504 
J. GEAHAM KERR. 
small and inconspicuous as compared with the left (text-fig. 8, 
B and c). 
By stage XXXV (text-fig. 8, d)^ the two cusps of the lung 
rudiment are seen to be growing actively. They are now 
approximately of equal length, and they extend back for a 
short distance along the dorsal side of the main mass of yolk, 
their tips lying in the shallow valley upon its dorsal surface. 
Subsequent stages in the development of the lungs will be 
made clear by text-figs. 4, a — d (pages 490, 491) repi-esenting 
dissections of young Lepidosirens of stages XXXII, XXXV, 
XXXVI, and XXXVII. It will be noticed how the lungs 
gradually extend backwards, at first in the shallow valley, 
already mentioned, on the dorsal side of the enteron. Later 
on they lie well above the surface of the enteron and, indeed, 
eventually, as will be shown later, dorsal to the entire 
splanchnoccele. 
The lung rudiment is at first quite solid, but a cavity soon 
begins to develop in it, the pharynx at this level still remain- 
ing solid. By about stage XXXII in Lepidosiren the lung 
has become hollow throughout, although the pharynx at the 
level of the glottis is still solid. It is not till about stage XXXV 
that the lumen of the pharynx is completed, and there is an 
open glottis leading into the lung. It was at this same stage 
that the young Lepidosii-ens were observed first to SAvallow air, 
so we may take it that the lung is functional practically from 
the time at which its communication Avith the exterior is 
established. Con-elated Avith this the mesodermal sheath of the 
lung is highly vascular by this stage (XXXV), and in places 
the blood-vessels are seen to penetrate the endodermal lining. 
Torsion of the lung. — During the course of its develop- 
ment the lung undergoes a complicated process of torsion, 
Avhich introduces considerable difficulties in the Avay of its 
investigation. Indeed, had not extensive material been 
available, it would in all probability have proved impossible 
to make out exactly Avhat happens. The difficulty is due to 
the fact that torsion of the lung rudiment takes place suc- 
cessively in two opposite directioiis. 
