182 EDWARD A. BOYDEN 
This includes a rectal ampulla passing below into a plicated 
‘zona columnaris.’ In the chick embryo it is bounded above by 
a single transverse plica and below by the urorectal fold already 
mentioned. Since this ampulla functions as a part of the cloaca 
in the adult bird, being the chamber in which both fecal matter 
and urine are retained, it seems better to keep the name copro- 
daeum, which Gadow applied to the most anterior of the three 
divisions of the cloaca. 
DEVELOPMENT OF THE BURSA OF FABRICIUS AND ASSOCIATED 
DIVERTICULA 
The primordium of the bursa is usually described as a swelling 
in the caudal wall of the cloaca, caused by the coalescence of 
vacuoles arising within the urodaeal membrane during the fifth 
and sixth days of incubation (figs. 31 and 18, bursa). While 
modeling earlier stages of the cloaca in relation to the develop- 
ment of the fenestra, I was much surprised to find that all chick 
embryos which had been incubated about four days showed a 
conspicuous diverticulum at the site of the caudal end of the 
cloacal fenestra, measured by its greatest extent (figs. 24 and 27, 
a; cf. figs. 16 and 18). The picture was further complicated by 
the occurrence, in several cases, of a second diverticulum (fig. 
24, b), arising as an outpocketing of the cloaca at the site of 
the cephalic end of the fenestra. Furthermore, diverticulum a, 
while originally developing as an invagination of the cloaca, soon 
became solid, then vacuolated, in continuity with the vacuoles in 
the developing urodaeal membrane (fig. 28, a), and then, by 
fusion of vacuoles, appeared to develop into the bursa itself 
(fig. 30, bursa). In view of these facts, it seemed not improbable 
that diverticulum a represented an earlier and more significant 
stage in the origin of the bursa than had hitherto been reported— 
a stage which had been overlooked because the cloaca had never 
been modeled during this period of its growth. This interpreta- 
tion, if true, would be of importance as bringing the origin of the 
organ into line with other derivatives of the gut. For it would 
show that it originated as an invagination of the entodermal tube, 
thus removing one more difficulty in the interpretation of an 
