258 EdmundB. Wilson: 
open; but the recent history of the subject shows, I think, that 
to identify any chromatie nucleolus-like body as a sex-chromosome, 
in the oocytes or elsewhere, without actually following out its 
whole history, may readily lead to quite erroneous conclusions. 
A confusion by some observers between the X-chromosome 
and an ordinary nucleolus is responsible for the surmise, made 
by certain writers, that the former is not a true‘ chromosome. 
This is contradieted by the fact that in the embryonie nuclei, 
and throughout the greater part of the life of the organism, the 
sex-chromosomes do not differ in behavior in any way from the 
others. Their peculiar behavior in the spermatocytes is a remar- 
kable and no doubt significant fact; but it gives no ground 
whatever for regarding them as anything other than true chromo- 
somes. There are, indeed, many well known cases in which all 
the chromosomes are condensed into a single nucleolus-like 
”karyosphere“ in the resting nuclei; and this is now known to 
oceur even in the growth-period of the spermatocytes in certain 
tracheates (Myriapoda, Coleoptera, Hemiptera), though this is 
exceptional.') 
’) Without attempting to trace in all its detail the devious history 
of views relating to the chromosome-nucleoli, I will briefly indicate in what 
way the entanglement of the subject arose. The first observers who positively 
recognized that the chromatic "nucleolus“ of the spermatocyte growth-period 
in insects is a true chromosome were Montgomery (1898) in "Pentatoma“ 
(Euschistus) and Me Olung (1899) in Xiphidium. The subsequent difficulties 
first grew out of a confusion on the part of Paulmier and Montgomery 
between the sex-chromosomes of the Hemiptera and certain others of quite 
different nature (the ”m-chromosomes“). In his original paper on Anasa, 
Paulmier (1899) reached the erroneous conclusion that the chromosome- 
nucleolus of the growth-period (which he correetly identified with the 
X-chromosome of the spermatocyte-divisions) was’ a bivalent body, formed 
by union of two very small spermatogonial chromosomes (the m-chromosomes); 
and the latter he believed to persist in the "rest stages“ of the spermatogonial 
nuclei in the form of two small nucleolus-like bodies. In this error he was 
followedby Montgomery (1901), later by Gross (1904), both of whom 
were thus led to additional misleading conclusions. Montgomery con- 
cluded that the m-chromosomes were of the same nature as the unequal 
”chromatin-nucleoli“ of the Pentatomidae (now known to be the XY pair) 
and subsequently included both, as ”"diplosomes‘“, among his ”heterochromo- 
somes‘“ (1904), later called ”allosomes“ (1906). The "heterochromosomes“ 
in general were characterized as those "which preserve to great extent 
their compact form during the whole growth-period, and during the rest 
