THE FQ@TAL MEMBRANES AND APPENDAGES. 47 
the disappearance of the bucco-pharyngeal and cloacal membranes it communicates, 
both anteriorly and posteriorly, with the alimentary canal of the embryo. 
The development of the amnion in higher mammals is closely associated with 
the attachment of the ovum to the uterine wall and with the subsequent formation 
of the placenta. 
Before the ecelom is formed, the ectoderm in the chorionic area becomes attached 
to the uterine tissues by small villous outgrowths which invade the uterine mucous 
membrane. This attach- 
ment is most complete in 
the placental region, that ae 
is, around the margins of ™ _~@ 
the amniotic area. As la Mee 
the embryo is folded off 
from the blastoderm and 
the ccelom develops, both 
the embryo and the am- 
niotic area remain quite 
free from the uterine 
tissues, indeed, it may be 
said that, at this period, 
the embryo is suspended 
from the margins of the 
placental area by the 
amniotic membrane. 
As development  pro- 
ceeds the amniotic area 
increases in extent by 
interstitial growth, and 
thereupon the embryo, the 
membrane which suspends LE 
it being relaxed, sinks Ze) eae 
more and more into the ies 26 Os 
interior of the ovum, or, Qe pee 
to be more precise, into 
the ccelomic space, which, 
in the meantime, has con- 
siderably increased. At 
the same time the growth 
of theplacental areacauses — Py. 36.—TRaNsveRSE SECTIONS oF THE UTERUS AND DEVELOPING 
all parts of its inner Ovum or a FERRET. 
margin to converge, and Showing the formation and closure of the amnion folds, the completion of 
‘as the inner margins of the amnion, and the coincident ingrowth of the inner margins of the 
placental area of the blastoderm. 
the placental area are con- : 
: : A. Amnion. EN. Entoderm. SC. Stratum compactum. 
tinuous with the outer AF. Amnion fold. M. Muscular wall SS. Stratum spongiosum. 
margins of the amniotic ©. Ccelom. of uterus. SoM. Somatic mesoderm. 
membrane, the amnion is Ch: Chorion. NG. Neural groove. SpM. Splanchnic mesoderm. 
y : EC. Ectoderm, PY. Placental villus. UL. Unchanged layer of 
gradually carried over the uterine mucosa, 
dorsal surface of the em- 
_bryo till its margins meet and fuse. After the fusion of its margins the amnion 
separates entirely from the chorionic area, henceforth known as the chorion, and 
forms a closed sac which poe tors surrounds the embryo. 
On reference to Figs. 19 and 25 it will be seen that as the wall of the blasto- 
dermic vesicle is carried inwards over the dorsal surface of the embryo it is folded ; 
the outer part of the fold consists of the chorionic portion, and the inner part of the 
amniotic portion of the blastoderm. ‘The fold is called the amnion fold ; it is quite 
continuous round the whole margin of the embryo, but some parts of it are more 
advanced than others, or in other words the conver gence of the inner margin 
of the placental area of the blastoderm over the dorsal surface of the embr yo does 
not take place at the same rate or to the same extent in all parts. For convenience 
