DEVELOPMENT OF THE RECTUM 33 



few of the outfoldings between them. Unlike in the preceding 

 stages the six distinct intramuscular glands found in this series 

 arise from the upper portion of the stratified squamous epithe- 

 lium, that is, from the zona intermedia. All are distinctly tubu- 

 lar and have distinct lumens throughout. For a short distance 

 they are lined with a stratified squamous epithelium but this 

 soon passes over into a stratified columnar which in the branches 

 becomes cuboidal. In all these glands evidences of a secretory 

 epithelium are apparent. One gland, of which a model is shown 

 in figure 23, extends more than half way through the longitudinal 

 muscle coat, giving off at various levels several branches. The 

 primary duct of the gland courses in an outward and downward 

 direction until it reaches the internal sphincter muscle. It then 

 assumes a horizontal direction and in the substance of the internal 

 sphincter, presents a flattened ampulla-like dilation. From this 

 ampulla branches arise which extend on through the internal 

 sphincter and terminate in the connective tissue between the 

 muscle layers. The gland labelled B in the same figure repre- 

 sents a simple tubular type which extends only to the internal 

 sphincter but does not pierce it. Another simple gland is shown 

 in figure 22. It has a somewhat ampullated ending at the outer 

 border of the internal sphincter. Several small knob-like pro- 

 tuberances mark what seem to be acini. They are lined with a 

 cuboidal epithelium which is distinctly of a secretory character. 



The zona intermedia is lined with a thick stratified squamous 

 epithelium which is cornified. This is thrown into a few large 

 folds which are directly continuous with the larger folds of the 

 zona columnaris above and with the anal skin folds below. The 

 zona cutanea presents no special features. 



The external sphincter muscle is of interest in that it gives off 

 vertical columns of striated muscle which extend up into some of 

 the larger folds for varying distances. Likewise small bundles 

 of smooth muscle fibers pass horizontally or obliquely toward the 

 epithelium and also turn upward into the large mucosal folds. 

 Both types of muscle fibers can be traced up into the zona colum- 

 naris, where they terminate. No evidence of such fibers are 

 apparent in this region of the previously-described embryos. 



THE AMERICAN JOURNAL OF ANATOMY, VOL. 16, NO. 1 



