94 Caroline McGill, 



may resemble ordinary connective tissue cells, is no argument against 

 their not being myofibrils. This statement will be clear when it is 

 remembered that in their development the myofibrils usually arise in 

 an embryonal connective tissue syncytium. In this syncytium, side by 

 side with the myofibrils, collagenous and elastic fibers are differentia- 

 ting. In the later development, the connective tissue fibrils are crowded 

 to the periphery of the protoplasm by the rapid development of myo- 

 fibrils around the nucleus. Even in the adult, however, both connec- 

 tive tissue fibrils and myofibrils may occur in the same protoplasmic 

 mass. The details of the development of smooth muscle, I have de- 

 scribed in a paper published in "Internationale Monatsschrift für Ana- 

 tomie und Physiologie", 1907, on "The Histogenesis of Smooth Muscle 

 in the Pig". 



In the development of smooth muscle there are many more coarse 

 fibrils present than in adult tissue. Often these coarse myofibrils do 

 not run parallel with the long axis of the cell, and they frequently 

 break up into bundles of finer fibrils, showing much the same ap- 

 pearance as fibroglia fibrils. The picture shown in Figures 2, 4, 5 

 and 6, very closely resembles some stages in the development of smooth 

 muscle in pig intestine. The presence of the fibroglia fibrils, staining 

 exactly like coarse myofibrils, in such large numbers in the tissue 

 described, seems to indicate more that it is of an embryonal muscle 

 type rather than that it is connective tissue. 



That the fibroglia fibrils in part of their course seem to lie free 

 in the intercellular substance, does not indicate that they are not myo- 

 fibrils. In the mass of myofibrils making up the smooth muscle bundle, 

 the granular protoplasm is restricted to a small amount immediately 

 surrounding the nucleus. It is impossible to demonstrate, by ordinary 

 methods, the sarcoplasm which is supposed to lie between the myo- 

 fibrils, so that the fibril bundles, to all appearances, seem to lie free 

 in the intercellular substance. Heidenhain considers the myofibrils living 

 protoplasm and there seems to me to be every reason to believe that 

 they are such. From the close resemblance of the fibroglia fibrils to 

 myofibrils there is no reason to doubt that they are living protoplasm 

 also, and not merely a protoplasmic product. The close relation which 



