THE CLOACA IN HUMAN EMBRYOS 9 



No. 16 (Mall 113; 8.0 mm.; =; 10= Fair). X 100. Stage of cloacal division 

 between than found in no. 12 and that found in no. 19. Cloacal membrane intact. 

 Division of ureteral pelvis. Ureter segment elongated. 



No. 17 (Mall 114; 10.0 mm.; =; 10; Fair). X 100. Stage of cloacal division 

 about the same as the foregoing. Cloacal membrane intact. Kidney a little 

 higher up. Upper and lower pelvis evident. 



No. 18 (Mall 109; 11.0 mm.; +; 20; Good). Sections directly through lower 

 part of cloacal region somewhat damaged. Otherwise about the same as no. 19. 



No. 19 (Mall 221; 12.0 mm.; = ; 20; Good). Model E. X 100. All traces of tail 

 gut lost. Cloacal membrane somewhat depressed from surface. Cloacal segment 

 of Wolffian duct shortened and opening of ureter and duct common into ventral 

 cloacal segment. Beginning formation of genital eminence and lengthening of the 

 ventral cloacal wall. Cloacal membrane intact. 



No. 20 (Mall 175; 13.0 mm.; +; 20; Good). X 100. Cloaca about divided. 

 Complete double ureter. Orifices of lateral, and medial ureter and Wolffian duct 

 at same level into cloaca. 



No. 21 (Mall 353; 11.0 mm.; +; 10; Good). Cloaca completely divided. Anal 

 and urogenital membranes intact. Ureteral orifice independent and on level with 

 Wolffian opening. 



No. 22 (]\Iall 317; 15.0 mm.; ||; 20; Good). About the same stage as in no. 21. 



No. 23 (Mall 350; 15.0 mm; i|; 10; Good). About the same as the foregoing, 

 and no. 24. 



No. 24 (Mall43; 16.0mm.; = ; 50; Good). Model F.X 50. Anal and urogenital 

 membranes separated. Genital eminence more marked. Ectodermic inclusion in 

 furrow on caudal surface. Ureteral orifice on same level but some distance lateral 

 to Wolffian orifice. 



No. 25 (Mall 256; 16.0 mm.; = ; 50; Good). Urogenital sinus open to outside 

 through rupture of urogenital membrane. Kidney nearly up to normal position 

 and rotated. 



No. 26 (Mall296; 17.0mm.; ||;20; Good). Slightly older than above. Appear- 

 ance of secondarj' genital folds at the sides of the phallus. 



No. 27 (Mall 128; 20.0 mm.; ||; 50; Good). About same as above. 



No. 28 (Mall 240; 20.0 mm.; ||; 20; Good). 



No. 29 (Mall 22; 20.0 mm.; +; 50; Good). One MuUerian duct has reached 

 the urogenital sinus. 



No. 30 (Embryo KP; 22.0 mm.; +; 15; Good). X 66. Both Mullerian ducts 

 at sinus. 



The last six embryos of this series have been introduced merely to carry out the 

 idea of a brief arrangement according to the development of the tract in so far as 

 it was possible and to indicate the position of Model of no. 30 referred to in refer- 

 ence to the measurements on the inter-ureteral and inter- Wolffian widths mentioned 

 in the discussion of the anlage for the trigonum. Embryo no. 30 has additional 

 interest in that it like no. 20 has a complete duplication of the ureter on one side. 



