CONNECTIVE AND VASIFACTIVE TISSUES OF LEECH. 311 
more minute brown-coloured granules which occur in various 
parts of the vaso-fibrous tissue. 
The vaso-Jlhrous tissues of the Medicinal Leech * — 
Whenever any portion of the muscular tissue or any organ 
of the leech is examined under the microscope^ a large 
quantity of dark brown fibres are sure to be noticed^ either 
forming an investment to the organ or ramifying over the 
field of the microscope (PI. XXVII, fig. 3). 
This system of fibres penetrates every region of the 
Leech’s body. It is continued superficially into the epi- 
dermis, beneath and between the cells of which it forms 
the characteristic pigment spots. It occurs between the 
bundles of muscular fibres, as may be seen in the section 
(fig. 13 g) of Mr. Bourne’s PI. XXV. It forms a 
loose sort of feltwork on the surface of the nephridium of 
the testes and of the alimentary canal. It is particularly 
rich and thickly developed on the wall of the blood-sinus in 
which the ventral nerve-cord is placed. 
When examined with a high power the fibres are seen 
to be of varying thickness, and many of them apparently 
hollow, so that the larger fibres are tubular rather than 
solid, and moreover, the tubular wall, according to the con- 
dition of tension or relaxation of the tissue, may be smooth 
or thrown into transverse rugae (PI. XXVII, fig. 1). Treat- 
ment with osmic acid, followed by picro-carmine, has the 
effect of demonstrating the nuclei of the cells by which this 
tissue is formed. The nuclei (PI. XXVII, fig. 4) are seen 
to be scattered at intervals in the course of the fibre, and 
when this is tubular they project internally into the lumen 
of the tube. The substance surrounding the nuclei is densely 
packed with very fine granules and does not stain red with 
picro-carmine as do the nuclei themselves. In many fibres 
(apparently old and fully differentiated) nuclei are either 
exceedingly rare or are not to be detected. It appears 
probable that in these fibres the nuclei of the original cells 
have disappeared and left only the finely granular cell- 
substance to play the part of a connective substance. 
In many of the tubular fibres, however, the nuclei are 
abundant, and show a series of transitional conditions, indi- 
cating that they gradually become freed from the granular 
cell-substance, and at first project into the cavity of the tube, 
and then become detached and lloat freely in it (PI. 
XXVII, fig. 4 and fig. 5). Some of the fibres of this 
system, which are definitely continuous with the darker and 
thick- walled fibres, have a pale aspect and relatively few 
granulations in the wall, whilst the nuclei are abundant 
VOL. XX. NEW SEK. X 
