THE CLOACA IN BIRDS 185 
of the latter is subdivided by longitudinal plicae into eleven 
(or twelve) grooved chambers. A cross-section of the bursa 
during the fifteenth day (fig. 12) shows that in the interval be- 
tween the eleventh and fifteenth days some of the primary plicae 
have cleft the central cavity deeper than others, so that the 
eleven primary cavities have become tributary to six or seven 
secondary channels, opening into the main cavity after the 
manner that minor and major calyces open into the pelvis of 
the kidney. 
Fig.12 Transverse section of a model of a 55-mm. chick embryo, H.E.C. 
1968:14 daysand5 hours. X 28. bl. v., blood vessel; cav., cavity of bursa; cor., 
cortex of follicle, derived from tunica propria; med., medulla of follicle, derived 
from epithelium; musc., muscularis; ¢t. p., tunica propria. 
Histogenesis begins with the appearance of the primary plicae 
and ends in the formation of spherical masses of lymphoid tissue 
(the ‘follicles’ of Stannius). Each follicle consists of a cortex 
and a medulla, the medullae or cores of the follicles (the ‘¥olli- 
kelkeime’ of Stieda) being the first to appear. These grow out 
into the tunica propria as solid buds of epithelium which soon 
become clothed peripherally with a cortical layer derived from 
the subjacent connective tissue (fig. 12, cor. and med.). In 
the course of development the follicles grow larger and larger 
until they meet, the resulting pressure molding them into a 
polyhedral shape. The walls of the bursa thus become greatly 
thickened, resembling somewhat in gross appearance the walls 
