Boyal Microscojoieal Society. 
9 
vessels, imbedded in this membrane, embracing, as it seemed, a con- 
siderable portion of the larger and smaller capillaries, consisted of 
tubes of about tB-q- to yio- mm., the wall of which represented a clear, 
very transparent membrane. In this, a number of oval and irregular 
oblong nuclei were observed. The irregular form of many of 
these bodies lead me to regard them as the remains of those mother- 
nuclei from which, in the chorion of the smallest embryos, those 
ceils, forming the primary embryonic blood-vessels, were seen to 
arise. The larger vessels were filled with blood-corpuscles, which, 
by the action of a weak chromic acid solution, in which the speci- 
mens had been preserved, had lost their colouring matter, and thus 
appeared now in the form of clear double-bordered cells. The 
smaller ones, on the contrary, contained only a few single corpuscles, 
which appeared to have been forcibly pressed into their interior. 
This circumstance points to the probability, already mentioned in 
connection with the development of the coloured blood-corpuscles in 
man, that these bodies penetrate with every contraction of the heart 
only gradually into the newly formed but still closed blood-vessels. 
Among the blood-corpuscles, contained within larger vessels, a 
number of mother-corpuscles, bearing one embryo-corpuscle, were 
always observed. 
In examining the larger vessels, imbedded in the pia mater, of 
this period, viz. the small arteries and veins, their structure was 
already found considerably more complicated. Their walls, namely, 
consisted no more of a homogeneous membrane with large nuclei, 
but of two distinct layers. In place of the large nuclei, often 
irregular in form, a larger number of smaller ones, of an oval or 
roundish form, and imbedded between the two membranes, are now 
observed. These I presume are the descendants of a small number 
of mother-nuclei with small vesicles budding from their surfaces, 
always found in company of the smaller nuclei, which they seem 
to have preceded. The inner membrane probably represented the 
original tube of the vessel, while the outer one, distinguished by fine, 
longitudinal, fibrillous lines, represented its later fibrous coat. " In 
the arteries, the fibrous character of the latter was more strongly 
marked than in the veins. In the larger arteries and veins of the 
pia mater these two layers, forming the wall of the vessel, and 
distinguished by their regular, sharply-defined double contour, were 
found to be separated from each other by a third layer of irregular 
thickness. (Figs. 1 0 and 11). The latter most probably represented 
the muscular coat of the vessel in progress of formation. In those 
cases where it was found, the nuclei were imbedded in it, aud a 
number of them were observed to assume a position in which their 
long axis comes to lay at right angles with the axis of the vessel. 
No muscular fibres, however, could as yet be distinguished. 
The greater number of the vessels just described were filled 
