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-fold, 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 31 s 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. Elsew^here 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 12 s 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, etc.). 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 



