444 MALL. [Vor. XII. 
mass of tissue has turned a quarter revolution. This is accom- 
panied by the extreme flexion of the head, as represented in 
embryo No. II. At this time the septum transversum has 
descended to the lower part of the cervical region. Now the 
septum begins to turn in the other direction again, for with the 

Fia. 43. — Section through the Point of Communication between the Pleural and Peritoneal Cavities 
in Embryo No. IX. Enlarged 15 times. 7, seventh rib; Z, lung; Zz, liver; 1%, ventral 
mesentery of liver; a, aorta. The diaphragm is complete on one side, X, while it is incomplete 
on the other. 
development of the neck the ventral end of the septum becomes 
the fixed point and the dorsal end moves more rapidly. The 
successive stages in the movement of the septum are best shown 
in the diagrammatic Fig. 41. 
Fig. 42 shows the septum transversum on the ventral side of 
the stomach and the pleural cavity communicating with the 
peritoneal at the point X. The Wolffian body and the supra- 
