ALIMENTARY TRACT AND ITS APPENDAGES 317 
median plate which is continuous ventrally with the cloacal mem- 
brane. 
This plate was interpreted by all the earlier observers (up to Wenck- 
ebach) as the hypertrophied cloacal membrane. It is, however, not 
difficult to demonstrate in good series of sections, that this is not the 
‘ase; the cloacal membrane forms only a small part of this plate, and 
its ectodermal component is thin. 
During the fifth and sixth days, vacuoles appear in the pos- 
terior and dorsal part of the fused portion of the cloaca, and 
these soon run together in the uppermost part, but remain as a 
chain of vacuoles ventrally (Fig. 184). The vacuolated portion 
is the primordium of the bursa Fabricii and its duct. Its cavity, 
which is extremely narrow and ill-defined at this time, may be 
regarded as a re-establishment of the cavity of the posterior 
division of the embryonic cloaca; its communication with the 
anterior portion of the cloacal cavity is soon closed. 
At this stage the lining epithelium of the rectum is much 
thickened, and the lumen has therefore become narrow (Fig. 184). 
During the seventh day the conditions change very rapidly 
and on the eighth day the relations are as shown in Figure 185. 
The anterior portion of the original cloaca, or urodzeum, has 
become compressed in an antero-posterior direction; the allantois 
leads off from it anteriorly and ventrally, and the rectum with 
its cavity now obliterated is attached to its anterior face; the dor- 
sal extension, above the rectum (see Fig. 185), is related to the 
urinogenital ducts. The bursa Fabricii has now a well-defined 
cavity that no longer communicates with the urodeum. The 
tissues surrounding the cloacal membrane have grown out to 
form a large perianal papilla, and the cloacal membrane is 
therefore invaginated; its direction also is so altered that the 
invaginated cavity or proctodzeum now hes behind it; the bursa 
‘abricii is on the point of opening into the highest point of the 
proctodeum. Vacuolization of the tissue between the cloacal 
membrane and the urodzum indicates its subsequent dis- 
appearance. 
At eleven days (Fig. 186) the general arrangement is essen- 
tially the same, but there are important differences in detail. 
The bursa Fabricii has now become a long-stalked sac, opening 
into the proctodeum at the level of the urodzal membrane. 
The latter is still quite a thick plate, but the vacuoles in it fore- 
