Royal Microscopical Society. 
i 
of the human embryo, I observed the nuclei, belonging to the pri- 
mitive fibrils of the axis cylinder, disappearing during the process 
of development by fusing with the latter. In the small ovum under 
discussion, the formation of the blood-vessels, as described above, 
was observed to occur throughout the entire chorion to the roots of 
the villi, but had as yet not extended into the latter. 
In examining the chorion of an embryo of about 16 mm. in 
length, we still find a considerable number of mother-nuclei with 
vesicles, budding from their surfaces, imbedded in the tissue. The 
structure of the membrane is on the whole further developed, for 
the fibres of the connective tissue are more definite in their form, 
and the cells of the epithelium placed nearer to each other. The 
stems of the tuberous villi, arising from the surface of the mem- 
brane, have gained in thickness, also their epithelium, which now 
consists of two layers of cells. (Figs. 6 and 7.) The fine fibres of 
the connective tissue of the villi themselves are fully developed and 
have lost their granular aspect. But the smaller blood-vessels, 
although more numerous, and having penetrated into the villi, still 
consist of cells, only partially fused with each other. (Fig. 6.) 
Among the blood-vessels of the pia mater of the spinal marrow of 
embryos of about the same period as the preceding, some, especially 
the larger ones, were found to contain a considerable number of 
coloured blood-corpuscles, showing that the circulation of the blood 
through them had already been established ; while others, especially 
the capillaries, still showed their original formation by the cellular 
structure of their walls, and the entire absence of blood-corpuscles 
in their interior. 
The muscular structure of the heart at this period consists of 
embryonic muscular fibrillae, each of them representing a row of 
simple granules. The walls of the larger blood-vessels, such as the 
aorta, umbilical vessels, &c., consisted of a granular, amorphous, 
already partially fibrous matrix, in which a considerable number of 
oval or even round nuclei were imbedded. On their inner surface a 
number of clear cells, containing a coarsely granular nucleus, devoid 
of an enclosing membrane, were met with. These cells, being 
present in various other tissues of the embryo, probably stand in 
some relationship to the multiplication of the nuclei. 
Judging from the development of the heart and the larger 
blood-vessels of this period, it would seem that the circulation of the 
blood were already complete ; that this is not the case is proved by 
the almost entire want of fully formed capillaries in a number of 
organs which I examined. For this reason I am inclined to 
believe that it occurs only through anastomosing branches of the 
smaller arteries and veins. Further, the fact that the smaller 
vessels of the chorion, and particularly those of its villi, still mostly 
consist of cells, the contiguous walls of . which are as yet not fused, 
