THE CLOACA IN HUMAN EMBRYOS 3 



membrane may be termed the tail gut and terminates in an un- 

 differentiated cell mass formed by itself, the chorda and neural 

 tube. Born emphasizes the length of the cloacal membrane as 

 follows: ''I call particular attention to the original extent of the 

 cloacal membrane. It reaches cephalward to the point where the 

 allantois leaves the body at right angles; i.e., as far as the caudal 

 border of the dermal navel." 



The cloacal membrane does not extend as far as the allantois 

 in the model of an 8.0 mm. human embryo presented by Keibel 

 ('91) . Mesodermic tissue has apparently wandered in from above 

 and separated the layers of epithelium. The cloacal membrane 

 is therefore shorter than in the 4.2 mm. stage. The tail gut shows 

 degeneration. The precloacal mesodermic tissue has increased 

 in amount but instead of displacing the cloacal membrane caudal- 

 ward, has folded it into the lower surface of the genital tubercle 

 as was first described by Tourneux ('89), and verified by Retterer 

 ('90) and Reichel ('93). No intermediate stage in the develop- 

 ment has been described up to this time ('93). With the increase 

 in size of the genital tubercle, the entodermal cloaca becomes 

 more deeply placed as was demonstrated by Reichel ('93). Born 

 states that the epithelial plate contained within the genital tu- 

 bercle is ectodermic and that it is continuous with the superficial 

 ectoderm. The epithelial plate occupies the caudal surface of 

 the eminence and is bordered laterally by folds of mesoderm (repli 

 ano-genitaux of Retterer) , while the cloacal membrane itself ter- 

 minates at the postanal fold (replis postanal). 

 , ''The depression arises (as in the mouth region) through in- 

 crease in the height of the limiting borders. The depression is 

 always closed in by epithelium, and the base of the depression is 

 never separated from the entodermal cloaca by mesodermic tis- 

 sue." (Born.) 



The cloaca is gradually divided into a ventral (bladder-uro- 

 genital sinus) segment, and a dorsal (gut) segment. As to the 

 manner of this division, Retterer and Born agree with the Tiede- 

 mann-Rathke idea of the gradual separation into two segments 

 through approximation of two lateral folds of mesoderm, while 

 Tourneux believes it to be accomplished by a septal (frontal) 



