312 
l’R(3rESS0R E. RAY LANKESTER. 
(fig. 5). The appearance of these pale hollow fibres at once 
impressed me with a close similarity to the developing 
capillaries of Vertebrata. I had previously been able to 
demonstrate (see this Journal, vol. xviii, '' The Vascular 
Fluid of the Earthworm a Corpusculated Fluid ”) that the 
delicate vessels of the red vascular system of the Earthworm 
contain corpuscles, which are nothing more nor less than the 
liberated nuclei of cells which line the walls of those vessels, 
and, accordingly, I was prepared to find in the vessels of the 
Leech (in the congeners of which the existence of blood* 
corpuscles has been generally asserted) corpuscles deve- 
loping by the liberation of nuclei belonging to the cells 
which constitute the vascular wall. 
At the same time the structures which I had before me 
(PI. XXVII, figs. 4, 5, 6, 7) were not vessels forming part 
of the red vascular system of the Leech, but tubular fibres 
connected with the general system of brown-coloured fibres 
pervading the body of the Leech. The vessels, in fact, in 
which I observed this displacement of the nuclei were not 
filled with haemoglobinous fluid, but had clear contents, and 
evidently had not yet — if ever they were destined to have — 
any communication with the red-coloured vascular system. 
My next care, therefore, was to search for any evidence of 
the connection of such tubular portions of the brown- 
coloured system of fibres with the blood-vessels. 
Thin-walled blood-vessels of the Leech . — The finer vessels 
of the Leech’s haemoglobinous vascular system are remark- 
able for the exceeding delicacy of their walls. In no in- 
stance have I succeeded, by means of reagents, in demon- 
strating any nuclei in those walls ; and, further, by the use 
of nitrate of silver I have not succeeded in obtaining any 
indication of cell-outlines on those walls which I expected 
to be able to demonstrate since Mr. D’Arcy Power had, when 
working with me at the histology of the Earthworm, ob- 
tained such cell-outlines in the finer vessels of that animal 
(this Journal, 1878), where, moreover, nuclei are always 
readily demonstrated, even in the very finest capillaries. 
The structureless character of the thin-walled blood-vessels 
of the medicinal Leech is, then, a peculiarity which separates 
them from the apparently similar vessels of the Earthworm, 
and renders it not improbable that the two sets of vessels 
may have a diverse developmental history. Rarely I have 
found corpuscles consisting of free nuclei in the vessels con- 
taining red fluid in the Leech. Two such instances are 
drawn in PI. XXVII, figs. 13 and 14. In the one case the 
