12 
Transactions of the 
vessels, as they are seen to distend the calibre of the vessel only to 
a certain distance. (Fig. 8.) The further development of the ves- 
sels is moreover promoted by the nutrient matters contained in 
the liquor sanguinis. While a number of blood-vessels are thus 
gradually approaching their final development, and in consequence 
the circulation of the blood is further extended, others are still 
formed by the same process. 
At what period the formation of new vessels in the human 
embryo ceases to take place I have not determined, as my examina- 
tions concerning this subject were extended only to the vessels of 
the foetus, about 5} months old, in the pia mater of the spinal 
marrow of which new capillary vessels were still seen in the 
process of formation. 
The further development of the newly-formed capillary vessels 
consists, as already mentioned, of the fusion of the granular fibrils 
of which they still consist, into a structureless membrane. In the 
small arteries and veins, formed by the fibrillous process, the gra- 
nular fibrils are developed into permanent, smooth fibrillae of con- 
nective tissue, while the diameter of the vessel increases by the 
continuous formation of new fibrils from the plasma of the blood 
circulating within them. The multiplication of the nuclei takes 
place by gemmation. 
In the pia mater of the embryo of about nine weeks we found, 
as will he remembered, the largest vessels already composed of three 
distinct layers, though in the middle and inner one no particular 
structure could as yet be recognized ; the outer layer alone was 
composed of delicate fibres of fibrous tissue. In the succeeding 
periods also, it is especially this layer which is most distinctly 
developed. The first traces of the formation of the muscular layer 
consist in the appearance of a larger number of oval nuclei, the long 
axis of which lays at right angles with that of the vessel. The 
inner layer, however, which in the beginning was equal in thickness 
to that of the outer one, is gradually rendered more indistinct, 
perhaps by the increasing development of the latter. 
In the foetus of months, finally, the outer coat of the larger 
vessels of the pia mater has attained sufficient consistency to permit 
its separation from the loose areolar tissue by which these vessels 
are surrounded, thus affording an opportunity for a closer examina- 
tion of their respective coats. In the walls of such vessels of about 
t £ mm. diameter, and freed from the loose fibrous tissue surround- 
ing them, only two layers, distinct from each other, are recognized 
(Fig. 12). The outer of these, representing the fibrous coat of the 
vessel, consists of delicate wave-like fibres of connective tissue, holding 
numerous, more or less oval nuclei, their long axis being parallel to 
that of the vessel. They are distinguished by a fine double contour, 
and filled with small granules. The inner layer, which appears 
