On IVIorphological Difference of the Chromosomes of 
Ascaris megalocephala. 
By 
Thomas H. Montgomery, jr. 
XTnivcrsity of Pennsylvania, Philadelphia. 
With plates VI— YII. 
The egg of Ascaris was made classical by vax Bexedex (1883), 
who first showed thereon that two of the chromosomes of the fertili- 
zed egg come from the sperm and two from the egg itself, a funde- 
mental fact in all our consideratious of heredity, and who also foun- 
ded the theory of the persisting iudividuality of the chromosomes. 
Boveri (1888) next worked out these phenomena with still greater 
detail, and confirmed the theory of the individuality of the chro- 
mosomes by analyzing the process through which the chromo- 
somes change into the resting nucleus, and by which they emerge 
from it. These two basic studies on this particiilar egg have laid 
the way for the now alniost generally accepted idea that chromosomes 
are in the same sense persisting entities as cells themselves are. 
Discoveries on other objects, mainly on cells of the spermato- 
genetic cycle of insects, have tended to fiirther strengthen this theory 
by showing that particiilar chromosomes can be constantly distinguish- 
ed from others through successive generations. This condition I 
have expressed (1906) by the term «chromosome dilference». It will 
be rccalled that in the germ cells of certain groups (insects, spiders, 
chilopods, chaetognaths) a large number of cases have been descri- 
bed of curiously modified chromosomes, elements that in size and 
form relations as well as in behavior can be constantly distinguish- 
ed from the others. Of these modified elements, or allosomes (he- 
terochromosomes) I have distinguished the two main kinds monosomes 
and diplosomes, the former occurring singly and the latter in pairs in 
the spermatogonia. These cases have been sumnied up by me (1906), 
