THE CLOACA IN HUMAN EMBRYOS 17 



does not seem to take place at the expense of the cloaca, but the 

 allantois appears to occupy a relatively fixed position at the lower 

 border of the dermal navel. This argument is strengthened by 

 the active increase in length of the hind gut, and by such condi- 

 tions of strictly pathological nature such as bladder exstrophy. 

 The line of demarkation however between what is allantois and 

 what is ventral cloacal segment is largely a matter of topography, 

 but is after all of some importance in a definite determination of 

 the anlage for the urinary bladder. 



THE DIVISION OF THE CLOACA 



The division of the cloaca manifests itself in a separation into 

 ventral and a dorsal segment. According to Tourneux, the divi- 

 sion is accomplished through the downgrowth of a single meso- 

 dermic fold (the saddle between the allantois and hindgut), 

 while Retterer maintains that two lateral mesodermic folds en- 

 croach upon the lumen of the cloaca and pinch it off, as it were, 

 into the two resulting segments. Minot ('97) and Keibel hold 

 the manner of division to be of little moment, and even Fleisch- 

 mann ('07) appears to have given up hope of solving the question 

 from the standpoint of comparative embryology. Tourneux's 

 idea is, according to the writer, a much better description of the 

 process than that of Retterer. 



Taking the Wolffian ducts as a fixed point, it is easily seen that 

 a coronal septum grows down dorsal to the Wolffian orifices 

 until finally the entoderm covering the septum comes into rela- 

 tion with the entoderm of the cloacal membrane. With the dis- 

 appearance of the epithelium in this fused area, the mesoderm 

 of the setpum touches the ectoderm of the surface and the cloacal 

 membrane is split into its two resulting segments. While this 

 division of the cloaca results in two segments that are about of 

 equal size as far as their antero-posterior measurements are con- 

 cerned, and inasmuch as the cloacal membrane does not materially 

 increase in length, it may be assumed that the cloacal membrane 

 is split into about equal parts in its transformation into the uro- 

 genital and anal membranes. This would mean that the increase 



THE AMERICAN JOURNAL OF ANATOMY, VOL. 12, NO. 1 



