210 Caroline Mcöill, 



have not touched upon the origin of myoflbrillae and intercellular 

 bridges, and other questions of histogenetic interest. 



Since the recent revival of the idea that all tissues, or at least 

 the embryonic tissues, are in the form of a syncytium, one or two papers, 

 notably one by Rohde (1905) call attention to the possibility of the 

 derivation of smooth muscle from a syncytium, rather than from in- 

 dividual cells; but none of these articles refer in more than a general 

 way to the subject, and none contain figures from embryonic material. 



Because of the incomplete knowledge of the development of 

 smooth muscle, it has seemed advisable to work out, as accurately as 

 possible, the histogenesis of the tissue in a continuous series of em- 

 bryos of one of the mammals. The subject, however, has been difficult 

 because of the uncertainty in some respects concerning the structure 

 of adult smooth muscle. Accordingly, in connection with the work on 

 histogenesis, it was found necessary to work out in detail the adult 

 histology in the form chosen for this investigation. As will appear 

 later, however, the study of development throws considerable light 

 upon the adult structure. It therefore seems advisable in this paper 

 to begin with the earliest stages of development and trace the process 

 through the various embryonic stages until the adult tissue is reached. 



This work was done in the Anatomical Laboratory of the Uni- 

 versity of Missouri, under the direction of Prof. C. M. Jackson. I wish 

 here to express my thanks to him for the kindly interest shown and 

 the many valuable suggestions offered in connection with this work. 



II. Review of the Literature. 



Before turning to a description of the development of smooth 

 muscle in the form studied, a resumé of the more important papers 

 dealing with the subject will be given. These papers, as will be seen, 

 might be divided into two classes: those dealing with the origin of 

 the tissue from the germ layers, and those which either support or 

 refute the cell theory. However, they will be considered in chrono- 

 logical order, without any attempt at classification. 



His (1868) advanced the view that all smooth muscle is deve- 



