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Table I. Table showing extent of cloacal membrane relative to the size of the embryo

Length of embryonic disc Embryo Presomite Cloacal membrane W.O. 0-25 mm. What von Mollendorlf describes as the cloacal (V. Mollendorlf, 1925) membrane is, according to Florian, the primitive streak, and von Mollendorif’s allantois is the cloacal membrane of Florian “Fetzer” 0-26 mm. The cloacal membrane is separated by an interval (Fetzer & Florian, 1930) from the primitive streak T. F. 0-4 mm. The cloacal membrane is a cord of cells which ex- (Florian (1928)) tends on to and connects the allantoic endoderm with the amniotic ectoderm Bi I ? Primitive streak reaches the cloacal membrane Bi 24 0-625 mm. Length 0-06 min. “Hugo” Stieve (1926) 0-635 mm. Length 0-035 mm. “Beneke” 0-75 mm. Cloacal membrane separated by an interval from (Strahl & Beneke, 1910) the primitive streak W.A. 17 0-85 mm. Length 0-13 mm. and extends on to the allantois (Grosser, 1930-1) “Dobbin” 0-98 mm. Length 0-08 mm. (Hill & Florian, 1931) ‘ Peh-Hochstetter 1-4 mm. Cloacal membrane extends well on to allantois Rossenbeck (1923) ' Heuser (1930) 1-6 mm. Cloacal membrane extends well on to allantois Ingalls (1918) 2-0 mm. Length 0-12 mm., and there is an additional con- nexion between the allantoic endoderm and am- niotic ectoderm Somites F. von Orts Llorca 2 Cloacal membrane in two parts—one of which is (1934) allantoic H 3 (Wilson) 2-3 Cloacal membrane at the commencement of al1an- tois Bi II 4 Cloacal membrane broken up—one portion in- volvingallantois Sternberg (1927) 4 Cloacal membrane is in four portions Pfannenstiel-K1-omer 5-6 Very short cloacal membrane extending some distance into the allantois Dandy (1910) 7 Not mentioned Payne (1925) 7 Not mentioned West (1930) 8 Length 0-09 mm. Etemod (1899) 8 Length 0-05 mm. Corner (1929) 10 Cloacal membrane terminates before reaching the allantois Embryo Bl. XI 10 Cloacal membrane terminates before reaching the allantois Pfannenstiel III 13-14 Has trebled its length compared to P£annenstiel- Kramer Heuser (1930) 14 Length 0-18 mm. Atwell (1930) 17 Length 0-18 mm. Extends to the allantois Davis (1924) 20 _ Length 0-16 mm.

Table I is a series of embryos arranged according to size, with, as far as can be obtained, the available data regarding the cloacal membrane.