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Transactions of the 
ment only attained the more perfect form in which they were dis- 
covered. In consequence, however, of the embryo being arrested 
in its development at a very early period, no heart or blood-vessels 
were formed to communicate with the system of canals of the umbi- 
lical vesicle for the establishment of the provisional circulation. The 
primitive blood-corpuscles, in the form of round nuclei therefore, 
being thus prevented from escaping from the canals, accumulated 
there, as well as in the interior of the follicles, where they were 
eventually developed into perfect coloured blood-corpuscles. The 
development of the umbilical vesicle with its follicles, and the 
coloured blood-corpuscles found within the walls of the latter, of 
course, must be regarded as an abnormal phenomenon of nature, 
almost in the same light as those isolated parts of embryos, as frag- 
ments of jaws with teeth, &c., found sometimes in the interior of 
ovarian cysts. 
As regards the presence of round granular nuclei within the 
follicles and canals of the umbilical vesicle, as well as within the 
rudimentary heart and the primary vessels of the chorion of the small 
normal embryo, it seems to indicate that at an early period, when 
the blood commences to circulate through the vessels of the provi- 
sional circulating apparatus, it carries small round granular nuclei, 
which are either gradually metamorphosed into coloured corpuscles, 
or replaced by such originating directly in the cells of the follicles. 
The latter seems to be the more probable, as a number of pale, 
smooth nuclei were observed in some of the cells. 
The formation of the primary blood-vessels in the chorion of 
this embryo was observed to be effected, as mentioned before, by the 
fusion of spindle-shaped cells. In the chorion of that abnormal 
ovum, however, the blood-vessels, as my former observations showed, 
were formed by the fusion of certain round cells, arising in the form 
of buds from the nuclei, distributed throughout the membrane. 
Now, it might be presumed that, as the development of the embryo 
in this case was abnormal, the mode of formation of the blood-vessels 
would probably be the same ; but remembering that I observed 
this same process still taking place in the chorion and the pia mater 
of much older embryos, of from 16 to 18 mm. in length, this argu- 
ment loses its force. With these facts before us, we can only pre- 
sume that the blood-vessels of the provisional circulatory apparatus, 
during the earliest period of development of the human embryo, 
are formed by the fusion of spindle-shaped cells, while somewhat 
later the formation of the permanent vessels is effected by the fusion 
of smaller or larger cells or vesicles arising by the process of gem- 
mation from the nuclei, as described in the first pages of this 
article. At a still later period this process, too, ceases in its turn, 
and the vessels are formed, as we have seen, by the fusion of granular 
spindle-shaped bodies. 
