74 > ANIMAL PHYSIOLOGY. 
other, meet after being joined by corresponding lateral folds. 
Fusion and absorption result at this meeting-point, in the 
inclosure of one cavity and the blending of two others. These 
we. 
Fig. 80.—Diagrammatic longitudinal section through the axis of an embryo chick (after Foster 
and Balfour). N.C, Neural canal; Ch, notochord; Fg, foregut; F. So, somatopleure ; 
F. Sp, splanchnopleure ; Sp, splanchnopleure, forming lower wall of foregut ; Ht, heart; 
pp, pleuroperitoneal cavity ; Am, amniotic fold ; E, epiblast ; 1, mesoblast ; H, hypoblast. 
folds constitute the amniotic membranes, the inner of which 
forms the true amnion, the outer the false amnion (serous mem- 
brane, subzonal membrane). Within the amnion proper is the 
amniotic cavity filled with fluid (liquor amnit), while the space 
between the true and false amniotic folds, which gradually in- 
creases in size as the yelk-sac diminishes, forms the pleuro- 
peritoneal cavity, body cavity, or celom. The amniotic cavity 
also extends, so that the embryo is surrounded by it or lies 
centrally within it. The enlargement of the coelom and exten- 
sion of the false amniotic folds lead finally to a similar meeting 
and fusion like that which occurred in the formation of the true 
amniotic cavity. The yelk-sac, gradually lessening, is at last 
withdrawn into the body of the embryo. 
Fig. 80 shows how the amniotic head fold arises, from a 
budding out of the epiblast and mesoblast at a point where the 
original cell layers of the embryo have separated into two folds, 
the somatopleure or body fold and the splanchnopleure or vis- 
ceral fold, owing to a division or cleavage of the mesoblast 
toward the long axis of the body. Remembering this, it is 
always easy to determine by a diagram the composition of any 
one of the membranes or folds of the embryo, for the compo- 
nents must be epiblast, mesoblast, or hypoblast; thus, the 
splanchnopleure is made up of hypoblast internally and meso- 
blast externally—a principle of great significance, since, as will 
be learned later, all the tissues of the body may be classified 
simply, and at the same time scientifically, according to their 
embryological origin. 
