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 

 case; 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 urodseum, 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 urodseum. 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 proctodseum now lies behind it; the bursa 

 Fabricii is on the point of opening into the highest point of the 

 proctodeum. Vacuolization of the tissue between the cloacal 

 membrane and the urodseum 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 proctoda^um at the level of the urodseal membrane. 

 The latter is still quite a thick plate, but the v^acuoles in it fore- 



