwise to its mate. A study 
of the variability of these grasshoppers should be made, to discover 
whether any corresponding somatic change can be detected in those 
that have the long chromosome. It is suggested that negative muta- 
tions might be due to the loss of a portion from the end of a chromo- 
some in this way. 
In the remarkable phenomena of mutation in Drosophila studied 
by Morgan and his pupils, the new forms appear to result from changes 
in the nature of certain portions of particular chromosomes. The 
new characters are grouped in four series according to their hereditary 
behavior, corresponding to the four pairs of chromosomes, and in 
giving rise to each mutation a particular part of one chromosome 
may be assumed to have undergone a change.^ This change is 
probably chemical in nature, at least in the series giving rise to the 
color varieties. The "crossing-over" phenomena, which have been 
studied in such detail by Morgan and his collaborators, are accounted 
for on the theory of the chiasma type of chromosome reduction, in 
which the chromosome pairs become looped around each other and 
so exchange segments of their substance. This process has not, I 
believe, been observed actually to take place in Drosophila.l 
In preparations of Drosophila chromosomes exhibited by Mr. 
Bridges, the somatic chromosomes appear to be remarkably closely 
paired and twisted about each other during the prophases of mitosis. 
Bii.^ Robertson, W. R. B. 1915. Chromosome studies III. Inequalities and 
deficiencies in homologous chromosomes: their bearing upon synapsis and the loss 
of unit characters. Journ. Morph. 26: 109-141, pis. 3. 
^ The recent observations of Chambers (Some physical properties of the cell 
nucleus, Science, n. sen, 40: 824-827. 1914) on the living spermatocytes of a grass- 
hopper lend direct support to the view that the chromosomes are composed of discrete 
and more or less independent particles. He found that stimulation of the cell in- 
duces chromosome formation in the resting nucleus, and that the chromosomes are 
formed by the aggregation of definite granules in bunches about a hyaline core. 
