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 delaA'ed 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 shortl}^ 

 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 entirelv 

 independent of each other and is not a matter of the imagination. 

 I have always described (orall)') 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*, 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 theorv 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. 



