The Histogenesis of Smooth Muscle in the Alimentary Canal etc. 227 



Sections of embryos from 5 mm to 15 mm in length, that is, during 

 the time when muscle is differentiating from the primitive mesenchyme, 

 and in the 15 mm embryo where the collagenous fibrillae are first 

 forming, give some very interesting results when stained with Mallory's 

 anilin-blue connective tissue stain. From the 5 mm to the 13 mm 

 embryo, the nuclei and scattered protoplasmic granules stain yellowish- 

 red; all other structures, the remaining protoplasmic granules of both 

 mesenchyme and muscle cells, as well as the myofibrillae, show a 

 blending of the fuchsin and anilin-blue stains, which gives them a pur- 

 pHsh color. In the 15 mm pig where the collagenous fibrillae make 

 their appearance, all of the structures, with the exception of the nuclei 

 and occasional protoplasmic granules stain blue (Fig. 9). With this 

 stain in embryos between 20 mm and 30 mm in length, the myofibrillae 

 begin to stain yellowish-red (Figs. 10 and 13) which is the diffe- 

 rential color for both coarse and fine myofibrillae in all later stages. 

 In Figs. 9, 10 and 13, the origin of both myofibrillae and collagenous 

 fibrillae in the same protoplasmic mass may be observed. 



5. The Development of the Interstitial Connective Tissue. 



A number of references have already been made to the develop- 

 ment of the connective tissue, in describing the histogenesis of the 

 smooth muscle tissue. A connected review of the process will be given 

 here, however, with a view of showing more clearly its relation to 

 the muscle. It has already been mentioned that in the formation of 

 muscle a few primitive mesenchyme or embryonal connective tissue cells 

 are left in between the muscle fibers {ct in Figs. 7, 11, 15 etc.). These 

 cells retain their stellate form, oval nuclei, and protoplasmic connection 

 with the muscle cells. They in part become the interstitial connective tissue 

 cells, and in part are later transformed into smooth muscle. It will be 

 seen from this that the interstitial connective tissue arises in situ and does 

 not grow in from the surrounding connective tissue as is described by 

 Szili (1902) to occur in the histogenesis of the intrinsic muscles of the eye. 



Collagenous fibrillae are first visible in about the 15 mm embryo, 



appearing throughout the entire mesenchymal and muscle syncytium 



surrounding the alimentary canal (Fig. 9). They show best stained 



15* 



