FROM TWELVE TO THIRTY-SIX SOMITES 139 
depression bounded in front by the ectamnion, and on the sides 
by the amnio-cardiac vesicles of the body-cavity (Fig. 65). The 
floor of the depression is the proamnion. Just before the for- 
mation of the head-fold proper, the ectamnion in front of the 
head becomes irregularly thickened to 
such an extent as sometimes to present 
an actually villous surface (Fig. 77; cf. 
Fig. 67). 
The head-fold of the amnion begins 
to form at about the same time as the 
cephalic flexure. The great expansion 
of the body-cavity on each side of the 
head (amnio-cardiac vesicles) causes an 
elevation of the anterior angle of the 
ectamnion, and a pocket is formed by 
fusion of its lateral limbs. This slips 
over the head of the embryo with aid 
of the ventral flexure of the head just 
developing. Inasmuch as the anterior 
angle of the ectamnion is in the pro- 
amnion, where there is no mesoderm, 
and where the ectoderm is in immediate | 
contact with the entoderm, the ento- s 
derm as well as the ectoderm of the pro- Fic. 76.— Entire embryo 
amnion is drawn into the head-fold, so of 15s, to show the rela- 
that the latter is not at first a fold’ of  “0?sOtthe ectamnion- 
; ; a. ¢., Inner margin of 
the-somatopleure. But im. the chick the. amacenrdinc vesicles: 
proamniotic part of the head-fold is & ae eae ae 
never very extensive and does not at any topleure destined to form 
time extend back of the beginning of the body-wall. 
: : She: ae: B. Amniogenous soma- 
the mid-brain. Moreover, it is soon in-  topleure. 
vaded (Fig. 75) by the body-cavity, and C. Choriogenous soma- 
. i ae ‘ topleure. 
then the entoderm is withdrawn and 
becomes part of the general splanchnopleure. The proamnion 
ventral to the head is not invaded by mesoderm until a much 
later period. 
The ectodermal thickening marking the junction of amniotic 
and chorionic somatopleure extends backwards very rapidly and 
~— 
always precedes the origin of folds in any region. The lateral 
folds themselves appear to owe their origin to the progressive 
