PLATE 3 
EXPLANATION OF FIGURES 
Graphic reconstructions of the cloaca of bird embryos drawn to the same scale 
(magnification, X 35). Dash lines indicate cavity; dotted lines, vacuoles. This 
plate represents chiefly a quantitative study of chick embryos made to demon- 
strate the origin of the bursa of Fabricius together with the identity and sig- 
nificance of a series of diverticula occurring on the back wall of the cloaca between 
the anal plate and the rectum. Diverticulum a represents an accessory diver- 
ticulum, arising from the caudal angle of the cloacal fenestra, which regularly 
becomes appended to the bursa of Fabricius; b represents an accessory diver- 
ticulum, only occasionally present, which arises from the cephalic angle of the 
cloacal fenestra and which becomes associated with the urodaeal sinus; ¢ repre- 
sents a diverticulum which regularly forms the medial component of the uro- 
daeal sinus. 
22 Chick embryo, H.E.C.1953. 3 days, 6 hours, 8.0 mm. 
23 Chick embryo, H.E.C. 2120. 3 days, 18 hours, 9.2 mm. 
24 Chick embryo, H.E.C. 2126. 3 days, 18 hours, 9.5 mm. 
25 Chick embryo, H.E.C. 2058. 4 days, 4 hours, 11.0 mm. 
26 Chick embryo, H.E.C. 2098. 4 days, 3 hours, 10.0 mm. 
27 Chick embryo, H.E.C. 1942. 4 days, 3 hours, 10.5 mm. 
28 Chick embryo, H.E.C. 2097. 4 days, 3 hours, 10.5 mm. 
29 Chick embryo, H.B.C. 2100. 4 days, 22 hours, 13.0 mm. 
30 Chick embryo, H.E.C. 1943. 4 days, 3 hours, 12.0 mm. 
31 Chick embryo, H.E.C. 1951. 5 days, 0 hours, 13.0 mm. 
32 Chick embryo, H.E.C. 2059. 4 days, 23 hours, 14.0 mm. 
33 Chick embryo, H.E.C. 2074. 5 days, 23 hours, 15.0 mm. 
34 Chick embryo, H.E.C. 2076. 6 days, 7 hours, 17.3 mm. 
35 Chick embryo, H.E.C. 1962. 8 days, 1 hours, 21.5 mm. 
386 Sterna hirundo (common tern) H.E.C. 2169. 8.0 mm. 
37 Sterna hirundo (common tern) H.E.C. 2115. 10.4 mm. 
38 Sterna hirundo (common tern) H.E.C. 2173. 13.4 mm. 
ABBREVIATIONS 
all., allantois Mull., Miillerian duct 
an., anus pelv., pelvis of kidney 
an. pl., cloacal membrane (anal plate) —_phal., phallus 
bursa, bursa cloacae (of Fabricius) proct., proctodaeum 
cauda, inner curvature of tail ps. v., ventral half of primitive streak 
cau. A., caudal artery rect., ampulla recti (coprodaeum) 
c.7., caudal intestine umb. A., umbilical artery 
copr., coprodaeum (ampulla recti) ur., ureter 
d., accessory rectal diverticulum urod., urodaeum 
div., complementary diverticula ur. m., urodaeal membrane 
fen., cloacal fenestra W. D., Wolffian duct 
198 
