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Transactions o f the 
to an imperfectly developed embryo of an ovum of very small 
dimensions. The embryo, arrested in its growth, consisted only of 
a mass of embryonic cells, nuclei, fibrils, &c. 
It was in the outer membrane of this small ovum, which, like 
the umbilical vesicle, appeared to be normal in its development, 
that I observed the primary formation of the smaller blood-vessels, 
as well as a certain process of multiplication of nuclei, which, as far 
as I know, had up to that time not been observed in the tissues of 
vertebrated animals. 
From the examinations made on the chorion of this ovum, 
together with others of older embryos in various stages of develop- 
ment, it appeared that the formation of the smaller blood-vessels 
occurred in two different modes. The one, belonging to the earlier 
period of development, was observed to consist in a coalescence of 
certain cells, while by the other, at a somewhat later period, the 
formation of the blood-vessels was effected by the fusion of certain 
spindle-shaped bodies. 
A short time ago, however, long after these observations were 
committed to the paper, a human ovum, still smaller than the one 
already mentioned, was put into my hands, only a few hours after 
its abortion. This specimen was the smallest I had ever seen, for 
the embryo which it contained scarcely measured 6 mm. in length ; 
it was perfectly normal and fresh, and still buried in the membrana 
decidua, from which I had to remove it. 
While the examination of the umbilical vesicle of this embryo 
corroborated my former statements regarding the origin of the 
coloured blood-corpuscles, as we shall see hereafter, that of the 
number of mother-blood corpuscles are seen, and also some free blood-crystals. 
Magnified 465 diam. 
Fig. 12. — Blood-vessel from the pia mater of the spinal marrow of a human 
foetus 5 1 months old ; a, nuclei belonging to the fibrous coat ; b, nuclei belonging 
to the muscular coat ; c, the same, bearing vesicles ; d, cells of the epithelium 
in its formation. Magnified 465 diam. 
Fig. 13. — Small human ovum ; a, membraua.decidua vera ; b, decidua reflexa, 
separated from the ovum and pinned down. The light spot upon it represents 
the embryo seen through the membrane. Natural size. 
Fig. 14. — The embryo of this ovum. Natural size. 
Fig. 15.— The same enlarged ; a, the elongated coccyx, making a spinal turn. 
Fig. 16.— The same, showing the left side. The spinal turn of the inferior 
portion of the spinal column has been straightened, in order to show its proper 
shape; also enlarged. 
Fig. 17. — The same embryo (enlarged), after having remained about eighteen 
hours in a weak solution of chromic acid. Its various organs described in the text 
are here exhibited ; the umbilical vesicle has, for the sake of completeness, been 
represented entire, though at this time a piece of it had already been removed for 
examination. 
Fig. 18.— Represents the structure of the umbilical vesicle of this embryo, as 
described in the text. Magnified 465 diam. 
Fig. 19.— Embryonic blood-vessels from the chorion of the same specimen, 
containing embryonic blood- corpuscles in the form of granular nuclei. Magnified 
465 diam. 
