Franklin P. Mall 359 



i. e., the granules are optical sections of fibrils of elastic tissue closely 

 packed around the bundles of white fibers. In the adventitia of the 

 umbilical artery, where the fibrils are cut parallel in this specimen, the 

 fibrils are all homogeneous and continuous. 



As the embryo grows the elastic tissue gradually extends along the 

 arteries to every part of the body, reaching those of the skin in embryos 

 about 20 cm. long. Shortly after the arteries of the skin have elastic 

 tissue in their wall, it can also be demonstrated in the loose tissue 

 below the hair follicles. 



From the study of the development of elastic tissue in the arteries it 

 is seen that the exoplasm of the connective-tissue syncytium forming 

 their walls differentiates into two kinds of fibrils, which give rise to 

 the white fibrous and elastic tissues, respectively. In other words, one 

 cell gives rise to both tissues. 



Arytenoid Cartilage. — In the arteries the elastic and white fibrous 

 tissues develop at the same time from the common exoplasm, as would 

 be expected in a region where the elastic tissue develops so early. In 

 cartilage, on the other hand, the exoplasm is converted completely into 

 the ground substance before the elastic fibers develop. A condition 

 which is parallel with that in cartilage is found in the skin, in bone and 

 in reticulated tissue when accompanied by elastic fibers. 



The arytenoid cartilage of the adult pig is partly hyaline and partly 

 elastic. Where the two kinds of tissue come together the fibrils course 

 in the ground substance between the fartilage cells. The hyaline car- 

 tilage near the elastic is infiltrated with granules which are sometimes 

 in rows but more frequently in clumps around one or more cartilage 

 cells. Generally the granules in the ground substance lie midway be- 

 tween the cells but where they begin to form masses they are usually 

 around a single cartilage cell. According to Kanvier the granules 

 form rows which coalesce to form elastic fibers. My own observations 

 show that whenever fibers or granules are in the same neighborhood 

 that they are separated and that one is never continuous with the other. 

 We have here to do with a special kind of elastic tissue composed only 

 of granules, as we have another form in the fenestrated membrane in 

 the smaller arteries. Conclusive proof is obtained when the develop- 

 ment of these structures is followed in the embryo pig. 



The arytenoid cartilage of a pig 12 cm long is a few millimeters long 

 and can easily be dissected out. It is then to be frozen and cut, stained 

 by Weigert's method, and mounted as usual. Such sections show that 

 most of the cartilage is hyaline, with some elastic fibers appearing at 

 one end of the cartilage. The fibrils are extremely delicate and lie 



