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proves sufficiently that even here the circulation is not fully esta- 
blished, and that accordingly the embryo does as yet not receive 
its nutriment from the mother directly through the circulation of 
its own blood. 
From these observations on the formation of the primary em- 
bryonic blood-vessels we may infer that the larger vessels are very 
probably formed, in the same manner and simultaneously with 
other parts and organs of the embryo, from the embryonic cells of 
the area germinativa, while those smaller ones, imbedded in mem- 
branes, are formed by the coalescence of larger and smaller cells, as 
above described. 
Continuing our examination of the blood-vessels of the pia 
mater of the spinal marrow, which membrane in regard to its 
delicate structure and transparency is better adapted than other 
tissues for this purpose, — at a later period, in embryos, about eight 
to nine weeks old, we find that the embryo has entered another 
stage of development, in which these vessels are no more formed by 
the coalescence of cells. The pia mater here is still represented by 
an amorphous, granular membrane, in which, however, a thin layer 
of delicate connective-tissue fibres has already been developed, and 
containing besides a considerable number of oval or round nuclei. 
The latter consist of small granules, surrounded by a thin mem- 
brane. Among them, however, a number of mother-nuclei, with 
small buds arising from their surface, are also observed, showing 
that multiplication of these nuclei still occurs by the process of 
budding or gemmation. The blood-vessels within the pia mater 
show different stages of development. 
The formation of those vessels described in the preceding pages 
was brought about, as we have seen, by the coalescence and subse- 
quent fusion of cells, and so were the greater number of those 
within the tissue of the pia mater of embryos from nine to ten 
weeks old, through which, as they were filled with blood-corpuscles, 
the blood had been evidently freely circulating. A smaller portion 
of these, however, still in process of development, were observed to 
be formed by another process. Where, in the former instance, the 
formation of the tube of the vessel was accomplished by the coales- 
cence of cells, it is now by the fusion of elementary fibrils. The 
process of gemmation, though still occurring, as far as the multipli- 
cation of nuclei is concerned, has ceased for the formation of vessels ; 
the fibrilhus formative process of the blood-vessels has now com- 
menced. This consists in the formation of granular fibrillse, lying 
parallel to each other, and becoming in the course of their develop- 
ment fused into the form of a tube. 
Postponing at present a more minute demonstration of this 
process, let us return to the already formed blood-vessels of the pia 
mater of the last-mentioned embryos. The greater part of the 
