436 DEAE: [VoL. XII. 
extend as two slits which encircle the lobes of the liver and 
separate them from the alimentary canal on the one hand and 
from the body wall on the other (Figs. 34-37). The two 

Fics. 35 and 36.— Sections through Embryo No. II. A, aorta; s, stomach; Z, liver; #, umbilical 
vein; x, bulbus aortae; 4, heart; 0, omphalomesenteric vein; g, lesser peritoneal cavity ;_/, 
foramen of Winslow; c, coeliac axis ; w 4, Wolffian body ; wd, Wolffian duct. 
pleural cavities do not communicate with each other around the 
lungs, leaving for them both a dorsal and a ventral mesentery. 
This appearance of the coelom about the lungs and the liver 
can be explained by the lungs and liver both growing into the 
two pleural cavities of embryo XII, and this has often made me 
think that the membrana reuniens of embryo XII is the main 
origin of what is called septum transversum in embryo II. If 
this proves to be the case, then the lower end of the mem- 
brana reuniens will form the ventral end of the diaphragm, and 
not the reverse. A stage between embryos XII and XVIII 
(Fig. 41) is required to elucidate this point. 
In the neighborhood of the stomach the peritoneal cavity on 
either side of it has become asymmetrical, as Fig. 36 shows. 
The mesentery has become bent to the left side, leaving a 
diverticulum from the right side which extends oralwards to 
the tip of the lung (Figs. 34 and 35) to form the beginning of 
