The Histogenesis of Smooth Muscle in the Alimentary Canal etc. 223 



in the protoplasm of the elongating cells. There are, as will appear 

 later, two kinds of myofibrillae, coarse and fine. The coarse myo- 

 fibrillae are first to develop. 



The myofibrillae appear in the circular layer of the oesophagus 

 at the level of the bifurcation of the trachea in the 9 mm pig. From 

 here the process of formation extends up and down the alimentary 

 canal. Plate Vin, Fig. 4, gs, from a 10 mm pig, shows them just 

 beginning to form in the lower end of the oesophagus. 



As previously described, the protoplasm of the stellate mesenchyme 

 cells apparently contains a granular reticulum (Plate VII, Fig. 1). As 

 the cells elongate to form muscle, the granules increase in number 

 and stain somewhat more intensely with the ordinary protoplasmic stains 

 (Fig. 4). As the elongation continues, the granular fibrils of the proto- 

 plasmic reticulum are stretched out more and more and finally appear 

 as more or less distinct longitudinal striatious (Plate VIII, Figs. 4 and 5). 

 The protoplasm of the cell body appears to be made up largely of 

 these irregular, longitudinal rows of granules instead of the fine-meshed, 

 protoplasmic reticulum. These granular fibrillae represent the anläge 

 of the myofibrillae. However, at the margins of the cell, in the proto- 

 plasmic processes connecting it with neighboring cells, and also around 

 the nucleus and between the myofibrillae, more or less ordinary granular 

 protoplasm remains (Plate Vili). These primitive, granular myofibrillae 

 occasionally branch and anastomose with each other. 



In the next stage of development, enlargements appear at certain 

 points on the granular myofibrillae. At these points, which usually 

 appear close to the nuclei, the granules become coarser, and arranged 

 in closely-packed groups. In this way; granular, spindle-shaped struc- 

 tures are formed (Plate VII, Fig. 3, Plate Vin, Fig. 4, gs), tapering off 

 at each end into the myofibrillae composed of a single row of granules. 

 In some cases, the end of the spindle appears to break up into several 

 branches of fine granular fibrillae, which may anastomose with neigh- 

 boring fibrillae (Plate VIII, Figs. 5 and 7, hr). For the most part, 

 however, the spindles are joined by the intermediate fibrillae into long, 

 varicose fibrils which pass through several cells, extending parallel to 

 the elongated nuclei (Fig. 3). 



