PLATE 2 
EXPLANATION OF FIGURES 
Projection-lantern drawings of microscopic sections through the cloacal 
fenestra of chick embryos, drawn to the same scale (magnification, X 340). 
19 H.E.C. 512 (section 121): 2 days, 20 hours? Obliquely-transverse section 
passing through cloaca at right angles to the long axis of the fenestra (ef. with 
imaginary line connecting letters y and all. in text fig.5). Note bilaterally sym- 
metrical gaps in cloacal wall; the concentration of mesenchyma around the gaps; 
the isolated floor of the cloaca, with necrotic cells on margin; the phagocytes in 
the cavity and the pyenotie nuclei in the epithelium bordering the gap. 
20 H.E.C. 2057 (section 736): 3 days, 4 hours; 6.8 mm. Section through 
fenestra during period of maximum extent (cf. embryos shown in figs. 14, 15 
and 22). Note complete resorption of disintegrating epithelium shown in pre- 
ceding figure, the rounded epithelial margins which fail to regenerate, the flat- 
tening out of the mesenchyma bordering exposed cavity. 
21 H.E.C. 2124 (section 749): 3 days, 18 hours; 8.5 mm. Last stage before 
closure showing section through fenestra reduced to small slit (same age as 
embryos shown in figs. 23 and 24). Note approximation of two side walls, the 
complete absence of regeneration along epithelial margins. 
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