The Histogenesis of Smooth Muscle in the Alimentary Canal etc. 215 



stain, I have also found it difficult to differentiate elastic fibers and 

 myofibrillae, both staining a reddish yellow. This is especially true 

 during development; yet even in the adult muscle where, according 

 to the* description of Mallory, elastic fibers stain yellow and myo- 

 fibrillae red, it is very difficult to get this differentiation, both usually 

 staining as they did during later development. At all times the 

 results obtained by this method are variable, depending upon the 

 length of time the material is stained, upon the extent of extraction etc. 

 In order to trace the various stages in the development of smooth 

 muscle, it was necessary to have both cross and longitudinal sections 

 through a given region of the alimentary canal, for embryos and 

 foetuses of various ages. For this region the lower oesophagus was 

 chosen. This was found to be the most favorable place from the fact 

 that here the smooth muscle appears very early and, further, in this 

 region during all early stages, the tissue is very loosely arranged, so 

 that it is much easier to make out the details of structure than in 

 the stomach or intestine where the tissue is more compact. The lower 

 portion of the oesophages was used, because little or no striated muscle 

 develops here to interfere with the results. In the adult, however, 

 the musculature of the oesophagus becomes denser, while that of the 

 stomach and intestine, due to the presence of large amounts of con- 

 nective tissue, is, in places, much looser. For this reason and also 

 for comparison, aside from the complete series from the lower end of 

 the oesophagus, sections from other portions of the alimentary canal 

 were found of service. A few of the drawings of the adult muscle 

 are made from regions other than the oesophagus. The development 

 of the smooth muscle in the trachea and bronchi was also studied. 

 In the description which follows, however, unless otherwise mentioned, 

 the explanations and figures refer to the lower end of the oesophagus. 



IV. Histogenesis of Smooth Muscle. 



1. Early Development. 



In the alimentary canal of the pig, smooth muscle first appears 



in the wall of the oesophagus. The differentiation begins just below 



the level of the bifurcation of the trachea. Kotzenberg (1902) has 



