356 E. R. HOSKINS 



In the 28.1 mm. stage the lumen of gut just posterior to the 

 spiral valve narrows suddenly, but is still present. Its narrow- 

 est diameter, about half way between the spiral valve and the 

 opening of the digitiform duct is two to four n. Anterior and 

 posterior to the portion of the gut with the narrow lumen the 

 wall of the gut is composed of five rows of cylindrical cells with 

 elongated nuclei. These gradually increase in number, and mid- 

 way between the two limits just mentioned as many as twenty 

 rows of nuclei are present, packed so closely together that they 

 seem almost to touch one another. From the periphery toward 

 the center of the gut, the first five or six rows of nuclei are 

 elongated, resembling the usual type seen in the gut, but the 

 nuclei lying internal to these are irregularly rounded. This 

 indicates that the occlusion is brought about by proliferation of 

 the cells lining the lumen. 



In the 33.1 mm. embryo the gut is solid for about 0.2 mm. 

 in the region midway between the spiral valve and the digiti- 

 form duct (figs. 24 and 16). At both ends of the solid portion 

 the cavity of the gut widens abruptly to its normal diameter, 

 but the transition in thickness of the epithelium is more gradual. 

 Both anterior and posterior to the solid portion the wall con- 

 tains from four to six rows of cylindrical cells with elongated 

 nuclei. This layer is continuous through the solid portion, but, 

 in addition, there is a large number of irregularly round cells 

 around the center of the gut. The ridges on the external sur- 

 face (fig. 24) appear to be produced by peripheral outpushing 

 of the epithelium due to pressure applied at the center, and to 

 be the result of central cell proliferation rather than of the 

 growth of the peripheral epithelium or pressure of the surround- 

 ing mesenchyma. However, cell division at the periphery may 

 aid in ridge formation. The mesenchyma covering the ridges 

 appears to be under considerable pressure, as the cells are very 

 flat as compared with those between the ridges. Smaller sec- 

 ondary ridges are found on the larger primary ones. 



In the 47.5 mm. stage the gut is solid (figs. 25 and 17) for 

 about 0.25 mm. just anterior to the point of entrance of the 

 digitiform duct. The main ridges are broken still more into 



