446 



mucosae (Fig. 3, M3I), in the shape of small groups of cells in the 

 Submucosa, just outside of the basement membrane. In the Mucosa, 

 the inner layer of cells are now swollen and filled with clear cyto- 

 plasm and have replaced the original columnar cells. The outer rows 

 of cells next to the basement membrane remain unchanged. Thus, the 

 character of the Mucosa has been transformed from a stratified co- 

 lumnar epithelium of the younger embryos, to a squamous epithelium 

 of a very simple type. The character of the Submucosa, beyond a 

 thickeoing of the fibrils particularly in the neighborhood of the Tunica 

 muscularis, where they are already gathered into dehcate trabeculae, 

 remains unchanged. Both layers of the Tunica muscularis have thicken- 

 ed, but retain their former character. 



The next step in the organogenesis of the oesophagus is well 

 shown in a pig 11 cm (Fig. 4) long, where the inner two or three 

 layers of cells of the mucosa are of an enlarged polygonal type, with 

 their long axes running parallel to the basement membrane. In the 

 Submucosa, the small groups of young cells forming the Muscularis 

 mucosae (Fig. 4, MM) are now more definitely seen and, beginning 

 with the preceding stage, there is an increase in the infolding of the 

 Mucosa and Submucosa, yielding usually five and often six longitudinal 

 folds projecting into the lumen of the organ. In the Tunica mus- 

 cularis, there is some further dififerentiation of the muscle fibres, which 

 now show evidences of simple striation in longitudinal sections, and 

 sarcostyles in cross sections of the cells. The latter, however, remain 

 spindle shaped. About the Tunica muscularis, the connective tissue 

 fibrils are gathered into distinct and firm trabeculae. 



In a pig 13 cm long (Fig. 5), the changes indicated above have 

 become even more marked. The epithelium is further flattened on 

 the surface of the Mucosa next to the lumen of the organ and the 

 Muscularis mucosae is now composed of definite bundles of smooth 

 muscle fibres. 



At 21,5 cm (Fig. 6), the Mucosa may be said to have reached a 

 stage of maturity, where the cells of the inner layer are desquamated 

 and may be seen lying free in the lumen of the oesophagus. The 

 Submucosa has undergone distinct changes and is now composed 

 almost entirely of trabeculae of connective fibrils in which the cell 

 bundles of the Muscularis mucosae are found. 



Numerous blood vessels of the submucosal plexus are seen in this 

 as in the earlier stages. Near the Tunica muscularis, the connective 

 trabeculae become thick and dense. The inner layer of the Tunica 

 muscularis is thicker and is composed of fibres running circularly, in 



