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Transactions of the 
with coloured blood-corpuscles, of which the greater portion had 
lost their colouring matter by the action of the chromic acid solu- 
tion, and appeared in the form of clear double-bordered cells, while 
the rest, comprising the younger corpuscles, had remained un- 
changed ; in many vessels they were crowded in such a degree that 
the former, by mutual pressure, had assumed a hexagonal form. 
A number of mother-blood corpuscles were always found among 
them ; they had also become discoloured, while their embryo - 
corpuscles had retained their colouring material. In many of the 
larger vessels, besides the blood-corpuscles, a number of coloured 
molecules and also small blood-crystals were found. Sometimes even 
blood-corpuscles were met with, in the substance of which small 
crystals were found to be imbedded. Many vessels were completely 
coloured by the hematin escaped from the blood-corpuscles. 
The blood-vessels were probably found, as already mentioned, by 
the coalescence of cells. But besides these, I observed in the same 
pia mater, a number of capillary vessels in progress of formation, 
_ the fibrinous composition of which attracted my attention. The 
result of a series of close examinations, which I subsequently made 
on the smaller vessels of the pia mater of this embryo, as well as of 
a number of others of different periods of development, convinced 
me that these vessels were formed in a different manner from the 
preceding, to which I have already alluded. This consists in the 
formation of granular fibrillae, laying parallel to each other and 
becoming eventually fused into the form of a tube. (Figs. 8 and 9.) 
In connection with the development of the coloured blood-corpus- 
cles, I have already pointed to the different modes of origin and 
development during the earlier and later periods of embryonic life. 
In the development of the embryonic blood-vessels, according to 
these observations an analogous change in the mode of formation 
seems to take place. The exact time at which the first or eellular 
process of formation is superseded by the second, or fibrillous, I am 
at present unable to state. 
In an embryo of 16 mm. in length, I found all the vessels of 
the liver still to consist of granular fibrillse, while those of the 
chorion and its villi, as well as those of the pia mater, were formed 
by the cellular process, above described. It appears, therefore, that 
the formative process of the vessels is not always the same in dif- 
ferent organs. In the pia mater, as far as I am able to judge, 
the fibrillous formative process seems to have already commenced 
about the eighth or ninth week, and to be in full operation some 
weeks later. We will now describe its course, as I observed it 
in the pia mater of human embryos of about twelve weeks, where, 
at this period, the small arteries and veins, as well as the capillaries, 
prove to be quite numerous. 
The primary elements of the vessels to be formed represent gra- 
