The Histogenesis of Smooth Muscle in the Alimentary Canal etc. 235 



and 28). The intimate relations of collagenous and elastic fibers with 

 the myofibrillae of the adult are clearly understood when their origin 

 from a common protoplasmic syncytium is remembered. 



V, Summary. 



1. The smooth muscle of the alimentary canal and respiratory 

 tract of the pig develops from primitive mesenchyme, which is made 

 up of stellate cells very intimately bound together by protoplasmic pro- 

 cesses into a syncytium. This syncytium persists throughout develop- 

 ment and in the adult muscle. 



2. In both alimentary canal and respiratory tract, just prior to 

 muscle formation, there occurs a proliferation and condensation of the 

 mesenchyme around the endodermal tubes. This appears in the respira- 

 tory tract and in the mid- oesophagus of the 5 mm pig embryo, and in 

 the 7 mm embryo has extended along the entire alimentary canal. 



3. The next step in muscle differentiation is an elongation of the 

 mesenchymal cells, especially of the nuclei. This occurs first in the 

 primitive bronchi and in the circular layer of the mid-oesophagus in 

 the 5 mm embryo, then appears in the circular layer of the lower 

 oesophagus in the 8 mm embryo and in the circular layer of the 

 stomach in the 9 mm embryo. In the 20 mm embryo, the circular 

 layer is found throughout the alimentary canal. The longitudinal layer 

 and muscularis mucosae of the alimentary canal appear later, first in 

 the mid-oesophagus and extending from there down the canal. Elonga- 

 tion for the longitudinal layer begins in the mid-oesophagus of the 

 20 mm pig, and in the stomach in the 30 mm pig. The muscularis 

 mucosae first appears in the 27 mm pig. The trachealis muscle is 

 beginning to differentiate in the pig embryo of 10 mm. 



4. After the layers of smooth muscle are established, the tissue 

 increases in amount in two ways: First, by a continuation of the pro- 

 cess of transformation of mesenchyme (or later of embryonal connective 

 tissue) into smooth muscle at the surface of the muscle layer, or from 

 interstitial cells. Tliis process predominates in earlier embryos. Second, 

 the nuclei of the already formed smooth muscle multiply by mitosis, 

 especially in the more advanced foetal stages. 



