Fig. 19. 



Fig. 20. 



Fig. 21. 



Fig. 22. 



Fig. 23. 



Fig. 24. 



Fig. 19. Schema of digestive tract of a 6 mm. pig embryo. 



Fig. 20. Schema of digestive tract of an 8 mm. pig embryo. 



Fig. 21. Schema of digestive tract of a 10 mm. pig embryo. 



Fig. 22. Schema of elastic band twisted as a left-handed spiral. This repre- 

 sents the mesenchyme of the straight digestive tract. 



Fig. 23. Schema of ventral flexure of an elastic band; represents the ventral 

 flexure of elongating tract of Fig. 20. 



Fig. 24 Schema of inevitable ventral looping of caudal element over the 

 cephalic element of an elastic band twisted as a left-handed spiral. This repre- 

 sents the reaction of the mesenchyme to the torsion produced by the spirally 

 growing epithelial tube as schematized in Fig. 21, The loop tends to relieve the 

 torsion to which the mesenchyme is subjected. 



t, trachea; e, esophagus; s, stomach; /, liver; d. p., dorsal pancreas; v.p., ventral 

 pancreas; s.i., small intestine; c, cecum; v.d., vitelline duct. 



77 



