450 MALL. [VoL. XII. 
lung and its apex away from it, as in embryo IX, shown in Figs. 
42 and 48. The pericardial space has now become separated 
completely from the pleural, although both have grown at 
about the same pace. From now on the pleural space grows 
more rapidly than the pericardial, as shown in Fig. 49. I have 
a number of embryos which represent intermediate stages 
between embryos IX and XXII, and all of them confirm the 
idea that the pleural space develops first and then is followed 
by a growth of the lung. Fig. 50, which is a section of 
embryo No. XLV, shows a marked increase in the size of the 
lung, but the heart and pericardial space are of about the same 
_ FR Ky 
Fic. 47. Fic. 48. Fic. 49. 
Fic. 47-49. — Outlines of the Pleural and Pericardial Cavities to show their Relative Position and 
Size. Enlarged 7 times. Fig. 47, Embryo No. II; Fig. 48, Embryo No. IX; Fig. 49, 
Embryo No. XXII. H, position of heart ; Z, position of lung. 
size as in embryo XXII. A much later stage is shown in 
Fig. 51. The scale of enlargement is only half that of Fig. 50, 
and when this is considered it is again seen that the heart has 
not grown very much but the lung has developed enormously. 
It is therefore seen that at first the pericardial cavity is on 
the oral side of the pleural, then on the ventral side, and is 
finally enclosed by the pleural cavity growing over it. 
The growth of the pleural cavity over the pericardial accounts 
for the location of the phrenic nerve in the adult. In Fig. 47 
the nerve passes to the septum transversum from the lateral 
body wall and it is gradually separated from it by the descent 
of the septum and by the growth of the pleural cavity between 
the nerve and the body wall, thus locating the nerve in a mem- 
brane, as Figs. 48 and 49 will readily explain. 
The expansion of the peritoneal cavity is by no means as 
simple. In it there are many bands and mesenteries, as 
