PLATE 4 
EXPLANATION OF FIGURES 
39 Model of chick embryo, H.E.C. 1945: 5 days, 15 hours; 15 mm. X 37. 
Showing especially the bladder-like enlargement of the allantois in the intra- 
embryonic body cavity, the lateral invaginations of the proctodaeum to meet the 
bursa of Fabricius (proct.), and the constriction of the metanephric pelvis into 
two parts by the umbilical artery. 
40 Model of chick embryo, H.E.C. 1962: 8 days, 1 hour; 21.5 mm. X 87. 
Note the occluded rectum, the prominent urodaeal sinus (wrod.), and the 
elongating bursa. 
41 Model of chick embryo, H.E.C. 1967:11 days;31mm. X21. Note differ- 
entiation of bursa into stalk and plicated gland, also division of cloaca into the 
three transverse parts characteristic of the adult: proctodaeum (ectodermal 
origin); urodacum, cloaca proper, receiving urogenital ducts; and the copro- 
daeum, rectal ampulla, with its ‘zona columnaris.’ 
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