DEVELOPMENT OF DIGITIFORM GLAND 353 



but tapers toward the posterior end, where it expands suddenly 

 to form the cloaca. Its contour is fairly regular, though marked 

 by several slight swellings, the largest of which will become the 

 digitiform gland. At 19 mm. (fig. 21) the general shape is the 

 same, but for the digitiform gland anlage and for a depression 

 which occurs just anterior to it. At 22.5 mm. (fig. 22) the por- 

 tion of the intestine between the spiral valve and the digitiform 

 gland is slightly swollen; as compared with the previous stage 

 and the digitiform gland is larger, but otherwise no changes of 

 importance have occurred. The growth in diameter has been 

 more rapid than the longitudinal growth. The post-valvular 

 portion of the intestine (fig. 23) in an embryo 28.1 mm. in length 

 is somewhat irregular in outline, the gut is constricted a' the 

 entrance of the duct of the digitiform gland and a few irregular 

 longitudinal ridges and furrows are present. Two dorsal out- 

 pouchings appear where the intestine joins the cloaca. 



In the 33 mm. embryo (fig. 24) the portion of the intestine in 

 question has undergone considerable change. The entire gut is 

 flattened dorso-ventrally. In the region of the entrance of the 

 digitiform duct, the gut is very much flattened and concave, 

 perhaps through pressure of the duct, and it is beginning to bend. 

 It is marked here by several irregular longitudinal ridges and 

 furrows. 



In the 47.5 mm. stage still greater alterations of the original 

 form of the post-valvular gut are evident (fig. 25) . The contour 

 is very irregular. Owing to the fact that the growth of this 

 part of the intestine has exceeded that of the body cavity, the 

 gut has bent in two places, each at right angles to the long 

 axis of the body. A twisting of the gut likewise has occurred 

 so that the duct of the digitiform gland extends laterally from it 

 instead of dorso-laterally as in preceding stages (figs. 23 and 24). 

 The dorso-ventral diameter of the gut in this region is small. 

 The concavity of the gut into which the duct of the digitiform 

 gland enters is very deep, the two sides nearly meeting, but 

 they are separated by the duct. The longitudinal furrows and 

 ridges are more numerous than in the previous stage and are 

 much more marked. There are four large ridges, each bearing 



