492 
Charles Edward Walker 
and upon which the determination of sex is supposed by several writers *) 
to depend. These ehromosomes have been variously called “accessorv”, 
“lagging”, “heterotropic”, “sex ehromosomes”, “diplosomes”, “gono- 
chromosomes”, “idiochromosomes” and “microchromosomes”. Thev 
have been demonstrated in a considerable number of animals, but thev 
apparentlv do not exist in the great majoritv. In the present eontri- 
bution an attempt is made to deal with ehromosomes of an ordinarv 
kind that are found in certain eells of the organism alwavs, and not with 
a special form which is found only in the eells of particular animals 
and of individuals of one sex and not in those of individuals of the 
opposite sex. In fact, it is hoped that the evidence produced here is 
favourable to the hypothesis that variations occur constantly among 
ehromosomes just as thev occur in organisms as a wliole. 
It would be impracticable under present conditions, to demonstrate 
variations among cliromosomes in somatic eells, for in the eells of the 
vast majoritv of organisms one chromosome is so much like another 
that it would be impossible to discriminate between them. In the first 
Meiotic (heterotype) division, however. in most of the organisms I have 
investigated, distinet and unmistakable ehromosomes appear with the 
greatest regidarity, both with regard to forms and the number of chromo- 
somes assuming each particular form * 2 ). Even in these cases great diffi- 
culties are encountered when any attempt is made to make observations 
which are sufficiently accurate to demonstrate without risk of error a 
definite Variation in the form and size of these Meiotic ehromosomes. 
In most organisms the ehromosomes are so small that even when a high 
power is used, the depth of focus is sufficient to take in the wliole chromo- 
some in spite of its position, and hence any difference in its position in 
relation to the optical axis will produce an apparent change in shape. 
The observations here described were made upon eells from the 
festes of Triton and Lepidosiren 3 ). In the Meiotic eells of these 
animals the ehromosomes are so large that the difficulty with regard 
1) Mc Clung, C. E., Biol. Bull., III, 1902; Stevens, N. M., Zool. Jahrb. Anat. u. 
Ont. XVIII, 1903; Joum. exper. Zool. III and IV, 1908; ibid. VI, 1909; Lefevre 
and Mc Gill, Amer. Joum. Anat. VIII, 4, 1908; Wilson, E. B., Joum. Exp. Zool. 
Vol. II, 2 and 4, 1905; ibid. Vol. III, 4, 1906; ibid. Vol. VI, 1 and 2, 1909. 
2 ) Baumgartner, Biol. Bull. 1904; Moore and Arnold, Proc. Roy. Soc. 1906. 
3 ) I am indebted to Dr. W. E. Agar, Zoological Department, Glasgow University, 
for the slides of the testis öf Lepidosiren which enabled me to make the obser- 
vations and without which the results obtained from Triton would have been unsup- 
ported. 
