May, 1909.] Chromosome Difference in Ascaris Megalocephala. 507 
This figure shows that there has been no pairing so far of the 
maternal and paternal chromosomes. This would seem to 
indicate that definite pairing is delayed until the prophase of the 
reduction division where it must take place before or at the time 
of the formation of the spirem. The two shorter chromosomes 
are situated at the ends of the long ones which form a more or 
less parallel pair. This arrangement can still be noticed shortly 
after the segmentation of the spirem and is sometimes quite 
striking. It is shown to some extent in figures 2, 3 and 5. 
There is a rather constant difference between the pair of 
smaller chromosomes, one of which is now known to be a mater¬ 
nal and one a paternal chromosome. It would be interesting if 
the pedigree of these two bodies could be determined. The 
writer has frequently pointed out this difference in appearance 
between the two smaller chromosomes in Ascaris. It was, 
therefore, a pleasure to read Montgomery’s statement of the 
same fact. It is evident that even though the difference is slight, 
it is distinct enough to be clearly seen by two observers entirely 
independent of each other and is not a matter of the imagination. 
I have always described (orally) the one chromosome as being 
more U-shaped or rounded at the head and the other as more 
pointed, V-shaped, or narrowed at the apex. Montgomery 
describes the difference as follows: “The smallest chromosome 
of all is very frequently hook-shaped and the other of the 
smaller pair often U-shaped, while the longer ones have usually 
two or three angles each, but none of these differences in form 
appear to be constant.” By long-continued study one might 
perhaps be able to acquire the ability to distinguish these bodies 
just as an expert systematist can readily determine two closely 
related species at sight which look exactly alike to the inexperi¬ 
enced. Thus we are led to hope that a plant or animal may be 
found which will present constant and recognizable differences 
between the maternal and paternal chromosomes. 
In my work on Agave 4 , a marked individuality was found in 
the twelve reduction chromosome loops as regards morphological 
character. The statement was also made that “Since these are 
bivalent chromosomes, it is evident that on the theory of the 
conjugation of maternal and paternal chromosomes, the con¬ 
jugating pairs must be quite similar in shape and activity. In 
the microsporocytes the bivalent chromosomes have an individ¬ 
ual shape and size easily distinguishable.” 
It is not necessary to assume, at the present time, that no 
interchange whatever of material takes place between the 
chromosomes even though we ascribe definite individuality to 
them. There might occasionally or even regularly be some 
4. Schaffner, John H. The Reduction Division in the Microsporo¬ 
cytes of Agave virginica. Bot. Gaz. 47.' 198-214. 1909. 
