DEVELOPMENT OF THE RECTUM 29 



most part is made up of large goblet cells. The villi are quite low 

 but everj^vhere in evidence. In passing aborally into the pars 

 analis recti the glands become further spread apart, the goblet 

 cells become fewer and fewer and the epithelium gradually changes 

 over into a stratified columnar type. Although the histological 

 ano-rectal line is not so sharply marked in this section as in 

 many others, nevertheless, it is clear that few goblet cells are 

 present in the upper part of the zona columnaris. 



The bulbus analis presents numerous folds as in the former 

 embryos but these are not seen in the figui'e, owing to the fact 

 that the section passed directly through one of the large infold- 

 ings of the epithelium. Extending into the connective tissue 

 subjacent to the epithelium are to be found here and there through 

 out the series, long tubular projections of the epithelium. In 

 some cases they extend only as far as the internal sphincter 

 while in others they pierce this muscular layer. One of them is 

 shown in figure 3, gl. iutrm. It extends in an outward and oral 

 direction and divides into three or foiu- branches, only one of 

 which penetrates the muscularis. These glands are lined with a 

 cuboidal epithelium of two to three layers of cells. The inner 

 laj'^er consists of cubical cells with distinct boundaries, a granu- 

 lar protoplasm and rounded nuclei. The outer cells are usually 

 small, they have no distinct boundaries, and their nuclei are 

 smaller and more deeply staining. For the most part the intra- 

 muscular tul^ules of this embryo project upward (orally) but not 

 always, for few were seen extending outwards and downwards. 

 The transition from the stratified columnar epithelium of the 

 zona columnaris to the stratified squamous epithelium of the zona 

 intermedia takes place gradually, forming the sinuous anal line. 



The zona intermedia presents a thickened region of the epithe- 

 lium. The stratified squamous epithelium is from ten to four- 

 teen layers of cells thick in the upper part of this zone, but in the 

 lower part it gradually becomes thinner. The basal two layers 

 of cells are granular, and have large round and ovtxl nuclei. The 

 middle layers of cells are vesicular and polygonal in shape. They 

 are made up of a faintly staining, finely granular protoplasm, and 

 contain small but deeply staining irregular nuclei. Tlie toj:) two 



