STUDIES IN MAMMALIAN EMBRYOLOGY. 301 
In the details of this process of metamorphosis of the tropho- 
blast there is no difference whether we consider the omphaloi- 
dean or the allantoidean diplotrophoblast. The latter, however, 
develops without having any other embryonic tissue immediately 
applied against it; the former is from the beginning contiguous 
with the splanchnic mesoblast of the yolk-sac, i. e. with the area 
vasculosa. This contiguity, as well as the relative significance 
of splanchnic and somatic mesoblast in the area vasculosa, which 
was already noticed above, and can be further gathered from 
fig. 43, not only facilitates the early interchange between foetal 
and maternal blood in this region, but also leads to the 
establishment of regular and massive villi, by which the 
yolk-sac is intimately interlocked with the omphaloidean tro- 
phoblast. An omphaloidean placentation is thus brought about, 
mesoblastic warts, ridges, and outgrowths being soon sur- 
rounded on three sides by the trophoblastic proliferation. In 
these yolk-villi the vessels of the area vasculosa send out loops. 
Certain of the chief afferent and efferent vessels of the area 
vasculosa, on the other hand, are found as if suspended by 
mesobiastic tissue in the cavity of the yolk-sac, adherent against 
the concave surface of the area vasculosa, and visible as pro- 
minent larger vessels, when these stages are inspected in the 
fresh state and looked at from the inside of the opened blasto- 
cyst. The extension of the area vasculosa is indicated in 
diagrams 82 to 35 by the thick red line; it is seen to adhere 
to the trophoblast along a zonary region, which embraces 
about one half of the entire surface of the blastosphere. In 
the further development of this region of vitelline circulation, 
intimately interweaving with the proliferating omphaloidean 
trophoblast, we must clearly distinguish a climax and from 
thence a diminution and a retrogressive development. 
The climax is attained at about the period that the allantois 
commences to spread out against the allantoidean trophoblast 
(diagrams 32 and 33). The thick omphaloidean villi and their 
surroundings are represented with a view to their histological 
detail in fig. 44; embryonic blood-corpuscles are seen to circu- 
late in the very top of these villi, It should be remarked that 
