76 DYNAMICS OF HISTOGENESIS. II 



The reverse relation takes place, however, in embryos ranging in 

 length from 14 to 24 mm. (Figs. 16 and 18). In the 24 mm. embryos 

 the esophagus measures 9,1 mm. in length and 0.15 mm. in diameter. 

 We see, therefore, that the length increases four times over that found 

 in the 14 mm. embryo, whereas the diameter only increases one-half. 

 These facts help us to coordinate intelligently, for it is now evident 

 that the inner close spiral musculature is in the process of formation 

 while the epithelial tube is growing relatively more rapidly in diameter 

 and that the outer longitudinal muscle is incepted during the rapid 

 elongation in length. At the 24 mm. stage the outer longitudinal 

 muscle is faintly indicated. This outer coat is found, throughout de- 

 velopment and in the mature state, less developed than the inner one. 



Interpretation of the Primary Intestinal Torsion. 



In embryos 5 to 10 mm. in length the alimentary tract undergoes 

 a torsion from left to right through approximately 90°. The left 

 aspect of the stomach becomes ventral and the right becomes dorsal. 

 This involves the esophagus, the small intestine, and the large intes- 

 tine. The result of this torsion in the large and the small intestines is 

 a reacting looping. The colic fraction becomes ventrally placed to the 

 small intestine. This loop whereby the colic portion is placed ventral 

 to the small intestinal element is a constant fundamental relation in 

 the Mamm.alia. No satisfactory explanation has been given for this 

 characteristic fundament of form. 



This characteristic loop is due to the reaction of the elastic mesen- 

 chyme, from left to right, reacting to the left-handed helicoidal growth 

 action of the epithelial tube. With intestinal elongation an inevitable 

 loop is produced in a characteristic manner. This is clearly demon- 

 strable in Figs. 19 to 24. The intestino-colic flexure, therefore, is 

 a mechanical torsional reaction of the mesenchyme. This right- 

 handed helicoidal reaction is due to the left-handed helicoidal growth 

 of the epithelial tube. 



