354 E. R. HOSKINS 



two or more smaller longitudinal ridges. The furrows between 

 the ridges often extend under them and hence, in transverse 

 section, appear as spaces in the epithelium filled with connective 

 tissue (fig. 17). The spiral valve ends abruptly anterior to this 

 region. The dorsal part of the cloaca into which the Wolffian 

 ducts enter is marked off from the remainder of this segment of 

 the gut. 



The structures just described have become more highly differ- 

 entiated in the 95 mm. stage (fig. 26). The angle of the two 

 bends has changed from 90 degrees to less than 30, giving the 

 intestine an 'S' shape. Three groups of ridges are present. 

 One group of six ridges extends forward from the cloaca, gradu- 

 ally disappearing anteriorly. Of these, three are ventral and 

 three dorsal. Of the latter, two are extensions of the lateral 

 corners of the dorsal outpouching which receives the Wolffian 

 ducts, and the other is midway between them. The middle 

 group of ridges is that described in the preceding stage. The 

 portion of the gut containing these ridges is greatly lengthened 

 in the present stage and the digitiform duct is carried posteriorly. 

 There are five principal ridges here, but each is subdivided at 

 the periphery and appears to branch when seen in transverse 

 sections (fig. 18). These ridges flatten posteriorly just above 

 the digitiform duct, and anteriorly they disappear abruptly just 

 behind the spiral valve (fig. 26). They are, in general, longi- 

 tudinal in direction, but they twist slightly in the same direc- 

 tion as the turns of the spiral valve. A third group of numerous 

 small ridges occurs on the exterior surface of the spiral valve, 

 running nearly at right angles to the group just described. 



The oldest embryo studied (200 mm.) was not modelled, but 

 from study of sections and dissections the post valvular intes- 

 tine appears to have straightened (fig. 9), while the solid ridges 

 are still present on the external surface of the gut (fig. 19). 



Occlusio?i of the post-valvular intestine 



In the first three stages described (15, 19 and 22 mm., figs. 

 20, 21, and 22) the lumen of the gut is relatively large and 

 uniform. In the wall of the gut (figs. 1 and 2) are five rows of 

 cells. 



