228 Caroline McGill, 



blue with Mallory's anilin-blue connective tissue stain. From the 

 description of the myofibrillae it will be remembered that at this time 

 they also stain blue by this method, so from the 15 mm to about the 

 25 mm embryo, in protoplasmic masses where both myofibrillae and 

 collagenous fibrilla,e are forming side by side, it is often impossible to 

 tell the two apart. Soon, however, the coarse myofibrillae become 

 larger and straighter than the collagenous fibrillae. After the embryo 

 has reached a length of 25 mm, when the myofibrillae begin to stain 

 yellowish-red, it is possible to differentiate the two. Moreover, the 

 collagenous fibrillae usually form an irregular loose network,- while the 

 myofibrillae extend parallel with the elongated muscle nuclei. 



Soon there is in places a rapid multiplication of the interstitial 

 embryonal connective tissue cells by mitosis {mt in Figs. 5 and 15) 

 so that the muscle usually becomes divided into irregular bundles 

 separated from each other by varying amounts of connective tissue 

 (Figs. 17 and 18). Also between the individual muscle fibers of these 

 bundles are scattered connective tissue cells and large numbers of 

 collagenous fibrillae, derived in part from the protoplasm of the con- 

 nective tissue cells, and in part, from that of the muscle cells themselves. 



In development, protoplasmic connections between the muscle cells 

 and the connective tissue cells are easily demonstrated (Fig. 19, d). 

 Through the connecting bands of protoplasm, myofibrillae may enter 

 the protoplasm of the connective tissue cells or collagenous fibrillae 

 make their way into the muscle cells. Thus myofibrillae and connec- 

 tive tissue fibrillae may develop side by side, in the common pro- 

 toplasmic syncytium {cf Figs. 13, 18, 19 etc.). In later development, 

 however, most of the collagenous fibrillae are crowded out of the pro- 

 toplasm of the muscle cells into the intercellular spaces. 



Elastic fibers cannot be demonstrated in the wall of the alimentary 

 canal of the pig, by any of the elastic tissue stains which I have 

 employed, before the embryo reaches a length of 10 cm (Plate X, 

 Fig. 15, ei), though they appear in the walls of the large blood vessels 

 as early as the 30 mm embryo. They arise in the general protoplasmic 

 syncytium, either along the margins of the muscle cells or in the pro- 

 toplasm of the connective tissue cells. When arising in the muscle 



