PLATE 1 
EXPLANATION OF FIGURES 
Models illustrating the formation of a cloacal fenestra and the early develop- 
ment of the cloaca in chick embryos. All figures are drawn to the same scale 
(magnification, X 50). H. F. Aitken, del. (plates 1 and 4). 
13 H. E. C. 2071: 2 days, 18 hours (ef. with text fig. 5, a sagittal reconstruc- 
tion of the same embryo). In passing across the picture from left to right at its 
upper level the organs are encountered in the following order: medullary tube, 
notochord, caudal intestine, primitive-streak mass, proctodaeum, allantois, 
rectum, dorsal aortae. This stage shows the persistence of a mass of primitive- 
streak tissue in the angle between the cloaca and caudal intestine; the mergence 
of four structures (medullary tube, notochord, caudal intestine, and anterior 
half of primitive-streak remnant) with the indifferent tail-bud mass; a cireclet of 
five or six complemental diverticula around the unattached terminal portion of 
the W. D.; a larger complemental diverticulum opposite its shaft; and the isolated 
foramina (in the back wall of the cloaca and adjacent portion of the caudal intes- 
tine) which mark the first step in the disintegration of the cloacal wall and the 
formation of a fenestra. 
14 and 15 H.E.C. 1958: 3 days, 6 hours; 8 mm. (ef. with fig. 22, a sagittal 
reconstruction of the same embryo). At the left of figure 13 are the remnants 
of the caudal intestine and primitive streak, each detached from the cloaca by a 
process of disintegration. The dash line indicates that portion of the cavity of 
the cloaca which has been denuded of epithelium. It is bounded by mesenchyma 
only, and indicates the maximum extent of the cloacal fenestra, shown to better 
advantage from below in figure 15. At the cephalic end of the fenestra in both 
figures is an aberrant diverticulum probably derived from one of the comple- 
mental diverticula shown in figure 12. 
16 H.E.C.1942:4 days, 3 hours; 10.5mm. (ef. with fig. 27, a sagittal recon- 
struction of the same embryo). Note diverticula lettered a and ¢ in fig. 26, 
together with accompanying legend. : 
17 H.E.C. 2097: 4 days, 3 hours; 10.5 mm. (cf. with fig. 28, a sagittal recon- 
struction of the same embryo). Note accessory diverticulum lettered b in figure 
27. 
18 H.E.C. 1951: 5 days, 13 mm. (ef. with fig. 31, a sagittal reconstruction of 
the same embryo). Note the swelling (bursa of Fabricius) caused by coalescence 
of vacuoles at the bottom of the cloaca; the distended cavity of the urodaeum 
connecting allantois and excretory ducts; the flattened and occluded area between 
the urodaeum and bursa (urodaeal membrane); the down-growing proctodaeum 
astride the cloacal membrane, reaching out to connect with the bursa; the con- 
striction in the metanephric pelvis marking the future division between the 
second and third lobes of the adult kidney (ef. with fig. 39). 
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