Pollen Molher-cells of Lilinm canadense. 225 
difference between the individual chromosomes derived from either germ- 
nucleus. 
In harmony with this evidence as to a difference in function is the fact 
that the chromosomes of a single nucleus often differ greatly among them- 
selves as to size and shape. Such differences have been noted, for example, 
by Guignard (’99) in the pollen mother-cells of Naias, and by Boveri (’04) 
in sea-urchin eggs. I have already spoken of the variations in size of the 
chromosomes of the lily in both the heterotypic and homoeotypic divisions. 
Strasburger (’00) finds in the pollen mother-cells of Funkia two sorts of 
chromosomes — some short, hardly longer than broad, and others several 
times as long as the former. Perhaps the most remarkable observations of 
similar phenomena are those of Sutton (’02) on the spermatogonial chromo- 
somes of Brachystola. For nine cell generations preceding the formation of 
the primary spermatocytes he finds in each nucleus twenty-three chromo- 
somes of varying size ; of these, one is the ‘ accessory chromosome.* The 
remaining twenty-two may be arranged, according to their size, in eleven 
pairs, those of each pair being approximately equal in size, but differing in 
this respect from those of any other pair ; and the eleven pairs form a pro- 
gression from smallest to largest, such that the proportional difference 
in size between any two pairs in one nucleus is practically the same as that 
between the corresponding pairs in any other nucleus. It seems very 
natural, at least, to suppose that such constant differences in appearance 
between the individual chromosomes correspond in some way to the 
functional differences which Boveri’s experiments indicate. Sutton’s ob- 
servations also seem to make it certain that of each pair of similar chromo- 
somes in a somatic nucleus, one was ultimately derived from each parent ; 
the two elements of a pair are, therefore, in a sense, homologous, and 
are probably to be thought of as covering the same portion of the field 
of individual development. Very similar size relations have been found by 
Montgomery (’04) in the spermatogonia of Plethodon and Desmognathus . 
Somewhat analogous phenomena are those which consist in the 
appearance of one or two chromosomes differing greatly in size from 
the other elements within the nucleus. To this class belongs the peculiar 
body called by McClung (’99) the ‘ accessory chromosome/ which seems 
to have been first discovered by Henking (’91) in the spermatocytes of 
Pyrrhocoris . McClung (’02) has suggested a possible function for this 
particular element, namely, that it determines the production of male repro- 
ductive cells in the individual in which it occurs. Montgomery (’04) has 
classed this and similar aberrant chromosomes under the general term 
‘ heterochromosomes.’ Schreiner (’04) also finds marked differences in size 
between the chromosomes in the spermatogonia and spermatocytes of 
Myxine. In the spermatogonia two chromosomes are especially distin- 
guished by their larger size from their fellows. 
