THE CLOACA IN BIRDS 167 
GRAPHIC RECONSTRUCTIONS ILLUSTRATING INITIAL STAGES IN THE FORMATION 
OF THE CLOACAL FENESTRA 
(Dotted lines and arabic numerals refer to somites; dash lines, to cavities of 
the cloaca; crosses, to the primitive-streak mass; periods, toscattered phagocytes; 
cross-hatching, to concentrated areas of disintegration on left side of embryo.) 
Fig.1 Tern embryo (Sterna hirundo) H.E.C. 2167:5. mm. X 42. all., 
allantois; an. pl., cloacal membrane; c. 7., caudal intestine; rect., rectum;-W. D., 
wolffian duct. 
Fig. 2 Duck embryo (Anas domestica) H.E.C. 2193:3 days, 21 hours. X 42. 
t. p., terminal portion of W. D.; y and z, primary and secondary foci of disinte- 
gration (z restricted to right side of embryo in this stage). 
Fig.3 Turtle embryo (Chrysemys marginata) H.E.C. 1067:6 mm. X 42. 
after R. F.Shaner. an.s., primordium of anal sacs (cf. div. c. of chick embryo in 
plate 3); x, point of rupture of caudal intestine. 
Fig.4 Duck embryo (Anas domestica) H.E.C. 2194:3 days, 21 hours. X 42. 
ps. v., ventral half of primitive-streak remnant; z, occluded segment of caudal 
intestine. 
Fig.5 Chick embryo (Gallus domesticus) H.E.GC. 2071: 3 days, 18 hours. 
X 42. (Compare with model of same embryo, fig. 13.) m, marginal sulcus sepa- 
rating thin-walled roof from thick-walled sides of cloaca. 
