Franklin P. Mall 357 



nuclei and endoplasni lie upon these bundles. The degree of develop- 

 ment of the exoplasm is practically of the stage I have termed pre- 

 iibrous above with numerous elastic fibrils, which stain with Weigert's 

 stain, added. This process is slightly more advanced in the umbilical 

 artery, which is especially suited for the study of early elastic fibrils, in 

 longitudinal or oblique sections (Fig. 1-1). In such sections all grades 

 of the development of elastic fibrils are easily found — from perfect 

 syncytium in the cord without elastic fibers to the finished elastic tissue 

 in the intima. At the point of juncture between the media and the 

 adventitia it is seen that the white fibrous tissue gradually passes over 



d^,- 



Fig. 14. Fig. 15. 



Fig. 14. Elastic tissue just beginning in the syncytium of tlie umbilical vein of a 

 pig 7 cm. long ( x 2.50 diameters). The specimen was first stained by Weigert's 

 method, then tinged with hcematoxylin and couuterstained with congo red. 



Fig. 15. Elastic fibers isolated from the skin of a pig 16 cm. long by means of 

 boiling acetic acid ( x 250 diameters). Stained with gentian violet. The fibrils form 

 baskets arouud the bundles of white fibrous tissue which are converted into a jelly- 

 like mass. 



into prefibrous tissue and this in turn over into typical exoplasm of 

 the syncytium of the cord. The degree of development of the elastic 

 tissue is exactly parallel with this. In the media the elastic network 

 encircles the bundles of white fibers, while in the region of prefibrous 

 tissue the network is in the periphery of the exoplasm. Farther out, in 

 the adventitia, the network of elastic fibrils is all through the exoplasm. 

 The fibrillated exoplasm in the walls of the arteries is composed of 

 two kinds of fibrils, destined to become the fibrils of white fibrous and 

 yellow elastic tissue. As this process of differentiation begins the white 

 fibers swell in acetic acid, are not digested in pancreatin, etc. While 

 the yellow elastic fibrils resist acids and dilute solutions of potassium 

 hydrate and stain intensely when treated by Weigert's method. At 

 25 



