THE CLOACA IN BIRDS 183 
organ which has been a bone of contention among anatomists 
since its discovery by Fabricius. The chief obstacle to this 
conclusion, however, arose from the examination of a single 
specimen pictured in figure 29. In this figure diverticulum a 
seemed farther removed from the anal plate than in other speci- 
mens, thereby leaving a vacuolated area between it and the anal 
plate (labeled bursa in the drawing) which might well develop 
into the bursa of figure 30, there recognized as the definitive 
bursa by the coalescence of the vacuoles. To solve this difficulty 
it became necessary to collect a series of graded embryos of other 
species of birds. Subsequent reconstruction of domestic duck 
and pheasant embryos left the matter still more confused, as in 
these forms the diverticula were present and similar to those in 
the chick, but less pronounced. Finally, an examination of 
tern embryos, birds some distance removed from the gallina- 
ceous tribe, brought the desired results. In these forms, as can 
be seen in figure 36 to 38 and reconstructions of earlier stages, 
no diverticula are developed at all, and the bursa arises directly 
from the region adjoining the anal plate, as a thickening of 
epithelium in continuity with that plate and restricted to the 
territory lying between it and the site of the caudal intestine 
(fig. 1). A reexamination of chick embryos in the light of these 
facts has led to the following conclusions. The bursa of Fabri- 
cius in the chick begins soon after the rupture of the caudal 
intestine, as early as the beginning of the fifth day, as a prolif- 
eration of entodermal epithelium on the caudal border of the 
cloaca adjoining the anal plate (fig. 26, bursa), but it does not 
develop from the epithelial elements which originally belonged 
to the caudal intestine, as maintained by Stieda. As the two 
walls of the cloaca, beginning at the anal plate, progressively 
fuse to form the urodaeal membrane, vacuoles appear in the 
solid plate thus formed (figs. 27, 28, and 29, bursa). Those on 
the free border adjoining the anal plate coalesce and distend the 
cloaca, forming the definitive bursa of Fabricius (fig. 30, bursa). 
Previous to these events, however, a diverticulum may appear at 
each end of the area marking the site of the cloacal fenestra. 
The caudal diverticulum (a) is always present in chick embryos, 
