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The result of the degeneration is first to bring the chorionic ecto- 
derm of the embryo into direct contact with the connective tissue of 
the mucosa uteri in consequence of the degeneration and resorption 
of the epithelium including the glands; second to allow the maternal 
vessels by simple expansion to come into contact with the foetal chorion. 
In the rodents the degeneration goes so far in the neighborhood of 
the chorion that all (or nearly all) the maternal tissue disappears, 
leaving the maternal blood to bathe the surface of the chorion or, to 
speak more exactly, of the chorionie villi. It is probable that similar 
changes take place in man, but in the earliest stages yet studied, 
they appear to have been already completed, so that in the region 
of the villi the maternal tissues have completely disappeared. HEINRI- 
cıus maintains that in the dog part of the glandular epithelium re- 
mains. 
Outgrowth of chorionic villi. The villi are restricted at 
first to the small placental area, but as that area may itself grow and 
take up more and more of the chorion, we get various modifications 
of the villous area. The more primitive types preserve the discoidal 
plan, illustrated by the rabbit; in other cases the placental or villous 
area expands until it forms a belt or zone around the ovum (carni- 
vora); but the development in the dog shows that the discoidal form 
is the earlier and changes into the zonary; in man the placental area 
spreads over the whole chorion. 
The villi appear to arise as outgrowths of the ectoderm only; 
after the outgrowths have attained .a certain size the mesoderm of 
the chorion grows into them. The villi grow into the maternal tissues 
and acquire great length and numerous branches by which their form 
becomes extremely complicated. Their form is highly characteristic of 
the various orders: it is known exactly only in man, but is certainly 
very different in various animals. 
The villi occupy only a part of the mucosa, there being always 
a considerable layer of decidual membrane left between the ends of 
the villi and the muscularis. 
The villi as here described consist of a cove of mesoderm covered 
by foetal ectoderm, and are essentially different from the ectodermal 
outgrowths assumed by DuvarL!) to exist in the rabbit. 
Union of the allantois with the chorion. We know two 
principal modifications of the union of the allantois with the chorion: 
1° The allantois joins the chorion early, and serves as the stalk con- 
1) Erroneously as I believe. 
