218 Caroline McGill, 



cells with anastomosing branches. The nuclei of the syncytium are 

 round or oval with distinct nuclear wall and heavy chromatic reticulum. 

 The protoplasm shows a fine reticular structure, the strands of the 

 reticulum being made up of rows of fine granules (Plate VII, Fig. 1, m). 

 Because of the persistence of the syncytium and the entire absence 

 of cell boundaries, even in the adult, it is (as will appear later) 

 really incorrect to use the term muscle cell, or even the term muscle 

 fiber, which since the time of Kölliker has been synonymous with 

 muscle cell. Both of these names were applied when it was believed 

 that smooth muscle is made up of separate and distinct elements. 

 In the description, these terms, from the lack of something better, 

 will be used; but when employed they must be understood to mean 

 merely the irregularly spindle-shaped masses of protoplasm which make 

 the nodal points of the syncytium, together with the enclosed nuclei. 

 Separate and distinct smooth muscle cells or fibers do not exist at 

 any stage of development. 



If Plate VII, Fig. 1 be carefully observed, it will be seen that 

 there is apparently no protoplasmic continuity between the epithelial 

 cells lining the alimentary canal and the mesenchymal syncytium, the 

 basement membrane (h) forming a distinct separation. On the other 

 hand, the intimate protoplasmic connection which exists between the 

 mesenchyme (m) and the mesothelium (mes) is apparent. There is, 

 at stages represented by Plate VII, Fig. 1 and Text figure 1, nothing to 

 indicate what portion of the mesenchyme will give rise to smooth 

 muscle. 



Before muscle development begins, there is a general condensation 

 of the mesenchyme in the neighborhood of the endodermal tube. Al- 

 ready in the 5 mm pig this condensation is pronounced in the alimen- 

 tary canal from the level of the bifurcation of the trachea to the 

 region where the liver anläge is being evaginated. As yet there is 

 little dilatation of the canal in this portion to form the stomach. Just 

 above the lung diverticula thé tube has not yet divided to form trachea 

 and oesophagus. Text figure 1, which is from this region, shows the 

 loosely arranged mesenchyme (w) around the dorsal portion of the 

 tube (a) which, in embryos a little older, will form the oesophagus. 



