30 FRANKLIN PARADISE JOHNSON 



or three layers of cells are flattened and in the lowei- part of the 

 zone, appear cornified. 



The ano-cutaneoiis line is marked by the beginning of the first 

 hair follicles. Beyond this line is the zona cutanea. The hair 

 follicles are accom])anied ))y beginning sebaceous glands. Still 

 further out are found the beginnings of a few scattered sweat 

 glands which undoubtedly represent the ' ' circum-anal glands" 

 of Gay. A few dermal papillae are also present in this zone. The 

 marked difference l:)etween this stage and the stages of 135 mm. 

 and 140 mm. in regard to the disposition of hair follicles can only 

 be explained as individual \'ariations. 



The muscularis mucosa (fig. 3) is distinctly seen in the region 

 of the Lieberklihn glands and a few fibers can be traced down into 

 the zona columnaris. The thickening of the circular muscle coat 

 to form the internal s}ihincter begins in the lower portion of the 

 am])ulla and extends down into the region of the zona intermedia 

 a distance of 3.5 to 4.5 mm. 



The external sphincter is placed below and to the outside of the 

 internal sphincter and is again separated into numerous bundles 

 of fibers by connective tissue septae. The fibers of the longitudi- 

 nal muscle layer seem to terminate just before reaching the exter- 

 nal sphincter b}' ending in the connective tissue septae. Above 

 and to the outside of the external sphincter is the levator ani. 



A model of the pars analis recti of an embryo of 240 mm. is 

 represented in figures 19 and 20. The ampulla is shown above as 

 cut off at a point where it has about reached its greatest width. 

 It is separated from the bulbiis analis by a constricted zone which, 

 as determined from the sections of this series, lies a little above 

 the ano-rectal line. This portion of the rectum (ampulla) pre- 

 sents only intestinal glands, \'illi having entirely disappeared. 

 Owing, however, to the difficulty met with in modeling the glands 

 at the magnification used (24 diameters) no attempt has been 

 made to represent them. All the folds of the pars analis recti 

 have been modelled. The primary folds can be best seen in cross 

 sections of this region l:>ut they are distinguishable in the model. 

 The posterior, right, and left primary outfoldings are present as 

 in the youngest stages, and the left, as seen in the figures, passes 



