356 The Development of the Connective Tissues 



of elastic tissue in the arteries and in cartilage. For this reason I shall 

 consider the development of elastic tissue in the skin at the end of the 

 discussion. 



Arteries. — At first it was extremely difficult to obtain any clear pic- 

 tures of young elastic fibers in the walls of the arteries by means of Wei- 

 gert's method, for the surrounding tissues were also stained somewhat 

 black. Finall}^ by staining the paraffin section upon the glass slide just 

 long enough, complete differentiation was obtained by subsequent treat- 

 ment with alcohol and hydrochloric acid, stronger than usual, and with a 

 saturated aqueous solution of picric acid. By this method numerous 

 sections were obtained with the elastic tissue only stained black. These 

 were then counterstained with congo red or first with a very dilute solu- 

 tion of Delafield's hematoxylin to tinge the nuclei a little and then 

 with Congo red. In this way perfect specimens were obtained with the 

 nuclei stained with hjematoxylin, the elastic fibers stained intensely 

 blue and the rest of the protoplasm red. 



Not any elastic fibers could be demonstrated by Weigert's method in 

 embryos less than 4 cm. long. As soon as the embryo has grown to this 

 length a delicate network of elastic fibrils is stained intensely blue-black 

 in the aorta and extends from the origin of the aorta into the arteries 

 arising from it. Here they are gradually lost. The arteries of the 

 skin do not have any elastic tissue in them. In a section of the 'carotid 

 artery it is seen that there is a thick layer of elastic fibrils in the intima 

 forming nearh' a complete membrane. The media is but a few cells 

 thick with a few individual fibrils between them. There are no elastic 

 fibrils in the adventitia. 



In an embryo 5 cm. long the elastic tissue is in the walls of the sub- 

 divisions of the main branches arising from the aorta. The walls of 

 the whole aorta and its main branches are filled with fibers which extend 

 into the adventitia. In the intima of the aorta the fibrils have coalesced 

 to form the Avell-known fenestrated membrane. In the carotid the 

 individual fibrils are present in the intima, the fenestrated membrane 

 appearing in an embryo somewhat older. The muscularis is filled with 

 most delicate elastic fibrils which together make a network of meshes 

 which are filled with nuclei. At the outer border of the muscularis 

 there is a gradual transition of the elastic tissue into the connective- 

 tissue syncytium of the adventitia. In a thin section stained with 

 Weigert's method and counterstained with congo red the relation of 

 the elastic fibers to the syncytium is especially well seen when examined 

 with the 2 mm. oil immersion lens of Zeiss. The elastic fibrils lie 

 within the exoplasm together with other fibrils and the spindle-shaped 



