224 Caroline McGill, 



The granular stage of the myofibrillae does not persist long, 

 however. Soon there is a condensation and fusion of the granules 

 both in the spindles and in the anastomosing flbrillae to form 

 apparently solid fibrils. The solidification appears fiist in the spindles 

 (Fig. 3, hs), and slightly later in the finer granular myofibrillae, 

 which are thus transformed into smooth fibrillae In this way, vari- 

 cose, smooth fibrillae like that shown at vf, in Fig. 7 are formed. 

 These varicose fibrillae run for long distances through the protoplasm, 

 passing through many muscle cells. In these early stages in the 

 circular layer they may run as much as one-half the distance around 

 the alimentary canal. Soon there may be several running through 

 each cell, causing a marked longitudinal striation. Between the 

 spindles, the fibrillae are at first slender, but they gradually become 

 thicker, so that the fibrillae become of a uniform, coarse caliber. In 

 embryos between 30 mm and 40 mm in length, the coarse fibrillae 

 become more even in caliber (Fig. 11). Figure 6 shows the circular 

 layer of muscle from the oesophagus of a 15 mm embryo cut in cross 

 section. The coarse fibrillae here appear as the large, black spots, 

 the finer, granular fibrillae as small black or red dots, embedded in 

 the granular protoplasm. 



This mode of formation of the coarse myofibrillae, i. e., primitive, 

 granular myofibrillae and spindles, later forming smooth, varicose 

 fibrils, continues until in the 27 mm or 30 mm pig. Here and there, 

 finer fibrillae do occur in the smooth muscle of the early embryo, but 

 they can usually be shown to be granular in structure and are pro- 

 bably merely stages in the formation of the coarse myofibrillae. In 

 the later embryos, the granular myofibrillae and spindles are absent 

 or inconspicuous, though smooth, varicose myofibrillae are not infrequent 

 (Figs. 13, 14, 15). From the 30 mm pig on, fine as well as coarse 

 myofibrillae are present in the smooth muscle cells. They are well 

 shown in Fig. 11 from a 38 mm embryo. In the late foetus and in 

 the adult, coarse myofibrillae are sometimes few in number or alto- 

 gether absent, but from the 30 mm pig up to and including adult 

 muscle, fine myofibrillae, to all appearances homogeneous, are always 

 present. 



