354 CHESTER H. HEUSER 



('98) showed that the primary coils of the intestine in the human 

 embryo could be recognized very early and that even the various 

 loops of the adult intestine, as well as their position, could be 

 made out in embryos of five weeks. The first three of the pri- 

 mary loops, I believe, can be identified in the 7-mm. opossum 

 embryo; the smaller distal coils, however, are not as yet indicated. 



Villi have developed in the first three loops of the small intes- 

 tine of the late opossum embryo. It was not determined whether 

 the villi originate singly as thickenings of the epithelium, as 

 Johnson ('10) has described for the human embryo, or whether 

 they are preceded by longitudinal folds. Four to six low folds 

 are present in the mucosa of the last slender coils and there are 

 a few diverticula of the mucosa similar to those found by Lewis 

 and Thyng ('08) in various mammals. One diverticulum of this 

 nature is shown in limb 12 of figure 12. The epithelial lining is 

 made up of undifferentiated cells of a low columnar type, filled 

 with homogeneous cytoplasm and with nuclei proximally placed 

 near the basement membrane. The entire width of the intestinal 

 wall, exclusive of the mucosa, is remarkably thin; in some places 

 the whole wall beneath the basement membrane is equal to about 

 one-third the height of the epithelial cells above it. The muscu- 

 lar coat particularly is very delicate; it consists of a fine strand 

 of mj'oblasts only one or two in thickness. 



In the pouch-young the small intestine has been crowded to the 

 right by the enlarged stomach. The primary coils, however, can 

 still be recognized; the numerals on figures 3 and 4 have been 

 placed on comparable limbs of the coils. Villi are present in all 

 parts of the small intestine, although they are older and more 

 advanced in the duodenum. The external coats are yet very 

 thin and the intestine is noteworthy for the very large surface 

 of the mucosa. The muscular layer is only faintly indicated by 

 a.few circular myoblasts, so that peristaltic contractions must be 

 very feeble. The intestinal blood-vessels are especially notice- 

 able on account of their large size and sinusoidal character, being 

 closely applied to the mucosa. 



The cytoplasm of the cells covering the villi is coarsely granular 

 and the large vesicular nuclei are placed centrally in the cell. 



