STUDIES IN MAMMALIAN EMBRYOLOGY. 341 
blood that fills the trophoblastic spaces (fig. 43). This inter- 
action between maternal and embryonic blood in the region of 
the yolk-sac is of course considerably increased and intensified 
when the latter is folded into villi as was described above and as 
are figured on Pl. XXIV, fig. 44, the trophoblast becoming (as 
was more fully described on p. 300) at the same time better 
adapted for a very intimate interlocking with these vitelline 
villi. During this period a very effective omphaloidean placen- 
tation is thus undoubtedly present. In a preceding chapter we 
have already described the anatomical and histological pecu- 
liarities by which it is characterised, and at the same time the 
gradual decline of the nutritory facilities on the surface of the 
yolk-sac and their substitution by the allantoidean placenta- 
tion. The yolk-sac ceases to grow and is folded up, although 
its circulation never wholly disappears. The trophoblast 
against which it was applied becomes membranous together 
with the rest of the omphaloidean trophosphere and the decidua 
reflexa. 
The circulation of the maternal blood inside the omphaloidean 
trophosphere is not materially affected by the changes of this 
region during the developmental stages that follow upon that 
of fig. 42, these being more essentially phenomena of growth. 
Nor is it in the allantoidean region of the trophosphere, 
although, as we have seen in the preceding chapter, the changes 
by which the trophoblast is there affected during the advance 
of pregnancy, though at the outset directly comparable to what 
occurs in the omphaloidean region, lead to a very different final 
stage in which the interlocking of allantoidean and tropho- 
blastic tissue is much more intimate than ever was that of 
omphaloidean villi and trophoblast. Moreover, no phenomena 
of reduction such as those concerning the omphaloidean tropho- 
sphere are ever noticed in this region. It may be asked 
whether the Jayer, which in figs. 44, 48, 53, 54 is marked s/., has 
not a certain significance in regulating the flow of blood from 
the trophospongian cavities into the trophoblastic lacune. The 
stretched nature of its cells gives it somewhat the character of 
an intervening membrane. If we now suppose the delicate 
