Royal Microscopical Society. 
11 
nular, spindle-shaped bodies, with an oval nncleus in their centre ; 
they are formed by the grannies of the plasma, arranging themselves 
into rows at the opposite poles of the nuclens. The latter, there- 
fore, forms the basis of development. After one row has thus been 
formed by the mutual arrangement and attachment of these gra- 
nules, and attained a certain length, a second one, starting also from 
the nucleus, begins to be formed in the same manner ; after this, a 
third and fourth, until their aggregate breadth equals that of the 
nucleus. But, as with the beginning formation of the succeed- 
ing rows of granules, or fibrils, those first formed continue to elon- 
gate by the attraction and appropriation of new granules, a spindle- 
shaped body is the result. In the formation of a vessel by a greater 
or smaller number of these bodies, they join — already during their 
own formation — each other laterally in such a manner that a part 
of the length of one overlaps another ; they finally are fused with 
each other into the form of a tube. 
In consequence of the various directions in which these spindle- 
formed bodies, or the bundles formed by them, are developed, they 
frequently cross or meet each other. TVhen this occurs, it is 
usually found that the point of the smaller body or bundle adapts 
itself to some extent to the lateral border of the larger. As the 
result of a number of such connections, or points of fusion, the 
meshes of the capillaries are formed. ( Fig. 8.) In this manner, 
capillaries, still in progress of development, are frequently observed 
to be fused with larger vessels, through which blood-corpuscles have 
already been circulating. The small arteries and veins are formed 
by the fusion of a number of bundles, composed of the spindle- 
shaped bodies. The contiguous borders of the latter can still be 
recognized on the vessels of the embryo of twelve weeks. Tig. 9. ) 
Frequently one of the larger vesssls is observed to be formed by 
the union of two smaller ones, the latter still being in process of 
development themselves. In the pia mater of human embryos of 
this age, the fusion of the spindle-formed primary elements into 
bundles, and of these into larger or smaller vessels, is observed to 
take place in an irregular manner, and also, as already mentioned, 
in various directions. It is owing to this circumstance that up to 
this period the form of the meshes, and even that of the vessels 
still in process of development, is quite indefinite. Equally irregu- 
lar are the numerous nuclei, contained in the walls of the arteries 
and veins, in size as well as distribution. 
The tube of the vessel being thus formed by a parallel apposi- 
tion and subsequent fusion of those fibrillous spindle-shaped bodies, 
is opened and distended by the blood-corpuscles penetrating into it 
from the neighbouring more fully developed vessels ; thus we meet 
in the newly-formed capillaries only a few blood-corpuscles, which 
have evidently been forced into them from larger neighbouring 
