16 FRANKLIN PARADISE JOHNSON 



which Keibel calls attention. A lighter staining zone was seen 

 just outside of the circular layer of muscle, probably the begin- 

 ning of the nervous layer. At 17 mm. a faint indication of the 

 longitudinal muscle coat is seen. In embryos of 19 mm. and 

 22.8 mm., both layers of muscle, with the intervening nervous 

 layer, have become more distinct. 



Apparently Keibel was the first to describe the two swellings of 

 the rectum referred to above. He shows that they are present 

 and distinct in embryos of 17.5, 18.5, and 25 mm. In the de- 

 scription of his figure 55 — a longitudinal section of the rectum 

 from an embryo of 25 mm. — he says: 



Wir sehen die Aftergrube, in welches sich die Deckschicht des Ecto- 

 derms fortsetzt. Es folgt weiter cranial eine kleine Anschwellung 

 des Darmrohres, die aber mit der Aftergrube noch nicht in offener 

 Verbindung steht. Die Grenze zwischen Ectoderm and Entoderm 

 tritt ziemlich deutlich hervor. Auf die kleine Anschwellung des 

 entodermalen Enddarmrohres folgt cranial eine stark ausgebildete An- 

 schwellung, wie sie ja an den Zeichungen nach den Modellen und auf 

 der Profilreconstruction klar genug zur Darstellung kommt. 



Tourneux ('90) who studied the cloacal region of the sheep, 

 does not refer to these two swellings so distinctly shown by 

 Keibel, and Pohlmann ('11) apparently failed to find them in the 

 human embryo. 



Concerning a sagittal section through the rectum of a 22 mm. 

 embryo, Lewis ('12) states that: 



Just before the rectum reaches the anal membrane it forms a bulbous 

 enlargement .... The terminal swelling extends beyond the mus- 

 cle layers as recorded by Keibel . . . . In a 32 mm. specimen the 

 anal membrane has disappeared. Along the dorsal wall there is a 

 slight indication of a terminal bulbous enlargement but it seems clear 

 that it is a transient structure. It is probable that the elongated swell- 

 ing above it gives rise to the rectal ampulla of the adult. 



Lewis accordingly has labelled the elongated upper swelling the 

 'ampulla recti.' Apparently, however, he disregards the accepted 

 use of 'ampulla recti,' for he applies the term to the bulbous pars 

 analis recti in the figure of a case of atresia ani cum fistula vulvari, 

 copied from Mackenzie. The conjecture that the elongated 

 swelling gives rise to the pars analis recti proves to be correct, but 



