Y O L K OF T H E H E N ' S EGG
257
but also because the yolk spheres undergo morphological changes
according to the surrounding medium; hence they might sometimes
appear to be living when examined microscopically.
Lepeschinskaya (1954) also used cinematography in her investigation,
but Zhinkin and Michailov (quoted by Balinsky, 1958) who repeated
her work were unable to confirm her results.
C. Further Differentiation of the Yolk Sac
The yolk has become covered by the yolk sac by about the 4th day of
incubation (Duval, 1884), except for a small region at the vegetal pole
which is known as the yolk sac umbilicus. About this time, the vitelline
membrane gives way over the surface of the embryo and slips down to
the region of the yolk sac umbilicus. It is in this region that the albumen
sac develops. This is a bag, embraced by the allantois and containing the
remains of the albumen, which becomes adherent to the lower wall of
allantois
shell
chorion
yolk sac
stalk'
shell
/membrane
ammon
yolk sac
stalk
extra
embryonic
coelom
Jf \ vitelline
membrane
folds of
yolk sac
vitelline
membrane
F I G . 24. Diagram illustrating t w o stages in the d e v e l o p m e n t of the yolk sac a n d
albumen sac. (After D u v a l , 1889.)