12 AUGUSTUS G. POHLMAN 



only a relative matter (see later). The tail gut has undergone 

 further degeneration and has lost its lumen in part. 



Model E. {No. 19; Mall no. 221; 12.0 mm.) This stage shows 

 the cloaca about half divided along the furrow line indicated in 

 Model A. The cloacal membrane has thickened but mostly 

 through addition of ectodermal cells, and has been displaced 

 through development of the ' precloacal mesoderm' from its prim- 

 itive position parallel to the dorsal line of the cloaca to one more 

 nearly at right angles to it. In so far as it was possible to ascer- 

 tain, this downward displacement of the cloacal membrane in no 

 way affects its caudal limit. The dorsal segment has retained the 

 original proportions while the ventral segment has widened — 

 most marked again at the level of the Wolffian orifices. The 

 further development of the kidney and ureter will be noted and 

 the gradual approach of the ureteral orifice to the cloaca proper 

 is evident. The ureter shows signs of shifting from its primitive 

 dorsal position on the Wolffian duct to a more lateral one. The 

 peritoneum has descended to the level of the ducts while the cloa- 

 cal division is relatively far advanced. The model agrees with 

 Keibel's model of an 11.5 mm. embrj^o. 



Model F. {No. 24; Mall no. 43; 16.0 mm. X 50.) Unfortunately 

 this embryo was cut too thick for minute reconstruction. The 

 stage however fills in another gap in the Keibel series. Here the 

 cloaca and the cloacal membrane are completely divided — the 

 ventral segment limited by the urogenital plate and the dorsal 

 segment by the anal plate. The ureter is displaced from its dorso- 

 lateral position on the Wolffian duct to a supero-lateral one and 

 opens distinctly into the ventral cloacal segment. The marked 

 increase in precloacal tissue has resulted not only in the large 

 genital eminence, but the urogenital plate has been dislocated 

 deeper into the base of the phallus and a marked heaping up of 

 ectodermal cells has occurred in the furrow on the caudal surface 

 of the eminence. The two resultant segments of the cloacal 

 membrane, the urogenital and anal plates are apparently no longer 

 than the}' were in much younger stages; a point that will be brought 

 out in greater detail later. 



