572 REPORT— 1905. 
or less definite thread-like structure and its aggregation into chromosomes. In 
any case such possibilities must be taken into account in considering the signifi- 
cance of such nuclear rearrangements, and if any of them can be definitely 
explained in this way the final solution of the problem may be much simplified. 
As Professor Jennings points out, ‘anything which promises a bridge from 
the inorganic to the organic, from the physical to the vital, demands attention.’ 
Such experimental investigations pave the way for further progress, and, if 
carefully considered, ‘ must result finally either in the discovery of the real factors 
at work, or in the recognition that we are dealing with a new class of factors not 
found in physics.’ 
Validity of Cell Structure as seen in Fixed and Stained Preparations. 
Our knowledge of the minute details of cell structure and nuclear differentia- 
tion depends upon the appearances presented by cells which have been fixed in 
various reagents and subsequently stained, and it is not an easy matter to deter- 
mine in how far these are artificial and in how far they are actual structures existing 
in the living cell. The researches of Fischer, Hardy, Mann, and others have shown 
that on the precipitation of proteids by reagents structures are produced which 
were certainly not present originally, and which resemble those often observed in 
fixed cells. From a consideration of such facts it has been suggested that many of 
the details revealed in fixed cells, such as centrosomes and centrospheres, with their 
fibrillar radiations, are produced artificially and have no real existence. It is 
unfortunate that so little attention has been paid to the examination of living cells, 
for the structures which can be seen in them are, so far as they can be revealed by 
the microscope, always like those seen in fixed preparations. 
Differentiation of Structure Visible in the Living Cell. 
The amount of differentiation visible in the living cell in favourable objects is 
very considerable. Not only can chloroplasts, starch-grains, nucleus, leucoplasts, 
pyrenoids, &c., be clearly seen, but also a very considerable amount of detailed 
structure. Chromosomes have been seen in the living cell by many observers— 
Treub, Strasburger, Behrens, Zacharias, and others. The series of figures pub- 
lished by Strasburger of nuclear division in the staminal hairs of Tradescantia 
show the whole process of chromosome formation and separation into two daughter- 
groups, except the longitudinal division. 
In the same object Demoor and de Wildeman have also been able to detect 
the spindle fibres and connecting fibres. These were not seen by Strasburger ; and 
Zacharias, who has more recently made observations on staminal hairs, was also 
not able to detect them. Nevertheless Strasburger mentions that in some cases 
connecting threads were visible at a late stage in the division between the daughter- 
nuclei, and Treub also describes a similar phenomenon in some cases during the 
nuclear division in the ovules of an orchid. 
In Spirogyra, Strasburger has given a full account of nuclear division in the 
living cell. Large species of this alza are very favourable objects for this work, 
and he has shown that in such species the spindle figure as well as the connecting 
fibres can be seen in the living cell. Wildeman has also seen and figured them ; 
but Behrens states that spindle fibres and connecting threads are not visible in 
Spirogyra during life. 
My own observations upon a large species of Spirogyra which I have had an 
opportunity of investigating entirely support the view that these structures are 
visible in the living condition. The resting nucleus is large, and contains a large 
pale refringent nucleolus (sometimes also a smaller one) which sometimes appears 
granular; the cavity of the nucleus is filled with a homogeneous-looking substance 
which appeared in some cases to be finely punctate or granular, The nucleus is 
surrounded on all sides by a granular cytoplasm the granules of which are in con- 
stant motion, especially just previous to nuclear division; it is suspended ta 
