346 CHESTER H. HEUSER 



terminal bars can be seen and the epithelial cells rest upon a 

 delicate but distinct basement membrane. There is some evi- 

 dence of a differentiation among the epithelial cells in the pyloric 

 region. Here groups of cells appear slightly more darkly stained 

 and more closely packed together. In each section there are 

 found a few cells which in form appear similar to their neighbors, 

 but are more brightly stained with the acid f uchsin ; they probably 

 represent a phase in cell activity in which the staining reaction 

 has been modified. In the more attenuated portion of the 

 stomach wall along the greater curvature, the distention has been 

 very pronounced and all layers of the wall have been greatly 

 reduced in thickness. The epithelial cells in some cases have 

 less than half the height of those along the lesser curvature. 

 In the thinnest places, the nuclei have been drawn out parallel 

 with the surface of the epithelium; they are also much farther 

 apart in these regions. The muscular layer is everywhere very 

 thin, being only one or two myoblasts in width and the elongated 

 nuclei are few in number. 



Small intestine 



A striking feature of the intestine in opossum embryos is the 

 abundance, size, and arrangement of the blood-vessels. In an 

 embryo 8.5 mm. in length — about nine days after the beginning 

 of segmentation and one day before birth — the vessels have a 

 very wide diameter and are also noteworthy for their close 

 approximation to the mucosa, as shown in figure 1 from a trans- 

 verse section of a loop of intestine. Closely hugging the mucosa, 

 they present the appearance of large sinusoids rather than that 

 of capillaries. The outer borders of the vessels, however, are 

 covered with the mesenchyme, and hence they are not true 

 sinusoids as defined by Professor Minot ('00). In many sections 

 nearly the whole extent of the epithelium of the mucosa is in 

 contact with these large blood-vessels. Four to six folds of the 

 epithelium extend into the lumen, and in each case the crypt 

 of the fold is filled with a large vessel. A reconstruction of a 

 segment of the intestine (fig. 2) indicates a rich plexus of vessels 

 spreading over the outer surface of the mucosa. The crypts of 



