138 THE DEVELOPMENT OF THE CHICK 
(Figs. 109 and 110). In each fold one can distinguish an amniotic 
or internal limb, and a chorionic or external limb meeting at or 
near the angle of the folds, the line of junction being marked 
by an ectodermal thickening, the ectamnion. Fusion of the 
right and left lateral folds begins at the head-cap, and progresses 
backwards in such a way that the right and left amniotic limbs 
become continuous with one another, similarly the right and 
left chorionic limbs; and, when fusion is complete, the amnion 
and chorion become separate continuous membranes. In this 
way the amnion extends, by the 27 s stage, back to the seventeenth 
somite (Fig. 105). At this time a new fold arises behind the 
rudimentary tail-bud and covers the tail precisely as the head- 
fold covers the head (Fig. 105); the tail-fold of the amnion then 
apparently is prolonged forward a short distance and soon meets 
the anterior lateral folds, forming a continuous lateral fold. Fu- 
sion continues until, about the 31s stage, the opening into the am- 
niotic cavity is reduced to a small elliptical aperture lying above 
the buds of the hind-limbs (Fig. 99). This then rapidly closes, 
but a connection, sero-amniotic connection, remains at the place 
of final closure. Elsewhere the separation of chorion and amnion 
is complete. 
The formation of the amnion is an extremely interesting 
process from the standpoint of developmental mechanics, and 
involves a number of details that are best understood after such 
a general review of the process as has been given in the preceding 
paragraphs. Returning then to the 12s stage for consideration 
of these details, we must first note that the extension of the meso- 
blast prior to this period has left an area situated in front of 
the head free from mesoblast (Figs. 65, 67, 71, 75, ete.). This 
area, in which the ectoderm and entoderm are in contact, is 
known as the proamnion. The formation of the amnion begins 
within this area by a thickening in the ectoderm (ectamnion) 
near the anterior boundary of the proamnion at a stage with 
about eight or nine somites. The thickening, which is very 
narrow, extends right and left, and turns backwards along the 
sides of the head to about the region of the middle of the heart, 
gradually becoming more peripheral in position and fading out 
(Fig. 76). It represents the junction of the amniogenous and 
choriogenous somatopleure and thus corresponds to the angle 
of the future amniotic folds. The head of the embryo lies in a 
