The Histogenesis of Smooth Muscle in the Alimentary Canal etc. 213 



opposition to the earlier findings of Kölliker, states that it arises from 

 mesenchyme and not from the endoderma! wall of the tract. His 

 figures very conclusively confirm his description. 



Manrer (1902) makes the statement that, in the alimentary canal 

 of Alytes, the mnscle is derived from the mesenchyme lying between 

 the endoderm and the serous membrane. 



Rohde (1905) calls attention to the fact that most of the investi- 

 gators who are working on smooth muscle hold to the view that each 

 smooth muscle fiber arises from a single embryonic cell and remains 

 separate and distinct from other cells. He makes the assertion that 

 the intercellular bridges, which are found present in the smooth muscle 

 of many adult forms, represent the remains of an embryonic syncytium. 

 He, however, gives no figures or descriptions of embryonic material 

 to support his view. 



III. Material and Methods. 



For material used in this work, the pig was chosen because of 

 the ease with which fresh material,, both embryonic and adult, can be 

 obtained, and also because of the large size and the clearness of 

 structure of the cells, especially in the embryo. The work is based 

 primarily upon observations on the development of the muscular tissue 

 in the alimentary canal, though the development in the respiratory 

 tract was also studied. 



Pig embryos from 4 mm in length up to full term were used. 

 For comparison, adult pig material was also employed. Stages between 

 the full term foetus and the adult pig Avere not available. In the 

 fuU term foetus, however, the smooth muscle has almost reached the 

 adult condition. 



All of the embryos used were fixed in Zenker's fluid. For adult 

 muscle, several fixatives were employed, Zenker's, Gilson's and Flem- 

 ming's solutions giving the best results. Gilson's fluid is especially 

 good because of the ease with which material fixed in it may be cut 

 and it apparently, in every way, gives as perfect fixation as do either 

 Zenker's or Flemming's mixtures. 



All of the younger embryos were embedded whole in paraffin and 



