6 
Transactions of the 
what older ova, as well as in the pia mater of the spinal marrow of 
a very small embryo. As has been remarked before, some of the 
vesicles or cells, springing from the mother-nuclei, were probably 
developed into ordinary nuclei, pertaining to the fibrous tissue of 
the membrane ; a portion of these might even assume the function 
of mother-nuclei and give rise to other vesicles. The rest, however, 
especially the larger ones, were observed to take part in the forma- 
tion of the embryonic vessels of the membrane. This seems to be 
effected by a number of these cells, arising from their respective 
nuclei at different points, and meeting each other in such a manner 
as to form a row. (Figs. 3, 4, and 5.) To form this row, however, 
they were not placed, as might be expected, in regular order, cell by 
cell ; on the contrary, the nuclei, from which they were seen to 
arise, were distributed throughout the membrane without any 
special arrangement or order ; neither was their size and form more 
regular. The formation of a vessel, of course, is brought about by 
the coalescence of cells arising from neighbouring nuclei, and oppo- 
site to each other. The tube of the vessel is not necessarily formed 
by one single row of cells ; for when, by the meeting of two cells 
unequal in diameter, a vacancy is left, this will be filled up by a 
third cell, arising from the nucleus of one of the former. Earely 
less than two cells were seen to arise from one nucleus, but more 
frequently five to six. Often, large irregularly-formed cells were 
met with in the course of a vessel, the nucleus of which was marked 
on its margin by a number of crescent-shaped indentations. In 
these cases, the large cell was formed by the absorption of the con- 
tiguous walls of a number of small ones, which originally had arisen 
from one and the same nucleus, of which the crescent-shaped inden- 
tations were indicating the place of their origin. (Fig. 4.) After 
the tube of the vessel is fully formed by the absorption of those 
portions of the cell-walls contiguous to each other, the nucleus 
assumes, as it seems, by a gradual rounding of its indentated 
margin, a round or oval form. Yery frequently, however, by the 
production of a number of cells, it is deprived of so much of its 
substance, that finally nothing of it remains but a long indentated 
body, which by the rounding of its angles assumes an oblong form. 
This form is often met with in the walls of young vessels, long after 
the cellular formation of the blood-vessels has ceased to occur. In 
some instances, especially with the smaller vessels, we meet with two 
cells arising from two opposite points of one and the same nucleus, 
the latter occupying the whole diameter of the row (Figs. 3 and 4), 
thus seeming to form an obstacle to the opening of the vessel. 
This, however, is overcome, I think, by the growth of the cells, at 
the expense of the substance of the nucleus, of which at the end of 
the process only little will be left. Thus the absorption of the 
entire nucleus will sometimes be effected. In the nervous tissues 
