236 Caroline McGill, 



5. Immediately following the elongation of the mesenchymal (or 

 embryonal connective tissue) cells, the formation of myofibrillae begins. 

 There are two varieties of myofibrillae, the coarse and the fine. 



6. The coarse fibrillae of the early embryo arise as follows: First, 

 granular myofibrillae appear as single rows of deeply staining granules, 

 apparently by a transformation of the granular protoplasmic reticulum. 

 The granular myofibrillae branch and anastomose and extend through 

 the muscle syncytium without regard to cell territories. At intervals 

 along the granular myofibrillae the granules increase in size and number, 

 forming spindle-shaped masses, so that the fibrillae appear varicose. 

 The granules of the myofibrillae next fuse into a homogeneous thread, 

 first in the spindles, and later in the slender intermediate portions. 

 The intermediate portions, between the spindles, then slowly enlarge, 

 so that the varicose myofibrillae become transformed into smooth, 

 coarse myofibrillae of uniform caliber. 



7. Fine myofibrillae do not arise until the pig embryo reaches a 

 length of about 30 mm. They are apparently homogeneous from the 

 beginning. In some cases, they seem to arise by a longitudinal split- 

 ting of the coarse myofibrillae. In development of smooth muscle in 

 the later embryo from the embryonal connective tissue, they appear 

 to arise as new formations in the protoplasm. In the later foetal 

 stages, the fine myofibrillae increase greatly in number. The coarse 

 myofibrillae, though variable in amount, usually decrease in number, 

 and in places may be entirely absent in the adult tissue. Rarely they 

 are abundant in the adult. 



8. The fine myofibrillae are scattered uniformly throughout the 

 cell protoplasm. The coarse myofibrillae are also from the beginning 

 similarly scattered. In the adult tissue, however, where the myofibrillae 

 are gathered into more sharply defined, compact bundles (fibers), the 

 coarse myofibrillae are often found lying in part near the surface of 

 the cell, resembling the „Grenzflbrillen" of Heidenhain. 



9. In the 15 mm pig embryo, throughout the mesenchyme and 

 also in the smooth muscle syncytium, collagenous fibrillae begin to form. 

 In the protoplasm of a single cell both myofibrillae and collagenous 

 fibrillae frequently differentiate side by side. From this it will be seen 



