The Histogenesis of Smooth Muscle in the Alimentary Canal etc. 229 



protoplasm they are, in sections stained with iron-haematoxylin , only 

 with difficulty distinguished from the coarse myofibrillae. They are 

 more nearly homogeneous and more wavy than are the coarse myo- 

 fibrillae, however (Figs. 15, 19 and 20, el). In the protoplasm of the 

 cell marked ct in the upper left-hand corner of Fig. 15, myofibrillae, 

 collagenous fibrillae, and an elastic fiber are visible. The number of 

 elastic fibers varies greatly in different regions, being abundant in some 

 places, and scanty or absent in others. 



6*. The Development of Smooth Muscle in the Respiratory Tract 



of the Pig. 



The 5 mm pig embryo is the youngest stage in which the respira- 

 tory tract was studied. At this time it has already undergone con- 

 siderable difierentiation. It consists of a deep groove, the pulmonary 

 groove, in the ventral wall of the oesophagus, which posteriorly, just 

 dorsal to the heart, ends abruptly in two small rounded evaginations, 

 the lung diverticula, or primitive bronchi. As yet, they are simple 

 endoderm al pouches lined by high stratified columnar epithelium. The 

 mesenchyme is already well condensed around the pulmonary groove 

 (Text figure 1, h) and the primitive bronchi. 



A little later, a constriction occurs between the oesophagus and 

 the pulmonary groove, so that the latter is separated from the oeso- 

 phagus and forms the trachea. This separation begins in the region 

 where the lung diverticula are given off and proceeds forwards. In 

 the 7 mm pig embryo the constriction has almost reached the region 

 of the larynx, so that the trachea and oesophagus are separate tubes, 

 and a small amount of mesenchyme has grown in between them. At 

 this stage the primitive bronchi have grown considerably but are still 

 simple pouches. 



In the respiratory tract, the smooth muscle first appears in the 

 walls of the primitive bronchi. It arises as a thin but continuous 

 layer from the mesenchyme just outside of the epithelial wall and so 

 closely applied to the wall as to lead Kölliker and others to the 

 erroneous conclusion that it is derived from the endoderm itself. In 

 the pig, the smooth muscle follows in all details of early histogenesis 



