No. 2.] DEVELOPMENT OF THE HUMAN COELOM. 427 
horseshoe-shaped ridge of tissue around the neck of the embryo 
to the ventral side of the pericardial and pleural cavities and 
parallel to them. The median portion is composed of the 
septum transversum, and each wing of the shoe is the mem- 
brana reuniens, one on either side of the embryo. Its general 
direction in this stage is parallel with the long axis of the 
embryo, and within each wing there is an omphalomesenteric 
vein. 
Origin of Pericardial Cavity. — With the pericardial cavity 
opening into the extraembryonic coelom on either side as a 
basis, it is possible to trace back the pericardial cavity to its 

Fic. 22. Fic. 23. Fic. 24. 
Fics. 22-24.— Three Stages to show the Development of the Blastodermic Layers at the Head End 
of the Embryo. Fig. 22, Hypothetical Stage. Fig. 23, Embryo No. III. Fig. 24, Embryo 
No. XII. V, vein; 4, pharynx; az, amnion; 7z Z, medullary plate; % c, pericardial 
cavity ; S, Seessel’s pocket ; #z, mouth; ¢, thyroid ; 7 and 2, first and second branchial pockets ; 
~, pleural cavity; # ~, membrana reuniens; wv o #, omphalomesenteric vein, which is 
expressed as a dotted line; O, communication between right and left body cavities on the 
ventral side of the umbilical vesicle. 
origin. Figs. 16 and 24 show that the ventral wall of the peri- 
cardial cavity is composed mostly of mesoderm. This is the 
portion of the membrana reuniens which is composed of meso- 
derm, as the sections, Figs. 18 and 19, show. An earlier stage 
is shown in the diagrammatic Fig. 23. It is taken from 
embryo No. III. In this specimen, since the ectoderm of the 
amnion has not reached completely around the body, as both 
the sagittal and transverse sections show (Figs. 23 and 25), 
it is evident that the pericardial space is first covered on 
the ventral side with mesoderm and later the ectoderm is added 
when the amnion begins to close over the head. In embryo 
III the canals communicating between the pericardial space 
and the extraembryonic coelom are not as long as in embryo 
