No. 2.] DEVELOPMENT OF THE HUMAN COELOM. 435 
The figures all illustrate this beautifully. But as the heart has 
rolled over the liver, and the septum transversum has undergone 
a quarter-revolution, the Cuvierian ducts and all have moved 
away from the head. This is by no means the end of the 
excursion of the septum transversum, as its dorsal end must move 
down and beyond the twelfth dorsal segment (compare Fig. 30). 
The pericardial cavity surrounds the whole heart, as the va- 
rious figures show. The cavity is traversed only where the 
large veins enter, and where the aorta leaves the heart. The 

Fic. 33. Fic. 34. 
Fics. 33 and 34.— Sections through Embryo No. II at the Points indicated in Fig. 30. Enlarged 
22 times. #&4, brachial plexus; a, aorta; 4 a, bulbus aortae; #2, pharynx; 4%, heart; 7, 
trachea; 0, oesophagus ; 7, jugular vein; 2, lung; vc, cardinal vein; C, Cuvierian duct. 
cavity completely surrounds the bulbus aortae to its origin 
(Figs. 32-35) in the ventricle. On the dorsal side of the 
heart the pericardial cavity is separated by a bridge for the 
transmission of the veins to the heart. Between the bulbus 
aortae and the entrance of the veins into the heart the peri- 
cardial cavity crosses the median line as three distinct open- 
ings, as expressed by the black areas in front of the trachea in 
Fig. 30. On the dorsal side of the heart on either side of the 
lungs the pericardial cavity communicates with the pleural 
cavities by means of two openings (Fig. 33), each of which is 
about .1 X .§ mm. in diameter. Farther on, the pleural cavities 
