The Histogenesis of Smooth Muscle in the Alimentary Canal etc. 217 



most portion of the oesophagus. I have studied a number of pig 

 embryos at this stage and in all find, as stated above, the first change 

 at the level of the bifurcation of the trachea. From here the gradual 

 formation of muscle tissue extends up and down the can al, differen- 

 tiating in the lower oesophagus shortly after its appearance in mid- 

 oesophagus, then forming in the stomach, small intestine, and large 

 intestine in the order named. 



In the respiratory tract, the smooth muscle appears in the walls of 

 the primitive bronchi almost as soon as they are evaginated from the wall 

 of the alimentary canal, very shortly before muscle formation in the wall 

 of the oesophagus itself. In the trachea, the tissue arises somewhat later. 



Before the muscle begins to differentiate, the wall of the alimen- 

 tary canal consists of an endoderma! lining of simple columnar epithe- 

 lium, surrounded by a more or less well developed layer of mesen- 

 chyme (Text fig. 1). Where it is in relation to the coelomic cavity 

 there is in addition, outside of the mesenchyme, the mesothelium, 

 which in the early embryo is of either cubical or columnar epithelium 

 (Plate VII, Fig. 1, mes). 



In the mesenchymal coats surrounding the epithelial tubes of 

 the alimentary and respiratory tracts, the smooth muscle difierentiates. 

 Throughout, there is a very distinct basement membrane between 

 endoderm and mesenchyme. In early development, it is formed by a 

 condensation of the protoplasm of the outer border of the epithelial 

 cells (Plate VII, Figs. 1 to 3, 6), showing soon, however, in places, 

 additions from the protoplasmic reticulum of the surrounding mesen- 

 chyme. Since the basement membrane is present in the embryo long 

 before the muscle begins to form in either the alimentary canal, 

 bronchi, or trachea, there is no possibility for the muscle to arise 

 from the endoderm. My observations as to the origin of the muscle 

 in the respiratory and alimentary tracts therefore confirm the obser- 

 vations of Kotzenberg (1902) on the mouse. 



The mesenchyme of the alimentary and respiratory canals as 

 elsewhere is in the form of a syncytium, there being protoplasmic 

 continuity throughout the entire mesenchymal mass (Plate VII, Figs. 1 

 to 3, ni). It is usually described as made up of nucleated, stellate 



