STRUCTURE OF CERTAIN CHROMOSOMES 7 
weaker ; and stain in a half per cent. (or weaker) solution of 
haematoxylin till the sections appear dark grey, not black 
(about twenty-five minutes in a virgin solution, or not more 
than four in one which has already had several slides passed 
through it); and differentiate in the iron solution for at least 
a couple of minutes after the sections, exammed in water, 
seem sufficiently extracted. For the stain always appears much 
lighter in water than in balsam. For the study of the sheath, 
mount in Gilson’s camsal balsam or euparal, rather than in 
balsam. 
(b) Descruptive. 
It will be best to begin with the study of some chromosomes 
taken at the anaphase, the most favourable moment, figs. 3 to 
18." The chromosome of fig. 6, which may be taken as typical, 
is from a spermatogonium of Salamandra maculosa. 
It shows the following two (not three) constituents, namely 
a chromatic (basophilous) axis, and an ‘achromatic’ (i.e. 
acidophilous) sheath enveloping this. The chromatic axis 
is by far the more conspicuous of the two ; so much so that, as 
the sheath is seldom conspicuous enough to compel attention, 
the axis alone is all that is usually seen, and is therefore generally 
taken as the whole of the chromosome. But the sheath (which 
is none other than the achromatic membrane described by 
Janssens, ‘La Cellule’, xxii, 1905, p. 413 and figs. 42 to 50 and 
52 to 55, as found in the auxocytes of Batrachoseps 
attenuatus), though it is a difficult object on account of 
its great tenuity, can generally be made out in well fixed 
specimens. 
The axis has approximately the form of a cylinder, showing 
a circular section. But it is not a cylinder of regular calibre, 
for it 1s generally somewhat dilated at the ends, as seen in 
figs. 6, 7, 14 (and to a slighter degree in figs. 3 and 4), thus 
becoming somewhat claviform. And it is generally notably 
narrower at the polar bend than elsewhere, figs. 3, 4, and 
1 For the objects from which these figs. are taken, see the Explanation 
of the Plates. 
