230 Caroline McGill, 



the description by Kotzenberg (1902) of the development in the primi- 

 tive bronchi of the mouse. 



Already in the 5 mm embryo, a few of the mesenchymal nuclei 

 close around the endodermal walls of the bronchi are seen in cross 

 section to be elongating to form muscle nuclei. In the 13 mm pig 

 there is a continuous layer of muscle one or two cells deep around 

 the entire circumference of the bronchus, and myofibrillae are forming 

 in the protoplasm. The smooth muscle around the primitive bronchi 

 therefore develops at about the same time, and in the same way, as 

 in the adjacent région of the oesophagus. 



The elongation of mesenchymal cells for the formation of the 

 trachealis muscle begins in the 10 mm pig. It appears at the bifur- 

 cation of the trachea and extends from there up the tube. The elon- 

 gation from the first is confined to the mesenchyme lying dorsal to 

 the tube and just outside the epithelial wall. The cells elongate in a 

 transverse direction, but the process never extends entirely around the 

 trachea. In the 13 mm pig, the area of elongation has extended half 

 way around the tube. Myofibrillae have, at this time, begun to form 

 in the protoplasm. 



Further differentiation of the smooth muscle, both in the trachea 

 and in the bronchi, follows the same course as described for the muscle 

 of the alimentary canal. It is therefore unnecessary to describe in 

 detail the histogenetic changes, or to give figures showing the various 

 details already discussed in connection with the alimentary canal. 



7. Comparisoii with Adult Smooth Muscle. 



No attempt will be made here to go into the details of the struc- 

 ture of adult smooth muscle. For a comprehensive review of the lite- 

 rature on this subject, the reader is referred to the work of Heiden- 

 hain, M. (1900). The subject is here discussed merely to trace the 

 developmental changes through which the structure of the adult tissue 

 is reached, and to consider the structure of adult smooth muscle in 

 the light of its histogenesis. 



From the oesophagus down to the rectum the muscle of the 

 alimentary canal of the adult pig is arranged in three layers, an outer 



