451 



of cells are desquamated into the lumen of the oesophagus by the 

 formation of new elements from the cells that rest against the base- 

 ment membrane. 



2. The Submucosa in developed primarily from the mesoderm 

 about the head gut. In the early stages, this consists of a syncitium 

 of anastamosing cells, which have differentiated relatively few con- 

 nective tissue fibrils. These fibrils increase rapidly in number and 

 density, anastamosing with each other and finally giving rise to the 

 trabeculae and form the framework of the oesophagus of which the 

 Submucosa is chiefly formed. In the meshes of these fibrils and tra- 

 beculae, the connective tissue cells are imbedded. 



3. The Muscularis mucosae may be recognized in embryos 7,5 cm 

 long, as a loosely packed group of cells lying well outside of the 

 basement membrane. These increase rapidly in number and size and, 

 at 13 cm form an indefinite layer composed of smooth muscle fibres, 

 lying approximately midway between the Mucosa and the Tunica 

 muscularis. 



4. The Tunica muscularis may be recognized in an embryo 13 mm 

 long, as a densely packed group of cells lying well outside of the 

 Mucosa. These cells increase rapidly in size and number, yielding in 

 embryos 7,5 cm long, two more or less well marked layers. The 

 course of the fibres at this level of the oesophagus cannot be said, 

 however, to be circular in the inner and longitudinal in the outer 

 layer, as they interlace with each other and run, in some segments, 

 circularly and, in others, longitudinally. At 11 cm, the first evi- 

 dences of the striations appear in the muscle fibres, while, at the 

 same time, sarcostyles may be observed in them when they are viewed 

 in cross section. From this period, their development and ditferentiation 

 progresses with the increasing age of the embryo. 



5. The glands of the oesophagus first appear at this level in pigs 

 about 21 cm long. They originate as downgrowths of the mucosal 

 epithelium, which pass through the muscle bundles of the Muscularis 

 mucosae into the deeper layers of the Submucosa. These first down- 

 growths have a double row of low cubical cells. From the divisions 

 at their ends are formed the primitive alveoli, while from the inner 

 row of cells of these acini the mucous cells which push the outer 

 row off towards the basement membrane are derived. 



29* 



